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<appendix id="hw">
  <title>PC Hardware compatibility</title>
  
  <para>Issues of hardware compatibility are among the most troublesome in the
    computer industry today and FreeBSD is by no means immune to trouble.  In
    this respect, FreeBSD's advantage of being able to run on inexpensive
    commodity PC hardware is also its liability when it comes to support for
    the amazing variety of components on the market.  While it would be
    impossible to provide a exhaustive listing of hardware that FreeBSD
    supports, this section serves as a catalog of the device drivers included
    with FreeBSD and the hardware each drivers supports. Where possible and
    appropriate, notes about specific products are included.  You may also
    want to refer to <link linkend="kernelconfig-config">the kernel
      configuration file</link> section in this handbook for a list of
    supported devices.</para>
      
  <para>As FreeBSD is a volunteer project without a funded testing department,
    we depend on you, the user, for much of the information contained in this
    catalog.  If you have direct experience of hardware that does or does not
    work with FreeBSD, please let us know by sending e-mail to the &a.doc;.
    Questions about supported hardware should be directed to the &a.questions;
    (see <link linkend="eresources-mail">Mailing Lists</link> for more
    information).  When submitting information or asking a question, please
    remember to specify exactly what version of FreeBSD you are using and
    include as many details of your hardware as possible.</para>
      
  <sect1>
    <title>Resources on the Internet</title>
    
    <para>The following links have proven useful in selecting hardware. Though
      some of what you see won't necessarily be specific (or even applicable)
      to FreeBSD, most of the hardware information out there is OS
      independent.  Please check with the FreeBSD hardware guide to make sure
      that your chosen configuration is supported before making any
      purchases.</para>

    <itemizedlist>
      <listitem>
	<para><ulink url="http://www.tomshardware.com/">The Pentium Systems
	    Hardware Performance Guide</ulink></para>
      </listitem>
    </itemizedlist>
  </sect1>
      
  <sect1 id="hw-configs">
    <title>Sample Configurations</title>
    
    <para>The following list of sample hardware configurations by no means
      constitutes an endorsement of a given hardware vendor or product by
      <emphasis>The FreeBSD Project</emphasis>.  This information is provided
      only as a public service and merely catalogs some of the experiences
      that various individuals have had with different hardware combinations.
      Your mileage may vary.  Slippery when wet. Beware of dog.</para>
    
    <sect2 id="hw-jordans-picks">
      <title>Jordan's Picks</title>
      
      <para>I have had fairly good luck building workstation and server
	configurations with the following components.  I can't guarantee that
	you will too, nor that any of the companies here will remain
	<quote>best buys</quote> forever.  I will try, when I can, to keep this
	list up-to-date but cannot obviously guarantee that it will be at any
	given time.</para>
	  
      <sect3 id="hw-mb">
	<title>Motherboards</title>

	<para>For Pentium Pro (P6) systems, I'm quite fond of the <ulink
	    url="http://www.tyan.com/html/products.html">Tyan</ulink> S1668
	  dual-processor motherboard as well as the Intel PR440FX motherboard
	  with on-board SCSI WIDE and 100/10MB Intel Etherexpress NIC.  You
	  can build a dandy little single or dual processor system (which is
	  supported in FreeBSD 3.0) for very little cost now that the Pentium
	  Pro 180/256K chips have fallen so greatly in price, but no telling
	  how much longer this will last.</para>
	    
	<para>For the Pentium II, I'm rather partial to the <ulink
	    url="http://www.asus.com.tw/">ASUS</ulink> <ulink
	    url="http://www.asus.com.tw/Products/Motherboard/Pentiumpro/P2l97-s/index.html">P2l97-S</ulink>
	  motherboard with the on-board Adaptec SCSI WIDE controller.</para>
	    
	<para>For Pentium machines, the ASUS <ulink
	    url="http://www.asus.com.tw/Products/Motherboard/Pentium/P55tp4/index.html">P55T2P4</ulink>
	  motherboard appears to be a good choice for mid-to-high range
	  Pentium server and workstation systems.</para>

	<para>Those wishing to build more fault-tolerant systems should also
	  be sure to use Parity memory or, for truly 24/7 applications, ECC
	  memory.</para>

	<note>
	  <para>ECC memory does involve a slight performance trade-off (which
	    may or may not be noticeable depending on your application) but
	    buys you significantly increased fault-tolerance to memory
	    errors.</para>
	</note>
      </sect3>
      
      <sect3>
	<title>Disk Controllers</title>
	    
	<para>This one is a bit trickier, and while I used to recommend the
	  <ulink url="http://www.buslogic.com/">Buslogic</ulink> controllers
	  unilaterally for everything from ISA to PCI, now I tend to lean
	  towards the <ulink url="http://www.adaptec.com/">Adaptec</ulink>
	  1542CF for ISA, Buslogic Bt747c for EISA and Adaptec 2940UW for
	  PCI.</para>
	    
	<para>The NCR/Symbios cards for PCI have also worked well for me,
	  though you need to make sure that your motherboard supports the
	  BIOS-less model if you're using one of those (if your card has
	  nothing which looks even vaguely like a ROM chip on it, you've
	  probably got one which expects its BIOS to be on your
	  motherboard).</para>
	    
	<para>If you should find that you need more than one SCSI controller
	  in a PCI machine, you may wish to consider conserving your scarce
	  PCI bus resources by buying the Adaptec 3940 card, which puts two
	  SCSI controllers (and internal busses) in a single slot.</para>

	<note>
	  <para>There are two types of 3940 on the market&mdash;the older
	    model with AIC 7880 chips on it, and the newer one with AIC 7895
	    chips.  The newer model requires <ulink
	      url="http://www.FreeBSD.org/pub/FreeBSD/development/cam/">CAM</ulink>
	    support which is not yet part of FreeBSD&mdash;you have to add it,
	    or install from one of the CAM binary snapshot release.</para>
	</note>
      </sect3>
      
      <sect3 id="hw-disks">
	<title>Disk drives</title>

	<para>In this particular game of Russian roulette, I'll make few
	  specific recommendations except to say <quote>SCSI over IDE whenever
	  you can afford it.</quote> Even in small desktop configurations, SCSI
	  often makes more sense since it allows you to easily migrate drives
	  from server to desktop as falling drive prices make it economical to
	  do so.  If you have more than one machine to administer then think
	  of it not simply as storage, think of it as a food chain! For a
	  serious server configuration, there's not even any
	  argument&mdash;use SCSI equipment and good cables.</para>
      </sect3>
      
      <sect3 id="hw-jordans-picks-cdrom">
	<title>CDROM drives</title>

	<para>My SCSI preferences extend to SCSI CDROM drives as well, and
	  while the <ulink url="http://www.toshiba.com/">Toshiba</ulink> drives
	  have always been favourites of mine (in whatever speed is hot that
	  week), I'm still fond of my good old <ulink
 	    url="http://www.plextor.com/">Plextor</ulink> PX-12CS drive.  It's
	  only a 12 speed, but it's offered excellent performance and
	  reliability.</para>
	    
	<para>Generally speaking, most SCSI CDROM drives I've seen have been
	  of pretty solid construction and you probably won't go wrong with an
	  HP or NEC SCSI CDROM drive either.  SCSI CDROM prices also appear to
	  have dropped considerably in the last few months and are now quite
	  competitive with IDE CDROMs while remaining a technically superior
	  solution.  I now see no reason whatsoever to settle for an IDE CDROM
	  drive if given a choice between the two.</para>
      </sect3>
      
      <sect3 id="hw-worm">
	<title>CD Recordable (WORM) drives</title>

	<para>At the time of this writing, FreeBSD supports 3 types of CDR
	  drives (though I believe they all ultimately come from Phillips
	  anyway): The Phillips CDD 522 (Acts like a Plasmon), the PLASMON
	  RF4100 and the HP 6020i.  I myself use the HP 6020i for burning
	  CDROMs (in 2.2 and alter releases&mdash;it does not work with
	  earlier releases of the SCSI code) and it works very well.  See
	  <ulink
	    url="file:/usr/share/examples/worm">/usr/share/examples/worm</ulink>
	  on your 2.2 system for example scripts used to created ISO9660
	  filesystem images (with RockRidge extensions) and burn them onto an
	  HP6020i CDR.</para>
      </sect3>
	  
      <sect3 id="hw-tape">
	<title>Tape drives</title>

	<para>I've had pretty good luck with both <ulink
	    url="http://www.Exabyte.COM:80/Products/8mm/8505XL/Rfeatures.html">8mm
	    drives</ulink> from <ulink
	    url="http://www.exabyte.com/">Exabyte</ulink> and  <ulink
	    url="http://www-dmo.external.hp.com:80/tape/_cpb0001.htm">4mm
	    (DAT)</ulink> drives from <ulink
	    url="http://www.hp.com/">HP</ulink>.</para>

	<para>For backup purposes, I'd have to give the higher recommendation
	  to the Exabyte due to the more robust nature (and higher storage
	  capacity) of 8mm tape.</para>
      </sect3>
      
      <sect3 id="hw-video">
	<title>Video Cards</title>

	<para>If you can also afford to buy a commercial X server for
	  US&#36;99 from <ulink url="http://www.xig.com/">Xi Graphics, Inc.
	    (formerly X Inside, Inc)</ulink> then I can heartily recommend the
	  <ulink url="http://www.matrox.com/">Matrox</ulink> <ulink
	    url="http://www.matrox.com/mgaweb/brochure.htm">Millenium
	    II</ulink> card.  Note that support for this card is also
	  excellent with the <ulink
	    url="http://www.xfree86.org/">XFree86</ulink> server, which is now
	  at version 3.3.2.</para>

	<para>You also certainly can't go wrong with one of <ulink
	    url="http://www.nine.com/">Number 9's</ulink> cards &mdash; their
	  S3 Vision 868 and 968 based cards (the 9FX series) also being quite
	  fast and very well supported by XFree86's S3 server.  You can also
	  pick up their Revolution 3D cards very cheaply these days,
	  especially if you require a lot of video memory.</para>
      </sect3>
      
      <sect3 id="hw-monitors">
	<title>Monitors</title>

	<para>I have had very good luck with the <ulink
	    url="http://cons3.sel.sony.com/SEL/ccpg/display/ms17se2.html">Sony
	    Multiscan 17seII monitors</ulink>, as have I with the Viewsonic
	  offering in the same (Trinitron) tube.  For larger than 17", all I
	  can recommend at the time of this writing is to not spend any less
	  than U.S.  &#36;2,000 for a 21" monitor or &#36;1,700 for a 20"
	  monitor if that's what you really need.  There are good monitors
	  available in the &gt;=20" range and there are also cheap monitors in
	  the &gt;=20" range.  Unfortunately, very few are both cheap and
	  good!</para>
      </sect3>
	  
      <sect3 id="hw-networking">
	<title>Networking</title>

	<para>I can recommend the Intel EtherExpress Pro/100B card first and
	  foremost, followed by the <ulink
	    url="http://www.smc.com/">SMC</ulink> Ultra 16 controller for any
	  ISA application and the SMC EtherPower or Compex ENET32 cards for
	  slightly cheaper PCI based networking.  In general, any PCI NIC
	  based around DEC's DC21041 Ethernet controller chip, such as the
	  Znyx ZX342 or DEC DE435/450, will generally work quite well and can
	  frequently be found in 2-port and 4-port version (useful for
	  firewalls and routers), though the Pro/100MB card has the edge when
	  it comes to providing the best performance with lower
	  overhead.</para>
	    
	<para>If what you're looking for is the cheapest possible solution
	  then almost any NE2000 clone will do a fine job for very little
	  cost.</para>
      </sect3>
      
      <sect3 id="hw-serial">
	<title>Serial</title>

	<para>If you're looking for high-speed serial networking solutions,
	  then <ulink url="http://www.dgii.com/">Digi International</ulink>
	  makes the <ulink
	    url="http://www.dgii.com/prodprofiles/profiles-prices/digiprofiles/digispecs/sync570.html">SYNC/570</ulink>
	  series, with drivers now in FreeBSD-current.  <ulink
	    url="http://www.etinc.com/">Emerging Technologies</ulink> also
	  manufactures a board with T1/E1 capabilities, using software they
	  provide.  I have no direct experience using either product,
	  however.</para>
	    
	<para>Multiport card options are somewhat more numerous, though it has
	  to be said that FreeBSD's support for <ulink
	    url="http://www.cyclades.com/">Cyclades</ulink>'s products is
	  probably the tightest, primarily as a result of that company's
	  commitment to making sure that we are adequately supplied with
	  evaluation boards and technical specs.  I've heard that the
	  Cyclom-16Ye offers the best price/performance, though I've not
	  checked the prices lately.  Other multiport cards I've heard good
	  things about are the BOCA and AST cards, and <ulink
	    url="http://www.stallion.com/">Stallion Technologies</ulink>
	  apparently offers an unofficial driver for their  cards at <ulink
	    url="ftp://ftp.stallion.com/drivers/unsupported/freebsd/stalbsd-0.0.4.tar.gz">this</ulink>
	  location.</para>
      </sect3>
      
      <sect3 id="hw-audio">
	<title>Audio</title>

	<para>I currently use a <ulink url="http://www.creaf.com/">Creative
	    Labs</ulink> AWE32 though just about anything from Creative Labs
	  will generally work these days.  This is not to say that other types
	  of sound cards don't also work, simply that I have little experience
	  with them (I was a former GUS fan, but Gravis's soundcard situation
	  has been dire for some time).</para>
      </sect3>
      
      <sect3 id="hw-vgrabbers">
	<title>Video</title>

	<para>For video capture, there are two good choices &mdash; any card
	  based on the Brooktree BT848 chip, such as the Hauppage or WinTV
	  boards, will work very nicely with FreeBSD.  Another board which
	  works for me is the <ulink
	    url="http://www.matrox.com/">Matrox</ulink> <ulink
	    url="http://www.matrox.com/imgweb/meteor.htm">Meteor</ulink> card.
	  FreeBSD also supports the older video spigot card from Creative
	  Labs, but those are getting somewhat difficult to find. Note that
	  the Meteor frame grabber card <emphasis>will not work</emphasis>
	  with motherboards based on the 440FX chipset! See the <link
	    linkend="hw-mb">motherboard reference</link> section for details.
	  In such cases, it's better to go with a BT848 based board.</para>
      </sect3>
    </sect2>
  </sect1>
  
  <sect1 id="hw-core">
    <title>Core/Processing</title>
    
    <sect2>
      <title>Motherboards, busses, and chipsets</title>
	  
      <sect3>
	<title>* ISA</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>* EISA</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>* VLB</title>

	<para></para>
      </sect3>
      
      <sect3 id="hw-mb-pci">
	<title>PCI</title>

	<para><emphasis>Contributed by &a.obrien; from postings by
	    &a.rgrimes;.  25 April 1995.</emphasis></para>

	<para><emphasis>Continuing updates by &a.jkh;.</emphasis>  Last 
          update on <emphasis>26 August 1996.</emphasis></para>

	<para>Of the Intel PCI chip sets, the following list describes various
	  types of known-brokenness and the degree of breakage, listed from
	  worst to best.</para>
	    
	<variablelist>
	  <varlistentry>
	    <term>Mercury:</term>

	    <listitem>
	      <para>Cache coherency problems, especially if there are ISA bus
		masters behind the ISA to PCI bridge chip. Hardware flaw, only
		known work around is to turn the cache off.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>Saturn-I <emphasis>(ie, 82424ZX at rev 0, 1 or
		2)</emphasis>:</term>
		  
	    <listitem>
	      <para>Write back cache coherency problems.  Hardware flaw, only
		known work around is to set the external cache to
		write-through mode.  Upgrade to Saturn-II.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>Saturn-II <emphasis>(ie, 82424ZX at rev 3 or
		4)</emphasis>:</term>
		  
	    <listitem>
	      <para>Works fine, but many MB manufactures leave out the
		external dirty bit SRAM needed for write back operation. Work
		arounds are either run it in write through mode, or get the
		dirty bit SRAM installed.  (I have these for the ASUS
		PCI/I-486SP3G rev 1.6 and later boards).</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>Neptune:</term>
	    
	    <listitem>
	      <para>Can not run more than 2 bus master devices. Admitted Intel
		design flaw.  Workarounds include do not run more than 2 bus
		masters, special hardware design to replace the PCI bus
		arbiter (appears on Intel Altair board and several other Intel
		server group MB's).  And of course Intel's official answer,
		move to the Triton chip set, we <quote>fixed it
		there</quote>.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>Triton <emphasis>(ie, 430FX)</emphasis>:</term>
		  
	    <listitem>
	      <para>No known cache coherency or bus master problems, chip set
		does not implement parity checking.  Workaround for parity
		issue.  Use Triton-II based motherboards if you have the
		choice.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>Triton-II <emphasis>(ie, 430HX)</emphasis>:</term>
	    
	    <listitem>
	      <para>All reports on motherboards using this chipset have been
		favorable so far.  No known problems.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term>Orion:</term>
	    
	    <listitem>
	      <para>Early versions of this chipset suffered from a PCI
		write-posting bug which can cause noticeable performance
		degradation in applications where large amounts of PCI bus
		traffic is involved.  B0 stepping or later revisions of the
		chipset fixed this problem.</para>
	    </listitem>
	  </varlistentry>
	  
	  <varlistentry>
	    <term><ulink
		url="http://developer.intel.com/design/pcisets/desktop.htm#440FX">440FX</ulink>:</term>
		  
	    <listitem>
	      <para>This <ulink
		  url="http://www.intel.com/procs/ppro/index.htm">Pentium
		  Pro</ulink> support chipset seems to work well, and does not
		suffer from any of the early Orion chipset problems.  It also
		supports a wider variety of memory, including ECC and parity.
		The only known problem with it is that the Matrox Meteor frame
		grabber card doesn't like it.</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </sect3>
    </sect2>
    
    <sect2>
      <title>CPUs/FPUs</title>
      
      <para><emphasis>Contributed by &a.asami;.  26 December
	  1997.</emphasis></para>
	  
      <sect3>
	<title>P6 class (Pentium Pro/Pentium II)</title>

	<para>Both the Pentium Pro and Pentium II work fine with FreeBSD. In
	  fact, our main ftp site <ulink
	    url="ftp://ftp.FreeBSD.org/">ftp.FreeBSD.org</ulink> (also known
	  as "<filename>ftp.cdrom.com</filename>", world's largest ftp site)
	  runs FreeBSD on a Pentium Pro.  <ulink
	    url="ftp://ftp.cdrom.com/archive-info/wcarchive.txt">Configurations
	    details</ulink> are available for interested parties.</para>
      </sect3>
      
      <sect3>
	<title>Pentium class</title>

	<para>The Intel Pentium (P54C), Pentium MMX (P55C), AMD K6 and
	  Cyrix/IBM 6x86MX processors are all reported to work with FreeBSD.
	  I will not go into details of which processor is faster than what,
	  there are zillions of web sites on the Internet that tells you one
	  way or another.  <!-- smiley --><emphasis>:)</emphasis></para>

	<note>
	  <para>Various CPUs have different voltage/cooling requirements. Make
	    sure your motherboard can supply the exact voltage needed by the
	    CPU.  For instance, many recent MMX chips require split voltage
	    (e.g., 2.9V core, 3.3V I/O).  Also, some AMD and Cyrix/IBM chips
	    run hotter than Intel chips.  In that case, make sure you have
	    good heatsink/fans (you can get the list of certified parts from
	    their web pages).</para>
	</note>

	<sect4>
	  <title>Clock speeds</title>
	  
	  <para><emphasis>Contributed by &a.rgrimes;.  1 October
	      1996.</emphasis></para>
	      
	  <para><emphasis>Updated by &a.asami;.  27 December
	      1997.</emphasis></para>
	  
	  <para>Pentium class machines use different clock speeds for the
	    various parts of the system.  These being the speed of the CPU,
	    external memory bus, and the PCI bus.  It is not always true that
	    a <quote>faster</quote> processor will make a system faster than a
	    <quote>slower</quote> one, due to the various clock speeds used.
	    Below is a table showing the differences:</para>

	  <informaltable frame="none">
	    <tgroup cols="4">
	      <thead>
		<row>
		  <entry>Rated CPU MHz</entry>
		  <entry>External Clock and Memory Bus MHz</entry>
		  <entry>External to Internal Clock Multiplier</entry>
		  <entry>PCI Bus Clock MHz</entry>
		</row>
	      </thead>

	      <tbody>
		<row>
		  <entry>60</entry>
		  <entry>60</entry>
		  <entry>1.0</entry>
		  <entry>30</entry>
		</row>
		
		<row>
		  <entry>66</entry>
		  <entry>66</entry>
		  <entry>1.0</entry>
		  <entry>33</entry>
		</row>
		
		<row>
		  <entry>75</entry>
		  <entry>50</entry>
		  <entry>1.5</entry>
		  <entry>25</entry>
		</row>
		
		<row>
		  <entry>90</entry>
		  <entry>60</entry>
		  <entry>1.5</entry>
		  <entry>30</entry>
		</row>
		
		<row>
		  <entry>100</entry>
		  <entry>50</entry>
		  <entry>2</entry>
		  <entry>25</entry>
		</row>
		
		<row>
		  <entry>100</entry>
		  <entry>66</entry>
		  <entry>1.5</entry>
		  <entry>33</entry>
		</row>
		
		<row>
		  <entry>120</entry>
		  <entry>60</entry>
		  <entry>2</entry>
		  <entry>30</entry>
		</row>
		
		<row>
		  <entry>133</entry>
		  <entry>66</entry>
		  <entry>2</entry>
		  <entry>33</entry>
		</row>
		
		<row>
		  <entry>150</entry>
		  <entry>60</entry>
		  <entry>2.5</entry>
		  <entry>30 (Intel, AMD)</entry>
		</row>
		
		<row>
		  <entry>150</entry>
		  <entry>75</entry>
		  <entry>2</entry>
		  <entry>37.5 (Cyrix/IBM 6x86MX)</entry>
		</row>
		
		<row>
		  <entry>166</entry>
		  <entry>66</entry>
		  <entry>2.5</entry>
		  <entry>33</entry>
		</row>
		
		<row>
		  <entry>180</entry>
		  <entry>60</entry>
		  <entry>3</entry>
		  <entry>30</entry>
		</row>
		
		<row>
		  <entry>200</entry>
		  <entry>66</entry>
		  <entry>3</entry>
		  <entry>33</entry>
		</row>
		
		<row>
		  <entry>233</entry>
		  <entry>66</entry>
		  <entry>3.5</entry>
		  <entry>33</entry>
		</row>
	      </tbody>
	    </tgroup>
	  </informaltable>
	  
	  <note>
	    <para>66MHz may actually be 66.667MHz, but don't assume so.</para>
	    
	    <para>The Pentium 100 can be run at either 50MHz external clock
	      with a multiplier of 2 or at 66MHz and a multiplier of
	      1.5.</para>
	  </note>
	  
	  <para>As can be seen the best parts to be using are the 100, 133,
	    166, 200 and 233, with the exception that at a multiplier of 3 or
	    more the CPU starves for memory.</para>
	</sect4>

	<sect4>
	  <title>The AMD K6 Bug</title>
	  
	  <para>In 1997, there have been reports of the AMD K6 seg faulting
	    during heavy compilation.  That problem has been fixed in 3Q '97.
	    According to reports, K6 chips with date mark <quote>9733</quote>
	    or larger (i.e., manufactured in the 33rd week of '97 or later) do
	    not have this bug.</para>
	</sect4>
      </sect3>
      
      <sect3>
	<title>* 486 class</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>* 386 class</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>286 class</title>
	    
	<para>Sorry, FreeBSD does not run on 80286 machines.  It is nearly
	  impossible to run today's large full-featured UNIXes on such
	  hardware.</para>
      </sect3>
    </sect2>
    
    <sect2>
      <title>* Memory</title>
      
      <para>The minimum amount of memory you must have to install FreeBSD is 5
	MB.  Once your system is up and running you can <link
	  linkend="kernelconfig-building">build a custom kernel</link> that
	will use less memory.  If you use the <filename>boot4.flp</filename>
	you can get away with having only 4 MB.</para>
    </sect2>
    
    <sect2>
      <title>* BIOS</title>
      
      <para></para>
    </sect2>
  </sect1>
  
  <sect1 id="hw-io">
    <title>Input/Output Devices</title>
    
    <sect2>
      <title>* Video cards</title>
      
      <para></para>
    </sect2>
    
    <sect2>
      <title>* Sound cards</title>
      
      <para></para>
    </sect2>
    
    <sect2>
      <title>Serial ports and multiport cards</title>
      
      <sect3 id="uart">
	<title>The UART: What it is and how it works</title>

	<para><emphasis>Copyright &copy; 1996 &a.uhclem;, All Rights
	    Reserved.  13 January 1996.</emphasis></para>

	<para>The Universal Asynchronous Receiver/Transmitter (UART)
	  controller is the key component of the serial communications
	  subsystem of a computer.  The UART takes bytes of data and transmits
	  the individual bits in a sequential fashion.  At the destination, a
	  second UART re-assembles the bits into complete bytes.</para>
	    
	<para>Serial transmission is commonly used with modems and for
	  non-networked communication between computers, terminals and other
	  devices.</para>
	    
	<para>There are two primary forms of serial transmission: Synchronous
	  and Asynchronous.  Depending on the modes that are supported by the
	  hardware, the name of the communication sub-system will usually
	  include a <literal>A</literal> if it supports Asynchronous
	  communications, and a <literal>S</literal> if it supports
	  Synchronous communications.  Both forms are described below.</para>
	    
	<para>Some common acronyms are:
	  
	  <blockquote>
	    <para>UART	Universal Asynchronous
	      Receiver/Transmitter</para>
	  </blockquote>
	  
	  <blockquote>
	    <para>USART	Universal Synchronous-Asynchronous
	      Receiver/Transmitter</para>
	  </blockquote></para>

	<sect4>
	  <title>Synchronous Serial Transmission</title>
	  
	  <para>Synchronous serial transmission requires that the sender and
	    receiver share a clock with one another, or that the sender
	    provide a strobe or other timing signal so that the receiver knows
	    when to <quote>read</quote> the next bit of the data.  In most
	    forms of serial Synchronous communication, if there is no data
	    available at a given instant to transmit, a fill character must be
	    sent instead so that data is always being transmitted.
	    Synchronous communication is usually more efficient because only
	    data bits are transmitted between sender and receiver, and
	    synchronous communication can be more more costly if extra wiring
	    and circuits are required to share a clock signal between the
	    sender and receiver.</para>
	      
	  <para>A form of Synchronous transmission is used with printers and
	    fixed disk devices in that the data is sent on one set of wires
	    while a clock or strobe is sent on a different wire. Printers and
	    fixed disk devices are not normally serial devices because most
	    fixed disk interface standards send an entire word of data for
	    each clock or strobe signal by using a separate wire for each bit
	    of the word.  In the PC industry, these are known as Parallel
	    devices.</para>
	      
	  <para>The standard serial communications hardware in the PC does not
	    support Synchronous operations.  This mode is described here for
	    comparison purposes only.</para>
	</sect4>

	<sect4>
	  <title>Asynchronous Serial Transmission</title>
	  
	  <para>Asynchronous transmission allows data to be transmitted
	    without the sender having to send a clock signal to the receiver.
	    Instead, the sender and receiver must agree on timing parameters
	    in advance and special bits are added to each word which are used
	    to synchronize the sending and receiving units.</para>
	      
	  <para>When a word is given to the UART for Asynchronous
	    transmissions, a bit called the "Start Bit" is added to the
	    beginning of each word  that is to be transmitted.  The Start Bit
	    is used to alert the receiver that a word of data is about to be
	    sent, and to force the clock in the receiver into synchronization
	    with the clock in the transmitter.  These two clocks must be
	    accurate enough to not  have the frequency drift by more than 10%
	    during the transmission of the remaining bits in the word.  (This
	    requirement was set in the days of mechanical teleprinters and is
	    easily met by modern electronic equipment.)</para>
	      
	  <para>After the Start Bit, the individual bits of the word of data
	    are sent, with the Least Significant Bit (LSB) being sent first.
	    Each  bit in the transmission is transmitted for exactly the same
	    amount of time as all of the other bits, and the receiver
	    <quote>looks</quote> at the wire at approximately halfway through
	    the period assigned to each bit to determine if the bit is a
	    <literal>1</literal> or a <literal>0</literal>.  For example, if
	    it takes two seconds to send each bit, the receiver will examine
	    the signal to determine if it is a <literal>1</literal> or a
	    <literal>0</literal> after one second has passed, then it will
	    wait two seconds and then examine the value of the next bit, and
	    so on.</para>
	      
	  <para>The sender does not know when the receiver has
	    <quote>looked</quote> at the value of the bit.  The sender only
	    knows when the clock says to begin transmitting the next bit of
	    the word.</para>
	      
	  <para>When the entire data word has been sent, the transmitter may
	    add a Parity Bit that the transmitter generates.  The Parity Bit
	    may be used by the receiver to perform simple error checking.
	    Then at least one Stop Bit is sent by the transmitter.</para>
	      
	  <para>When the receiver has received all of the bits in the data
	    word, it may check for the Parity Bits (both sender and receiver
	    must agree on whether a Parity Bit is to be used), and then the
	    receiver looks for a Stop Bit.  If the Stop Bit does not appear
	    when it is supposed to, the UART considers the entire word to be
	    garbled and will report a Framing Error to the host processor when
	    the data word is read.  The usual cause of a Framing Error is that
	    the sender and receiver clocks were not running at the same speed,
	    or that the signal was interrupted.</para>
	      
	  <para>Regardless of whether the data was received correctly or not,
	    the UART automatically discards the Start, Parity and Stop bits.
	    If the sender and receiver are configured identically, these bits
	    are not passed to the host.</para>
	      
	  <para>If another word is ready for transmission, the Start Bit for
	    the new word can be sent as soon as the Stop Bit for the previous
	    word has been sent.</para>
	      
	  <para>Because asynchronous data is <quote>self synchronizing</quote>,
	    if there is no data to transmit, the transmission line can be
	    idle.</para>
	</sect4>

	<sect4>
	  <title>Other UART Functions</title>
	  
	  <para>In addition to the basic job of converting data from parallel
	    to serial for transmission and from serial to parallel on
	    reception, a UART will usually provide additional circuits for
	    signals that can be used to indicate the state of the transmission
	    media, and to regulate the flow of data in the event that the
	    remote device is not prepared to accept more data.  For example,
	    when the  device connected to the UART is a modem, the modem may
	    report the presence of a carrier on the phone line while the
	    computer may be able to instruct the modem to reset itself or to
	    not take calls by asserting or deasserting one more more of these
	    extra signals. The function of each of these additional signals is
	    defined in the EIA RS232-C standard.</para>
	</sect4>

	<sect4>
	  <title>The RS232-C and V.24 Standards</title>
	  
	  <para>In most computer systems, the UART is connected to circuitry
	    that generates signals that comply with the EIA RS232-C
	    specification.  There is also a CCITT standard named V.24 that
	    mirrors the specifications included in RS232-C.</para>
	      
	  <sect5>
	    <title>RS232-C Bit Assignments (Marks and Spaces)</title>
	    
	    <para>In RS232-C, a value of <literal>1</literal> is called a
	      <literal>Mark</literal> and a value of <literal>0</literal> is
	      called a <literal>Space</literal>.  When a communication line is
	      idle, the line is said to be <quote>Marking</quote>, or
	      transmitting continuous <literal>1</literal> values.</para>
		
	    <para>The Start bit always has a value of <literal>0</literal> (a
	      Space).  The Stop Bit always has a value of <literal>1</literal>
	      (a Mark).  This means that there will always be a Mark (1) to
	      Space (0) transition on the line at the start of every word,
	      even when multiple word are transmitted back to back.  This
	      guarantees that sender and receiver can resynchronize their
	      clocks regardless of the content of the data bits that are being
	      transmitted.</para>
		
	    <para>The idle time between Stop and Start bits does not have to
	      be an exact multiple (including zero) of the bit rate of the
	      communication link, but most UARTs are designed this way for
	      simplicity.</para>
		
	    <para>In RS232-C, the "Marking" signal (a <literal>1</literal>) is
	      represented by a voltage between -2 VDC and -12 VDC, and a
	      "Spacing" signal (a <literal>0</literal>) is represented by a
	      voltage between 0 and +12 VDC.  The transmitter is supposed to
	      send +12 VDC or -12 VDC, and the receiver is supposed to allow
	      for some voltage loss in long cables.  Some transmitters in low
	      power devices (like portable computers) sometimes use only +5
	      VDC and -5 VDC, but these values are still acceptable to a
	      RS232-C receiver, provided that the cable lengths are
	      short.</para>
	  </sect5>
	      
	  <sect5>
	    <title>RS232-C Break Signal</title>
	    
	    <para>RS232-C also specifies a signal called a
	      <literal>Break</literal>, which is caused by sending continuous
	      Spacing values (no Start or Stop bits).  When there is no
	      electricity present on the data circuit, the line is considered
	      to be sending <literal>Break</literal>.</para>
		
	    <para>The <literal>Break</literal> signal must be of a duration
	      longer than the time it takes to send a complete byte plus
	      Start, Stop and Parity bits.  Most UARTs can distinguish between
	      a Framing Error and a Break, but if the UART cannot do this, the
	      Framing Error detection can be used to identify Breaks.</para>
		
	    <para>In the days of teleprinters, when numerous printers around
	      the country were wired in series (such as news services), any
	      unit could cause a <literal>Break</literal> by temporarily
	      opening the entire circuit so that no current flowed.  This was
	      used to allow a location with urgent news to interrupt some
	      other location that was currently sending information.</para>
		
	    <para>In modern systems there are two types of Break signals. If
	      the Break is longer than 1.6 seconds, it is considered a "Modem
	      Break", and some modems can be programmed to terminate the
	      conversation and go on-hook or enter the modems' command mode
	      when the modem detects this signal.  If the Break is smaller
	      than 1.6 seconds, it signifies a Data Break and it is up to the
	      remote computer to respond to this signal.  Sometimes this form
	      of Break is used as an Attention or Interrupt signal and
	      sometimes is accepted as a substitute for the ASCII CONTROL-C
	      character.</para>
		
	    <para>Marks and Spaces are also equivalent to <quote>Holes</quote>
	      and <quote>No Holes</quote> in paper tape systems.</para>

	    <note>
	      <para>Breaks cannot be generated from paper tape or from any
		other byte value, since bytes are always sent with Start and
		Stop bit.  The UART is usually capable of generating the
		continuous Spacing signal in response to a special command
		from the host processor.</para>
	    </note>
	  </sect5>
	  
	  <sect5>
	    <title>RS232-C DTE and DCE Devices</title>
	    
	    <para>The RS232-C specification defines two types of equipment:
	      the Data Terminal Equipment (DTE) and the Data Carrier Equipment
	      (DCE).  Usually, the DTE device is the terminal (or computer),
	      and the DCE is a modem.  Across the phone line at the other end
	      of a conversation, the receiving modem is also a DCE device and
	      the computer that is connected to that modem is a DTE device.
	      The DCE device receives signals on the pins that the DTE device
	      transmits on, and vice versa.</para>
		
	    <para>When two devices that are both DTE or both DCE must be
	      connected together without a modem or a similar media translater
	      between them, a NULL modem must be used.  The NULL modem
	      electrically re-arranges the cabling so that the transmitter
	      output is connected to the receiver input on the other device,
	      and vice versa.  Similar translations are performed on all of
	      the control signals so that each device will see what it thinks
	      are DCE (or DTE) signals from the other device.</para>
		
	    <para>The number of signals generated by the DTE and DCE devices
	      are not symmetrical.  The DTE device generates fewer signals for
	      the DCE device than the DTE device receives from the DCE.</para>
	  </sect5>
	  
	  <sect5>
	    <title>RS232-C Pin Assignments</title>
	    
	    <para>The EIA RS232-C specification (and the ITU equivalent, V.24)
	      calls for a twenty-five pin connector (usually a DB25) and
	      defines the purpose of most of the pins in that
	      connector.</para>
		
	    <para>In the IBM Personal Computer and similar systems, a subset
	      of RS232-C signals are provided via nine pin connectors (DB9).
	      The signals that are not included on the PC connector deal
	      mainly with synchronous operation, and this transmission mode is
	      not supported by the UART that IBM selected for use in the IBM
	      PC.</para>
		
	    <para>Depending on the computer manufacturer, a DB25, a DB9, or
	      both types of connector may be used for RS232-C communications.
	      (The IBM PC also uses a DB25 connector for the parallel printer
	      interface which causes some confusion.)</para>
		
	    <para>Below is a table of the RS232-C signal assignments in the
	      DB25 and DB9 connectors.</para>

	    <informaltable frame="none">
	      <tgroup cols="7">
		<thead>
		  <row>
		    <entry>DB25 RS232-C Pin</entry>
		    <entry>DB9 IBM PC Pin</entry>
		    <entry>EIA Circuit Symbol</entry>
		    <entry>CCITT Circuit Symbol</entry>
		    <entry>Common Name</entry>
		    <entry>Signal Source</entry>
		    <entry>Description</entry>
		  </row>
		</thead>
		
		<tbody>
		  <row>
		    <entry>1</entry>
		    <entry>-</entry>
		    <entry>AA</entry>
		    <entry>101</entry>
		    <entry>PG/FG</entry>
		    <entry>-</entry>
		    <entry>Frame/Protective Ground</entry>
		  </row>
		  
		  <row>
		    <entry>2</entry>
		    <entry>3</entry>
		    <entry>BA</entry>
		    <entry>103</entry>
		    <entry>TD</entry>
		    <entry>DTE</entry>
		    <entry>Transmit Data</entry>
		  </row>
		  
		  <row>
		    <entry>3</entry>
		    <entry>2</entry>
		    <entry>BB</entry>
		    <entry>104</entry>
		    <entry>RD</entry>
		    <entry>DCE</entry>
		    <entry>Receive Data</entry>
		  </row>
		  
		  <row>
		    <entry>4</entry>
		    <entry>7</entry>
		    <entry>CA</entry>
		    <entry>105</entry>
		    <entry>RTS</entry>
		    <entry>DTE</entry>
		    <entry>Request to Send</entry>
		  </row>
		  
		  <row>
		    <entry>5</entry>
		    <entry>8</entry>
		    <entry>CB</entry>
		    <entry>106</entry>
		    <entry>CTS</entry>
		    <entry>DCE</entry>
		    <entry>Clear to Send</entry>
		  </row>
		  
		  <row>
		    <entry>6</entry>
		    <entry>6</entry>
		    <entry>CC</entry>
		    <entry>107</entry>
		    <entry>DSR</entry>
		    <entry>DCE</entry>
		    <entry>Data Set Ready</entry>
		  </row>
		  
		  <row>
		    <entry>7</entry>
		    <entry>5</entry>
		    <entry>AV</entry>
		    <entry>102</entry>
		    <entry>SG/GND</entry>
		    <entry>-</entry>
		    <entry>Signal Ground</entry>
		  </row>
		  
		  <row>
		    <entry>8</entry>
		    <entry>1</entry>
		    <entry>CF</entry>
		    <entry>109</entry>
		    <entry>DCD/CD</entry>
		    <entry>DCE</entry>
		    <entry>Data Carrier Detect</entry>
		  </row>
		  
		  <row>
		    <entry>9</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>Reserved for Test</entry>
		  </row>
		  
		  <row>
		    <entry>10</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>Reserved for Test</entry>
		  </row>
		  
		  <row>
		    <entry>11</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>Reserved for Test</entry>
		  </row>
		  
		  <row>
		    <entry>12</entry>
		    <entry>-</entry>
		    <entry>CI</entry>
		    <entry>122</entry>
		    <entry>SRLSD</entry>
		    <entry>DCE</entry>
		    <entry>Sec. Recv. Line Signal Detector</entry>
		  </row>
		  
		  <row>
		    <entry>13</entry>
		    <entry>-</entry>
		    <entry>SCB</entry>
		    <entry>121</entry>
		    <entry>SCTS</entry>
		    <entry>DCE</entry>
		    <entry>Secondary Clear to Send</entry>
		  </row>
		  
		  <row>
		    <entry>14</entry>
		    <entry>-</entry>
		    <entry>SBA</entry>
		    <entry>118</entry>
		    <entry>STD</entry>
		    <entry>DTE</entry>
		    <entry>Secondary Transmit Data</entry>
		  </row>
		  
		  <row>
		    <entry>15</entry>
		    <entry>-</entry>
		    <entry>DB</entry>
		    <entry>114</entry>
		    <entry>TSET</entry>
		    <entry>DCE</entry>
		    <entry>Trans. Sig. Element Timing</entry>
		  </row>
		  
		  <row>
		    <entry>16</entry>
		    <entry>-</entry>
		    <entry>SBB</entry>
		    <entry>119</entry>
		    <entry>SRD</entry>
		    <entry>DCE</entry>
		    <entry>Secondary Received Data</entry>
		  </row>
		  
		  <row>
		    <entry>17</entry>
		    <entry>-</entry>
		    <entry>DD</entry>
		    <entry>115</entry>
		    <entry>RSET</entry>
		    <entry>DCE</entry>
		    <entry>Receiver Signal Element Timing</entry>
		  </row>
		  
		  <row>
		    <entry>18</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>141</entry>
		    <entry>LOOP</entry>
		    <entry>DTE</entry>
		    <entry>Local Loopback</entry>
		  </row>
		  
		  <row>
		    <entry>19</entry>
		    <entry>-</entry>
		    <entry>SCA</entry>
		    <entry>120</entry>
		    <entry>SRS</entry>
		    <entry>DTE</entry>
		    <entry>Secondary Request to Send</entry>
		  </row>
		  
		  <row>
		    <entry>20</entry>
		    <entry>4</entry>
		    <entry>CD</entry>
		    <entry>108.2</entry>
		    <entry>DTR</entry>
		    <entry>DTE</entry>
		    <entry>Data Terminal Ready</entry>
		  </row>
		  
		  <row>
		    <entry>21</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>RDL</entry>
		    <entry>DTE</entry>
		    <entry>Remote Digital Loopback</entry>
		  </row>
		  
		  <row>
		    <entry>22</entry>
		    <entry>9</entry>
		    <entry>CE</entry>
		    <entry>125</entry>
		    <entry>RI</entry>
		    <entry>DCE</entry>
		    <entry>Ring Indicator</entry>
		  </row>
		  
		  <row>
		    <entry>23</entry>
		    <entry>-</entry>
		    <entry>CH</entry>
		    <entry>111</entry>
		    <entry>DSRS</entry>
		    <entry>DTE</entry>
		    <entry>Data Signal Rate Selector</entry>
		  </row>
		  
		  <row>
		    <entry>24</entry>
		    <entry>-</entry>
		    <entry>DA</entry>
		    <entry>113</entry>
		    <entry>TSET</entry>
		    <entry>DTE</entry>
		    <entry>Trans. Sig. Element Timing</entry>
		  </row>
		  
		  <row>
		    <entry>25</entry>
		    <entry>-</entry>
		    <entry>-</entry>
		    <entry>142</entry>
		    <entry>-</entry>
		    <entry>DCE</entry>
		    <entry>Test Mode</entry>
		  </row>
		</tbody>
	      </tgroup>
	    </informaltable>
	  </sect5>
	</sect4>

	<sect4>
	  <title>Bits, Baud and Symbols</title>
	  
	  <para>Baud is a measurement of transmission speed in asynchronous
	    communication.  Because of advances in modem communication
	    technology, this term is frequently misused when describing the
	    data rates in newer devices.</para>
	      
	  <para>Traditionally, a Baud Rate represents the number of bits that
	    are actually being sent over the media, not the amount of data
	    that is actually moved from one DTE device to the other. The Baud
	    count includes the overhead bits Start, Stop and Parity that are
	    generated by the sending UART and removed by the receiving UART.
	    This means that seven-bit words of data actually take 10 bits to
	    be completely transmitted.  Therefore, a modem capable of moving
	    300 bits per second from one place to another can normally only
	    move 30 7-bit words if Parity is used and one Start and Stop bit
	    are present.</para>
	      
	  <para>If 8-bit data words are used and Parity bits are also used,
	    the data rate falls to 27.27 words per second, because it now
	    takes 11 bits to send the eight-bit words, and the modem still
	    only sends 300 bits per second.</para>
	      
	  <para>The formula for converting bytes per second into a baud rate
	    and vice versa was simple until error-correcting modems came
	    along.  These modems receive the serial stream of bits from the
	    UART in the host computer (even when internal modems are used the
	    data is still frequently serialized) and converts the bits back
	    into bytes.  These bytes are then combined into packets and sent
	    over the phone line using a Synchronous transmission method.  This
	    means that the Stop, Start, and Parity bits added by the UART in
	    the DTE (the computer) were removed by the modem before
	    transmission by the sending modem. When these bytes are received
	    by the remote modem, the remote modem adds Start, Stop and Parity
	    bits to the words, converts them to a serial format and then sends
	    them to the receiving UART in the remote computer, who then strips
	    the Start, Stop and Parity bits.</para>
	      
	  <para>The reason all these extra conversions are done is so that the
	    two modems can perform error correction, which means that the
	    receiving modem is able to ask the sending modem to resend a block
	    of data that was not received with the correct checksum.  This
	    checking is handled by the modems, and the DTE devices are usually
	    unaware that the process is occurring.</para>
	      
	  <para>By striping the Start, Stop and Parity bits, the additional
	    bits of data that the two modems must share between themselves to
	    perform error-correction are mostly concealed from the effective
	    transmission rate seen by the sending and receiving DTE equipment.
	    For example, if a modem sends ten 7-bit words to another modem
	    without including the Start, Stop and Parity bits, the sending
	    modem will be able to add 30 bits of its own information that the
	    receiving modem can use to do error-correction without impacting
	    the transmission speed of the real data.</para>
	      
	  <para>The use of the term Baud is further confused by modems that
	    perform compression.  A single 8-bit word passed over the
	    telephone line might represent a dozen words that were transmitted
	    to the sending modem.  The receiving modem will expand the data
	    back to its original content and pass that data to the receiving
	    DTE.</para>
	      
	  <para>Modern modems also include buffers that allow the rate that
	    bits move across the phone line (DCE to DCE) to be a different
	    speed than the speed that the bits move between the DTE and DCE on
	    both ends of the conversation.  Normally the speed between the DTE
	    and DCE is higher than the DCE to DCE speed because of the use of
	    compression by the modems.</para>
	      
	  <para>Because the number of bits needed to describe a byte varied
	    during the trip between the two machines plus the differing
	    bits-per-seconds speeds that are used present on the DTE-DCE and
	    DCE-DCE links, the usage of the term Baud to describe the overall
	    communication speed causes problems and can misrepresent the true
	    transmission speed.  So Bits Per Second (bps) is the correct term
	    to use to describe the transmission rate seen at the DCE to DCE
	    interface and Baud or Bits Per Second are acceptable terms to use
	    when a connection is made between two systems with a wired
	    connection, or if a modem is in use that is not performing
	    error-correction or compression.</para>
	      
	  <para>Modern high speed modems (2400, 9600, 14,400, and 19,200bps)
	    in reality still operate at or below 2400 baud, or more
	    accurately, 2400 Symbols per second.  High speed modem are able to
	    encode more bits of data into each Symbol using a technique called
	    Constellation Stuffing, which is why the effective bits per second
	    rate of the modem is higher, but the modem continues to operate
	    within the limited audio bandwidth that the telephone system
	    provides.  Modems operating at 28,800 and higher speeds have
	    variable Symbol rates, but the technique is the same.</para>
	</sect4>

	<sect4>
	  <title>The IBM Personal Computer UART</title>
	  
	  <para>Starting with the original IBM Personal Computer, IBM selected
	    the National Semiconductor INS8250 UART for use in the IBM PC
	    Parallel/Serial Adapter.  Subsequent generations of compatible
	    computers from IBM and other vendors continued to use the INS8250
	    or improved versions of the National Semiconductor UART
	    family.</para>
	      
	  <sect5>
	    <title>National Semiconductor UART Family Tree</title>
	    
	    <para>There have been several versions and subsequent generations
	      of the INS8250 UART.  Each major version is described
	      below.</para>

	    <!-- This should really be a graphic -->
	    <programlisting>
INS8250  -&gt; INS8250B
  \
   \
    \-&gt; INS8250A -&gt; INS82C50A
             \
              \
               \-&gt; NS16450 -&gt; NS16C450
                        \
                         \
                          \-&gt; NS16550 -&gt; NS16550A -&gt; PC16550D</programlisting>
		
	    <variablelist>
	      <varlistentry>
		<term>INS8250</term>

		<listitem>
		  <para>This part was used in the original IBM PC and IBM
		    PC/XT.  The original name for this part was the INS8250
		    ACE (Asynchronous Communications Element) and it is made
		    from NMOS technology.</para>
			
		  <para>The 8250 uses eight I/O ports and has a one-byte send
		    and a one-byte receive buffer.  This original UART has
		    several race conditions and other flaws. The original IBM
		    BIOS includes code to work around these flaws, but this
		    made the BIOS dependent on the flaws being present, so
		    subsequent parts like the 8250A, 16450  or 16550 could not
		    be used in the original IBM PC or IBM PC/XT.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>INS8250-B</term>
		
		<listitem>
		  <para>This is the slower speed of the INS8250 made from NMOS
		    technology.  It contains the same problems as the original
		    INS8250.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>INS8250A</term>
		
		<listitem>
		  <para>An improved version of the INS8250 using XMOS
		    technology with various functional flaws corrected. The
		    INS8250A was used initially in PC clone computers by
		    vendors who used <quote>clean</quote> BIOS designs. Because
		    of the corrections in the chip, this part could not be
		    used with a BIOS compatible with the INS8250 or
		    INS8250B.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>INS82C50A</term>
		
		<listitem>
		  <para>This is a CMOS version (low power consumption) of the
		    INS8250A and has similar functional
		    characteristics.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>NS16450</term>
		
		<listitem>
		  <para>Same as NS8250A with improvements so it can be used
		    with faster CPU bus designs.  IBM used this part in the
		    IBM AT and updated the IBM BIOS to no longer rely on the
		    bugs in the INS8250.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>NS16C450</term>
		
		<listitem>
		  <para>This is a CMOS version (low power consumption) of the
		    NS16450.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>NS16550</term>
		
		<listitem>
		  <para>Same as NS16450 with a 16-byte send and receive buffer
		    but the buffer design was flawed and could not be reliably
		    be used.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>NS16550A</term>
		
		<listitem>
		  <para>Same as NS16550 with the buffer flaws corrected. The
		    16550A and its successors have become the most popular
		    UART design in the PC industry, mainly due it its ability
		    to reliably handle higher data rates on operating systems
		    with sluggish interrupt response times.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>NS16C552</term>
		
		<listitem>
		  <para>This component consists of two NS16C550A CMOS UARTs in
		    a single package.</para>
		</listitem>
	      </varlistentry>
	      
	      <varlistentry>
		<term>PC16550D</term>
		
		<listitem>
		  <para>Same as NS16550A with subtle flaws corrected. This is
		    revision D of the 16550 family and is the latest design
		    available from National Semiconductor.</para>
		</listitem>
	      </varlistentry>
	    </variablelist>
	  </sect5>
	  
	  <sect5>
	    <title>The NS16550AF and the PC16550D are the same thing</title>
		
	    <para>National reorganized their part numbering system a few years
	      ago, and the NS16550AFN no longer exists by that name. (If you
	      have a NS16550AFN, look at the date code on the part, which is a
	      four digit number that usually starts with a nine.  The first
	      two digits of the number are the year, and the last two digits
	      are the week in that year when the part was packaged.  If you
	      have a NS16550AFN, it is probably a few years old.)</para>
		
	    <para>The new numbers are like PC16550DV, with minor differences
	      in the suffix letters depending on the package material and its
	      shape.  (A description of the numbering system can be found
	      below.)</para>
		
	    <para>It is important to understand that in some stores, you may
	      pay &#36;15(US) for a NS16550AFN made in 1990 and in the next
	      bin are the new PC16550DN parts with minor fixes that National
	      has made since the AFN part was in production, the PC16550DN was
	      probably made in the past six months and it costs half (as low
	      as &#36;5(US) in volume) as much as the NS16550AFN because they
	      are readily available.</para>
		
	    <para>As the supply of NS16550AFN chips continues to shrink, the
	      price will probably continue to increase until more people
	      discover and accept that the PC16550DN really has the same
	      function as the old part number.</para>
	  </sect5>
	  
	  <sect5>
	    <title>National Semiconductor Part Numbering System</title>
	    
	    <para>The older  NS<replaceable>nnnnnrqp</replaceable>  part
	      numbers are now of the format
	      PC<replaceable>nnnnnrgp</replaceable>.</para>
		
	    <para>The <replaceable>r</replaceable> is the revision field.  The
	      current revision of the 16550 from National Semiconductor is
	      <literal>D</literal>.</para>
		
	    <para>The <replaceable>p</replaceable> is the package-type field.
	      The types are:</para>
	    
	    <informaltable frame="none">
	      <tgroup cols="3">
		<tbody>
		  <row>
		    <entry>"F"</entry>
		    <entry>QFP</entry>
		    <entry>(quad flat pack) L lead type</entry>
		  </row>
		  
		  <row>
		    <entry>"N"</entry>
		    <entry>DIP</entry>
		    <entry>(dual inline package) through hole straight lead
		      type</entry>
		  </row>

		  <row>
		    <entry>"V"</entry>
		    <entry>LPCC</entry>
		    <entry>(lead plastic chip carrier) J lead type</entry>
		  </row>
		</tbody>
	      </tgroup>
	    </informaltable>
	    
	    <para>The <replaceable>g</replaceable> is the product grade field.
	      If an <literal>I</literal> precedes the package-type letter, it
	      indicates an <quote>industrial</quote> grade part, which has
	      higher specs than a standard part but not as high as Military
	      Specification (Milspec) component.  This is an optional
	      field.</para>
		
	    <para>So what we used to call a NS16550AFN (DIP Package) is now
	      called a PC16550DN or PC16550DIN.</para>
	  </sect5>
	</sect4>

	<sect4>
	  <title>Other Vendors and Similar UARTs</title>
	  
	  <para>Over the years, the 8250, 8250A, 16450 and 16550 have been
	    licensed or copied by other chip vendors.  In the case of the
	    8250, 8250A and 16450, the exact circuit (the
	    <quote>megacell</quote>) was licensed to many vendors, including
	    Western Digital and Intel. Other vendors reverse-engineered the
	    part or produced emulations that had similar behavior.</para>
	      
	  <para>In internal modems, the modem designer will frequently emulate
	    the 8250A/16450 with the modem microprocessor, and the emulated
	    UART will frequently have a hidden buffer consisting of several
	    hundred bytes.  Because of the size of the buffer, these
	    emulations can be as reliable as a 16550A in their ability to
	    handle high speed data.  However, most operating systems will
	    still report that the UART is only a 8250A or 16450, and may not
	    make effective use of the extra buffering present in the emulated
	    UART unless special drivers are used.</para>
	      
	  <para>Some modem makers are driven by market forces to abandon a
	    design that has hundreds of bytes of buffer and instead use a
	    16550A UART so that the product will compare favorably in market
	    comparisons even though the effective performance may be lowered
	    by this action.</para>
	      
	  <para>A common misconception is that all parts with
	    <quote>16550A</quote> written on them are identical in performance.
	    There are differences, and in some cases, outright flaws in most
	    of these 16550A clones.</para>
	      
	  <para>When the NS16550 was developed, the National Semiconductor
	    obtained several patents on the design and they also limited
	    licensing, making it harder for other vendors to provide a chip
	    with similar features.  Because of the patents, reverse-engineered
	    designs and emulations had to avoid infringing the claims covered
	    by the patents.  Subsequently, these copies almost never perform
	    exactly the same as the NS16550A or PC16550D, which are the parts
	    most computer and modem makers want to buy but are sometimes
	    unwilling to pay the price required to get the genuine
	    part.</para>
	      
	  <para>Some of the differences in the clone 16550A parts are
	    unimportant, while others can prevent the device from being used
	    at all with a given operating system or driver.  These differences
	    may show up when using other drivers, or when particular
	    combinations of events occur that were not well tested or
	    considered in the Windows driver.  This is because most modem
	    vendors and 16550-clone makers use the Microsoft drivers from
	    Windows for Workgroups 3.11 and the Microsoft MSD utility as the
	    primary tests for compatibility with the NS16550A.  This
	    over-simplistic criteria means that if a different operating
	    system is used, problems could appear due to subtle differences
	    between the clones and genuine components.</para>
	      
	  <para>National Semiconductor has made available a program named
	    <application>COMTEST</application> that performs compatibility
	    tests independent of any OS drivers.  It should be remembered that
	    the purpose of this type of program is to demonstrate the flaws in
	    the products of the competition, so the program will report major
	    as well as extremely subtle differences in behavior in the part
	    being tested.</para>
	      
	  <para>In a series of tests performed by the author of this document
	    in 1994, components made by National Semiconductor, TI, StarTech,
	    and CMD as well as megacells and emulations embedded in internal
	    modems were tested with COMTEST.  A difference count for some of
	    these components is listed below. Because these tests were
	    performed in 1994, they may not reflect the current performance of
	    the given product from a vendor.</para>
	      
	  <para>It should be noted that COMTEST normally aborts when an
	    excessive number or certain types of problems have been detected.
	    As part of this testing, COMTEST was modified so that it would not
	    abort no matter how many differences were encountered.</para>

	  <informaltable frame="none">
	    <tgroup cols="3">
	      <thead>
		<row>
		  <entry>Vendor</entry>
		  <entry>Part Number</entry>
		  <entry>Errors (aka "differences" reported)</entry>
		</row>
	      </thead>
	      
	      <tbody>
		<row>
		  <entry>National</entry>
		  <entry>(PC16550DV)</entry>
		  <entry>0</entry>
		</row>
		
		<row>
		  <entry>National</entry>
		  <entry>(NS16550AFN)</entry>
		  <entry>0</entry>
		</row>
		
		<row>
		  <entry>National</entry>
		  <entry>(NS16C552V)</entry>
		  <entry>0</entry>
		</row>
		
		<row>
		  <entry>TI</entry>
		  <entry>(TL16550AFN)</entry>
		  <entry>3</entry>
		</row>
		
		<row>
		  <entry>CMD</entry>
		  <entry>(16C550PE)</entry>
		  <entry>19</entry>
		</row>
		
		<row>
		  <entry>StarTech</entry>
		  <entry>(ST16C550J)</entry>
		  <entry>23</entry>
		</row>
		
		<row>
		  <entry>Rockwell</entry>
		  <entry>Reference modem with internal 16550 or an
		    emulation (RC144DPi/C3000-25)</entry>
		  <entry>117</entry>
		</row>
		
		<row>
		  <entry>Sierra</entry>
		  <entry>Modem with an internal 16550
		    (SC11951/SC11351)</entry>
		  <entry>91</entry>
		</row>
	      </tbody>
	    </tgroup>
	  </informaltable>
	  
	  <note>
	    <para>To date, the author of this document has not found any
	      non-National parts that report zero differences using the
	      COMTEST program.  It should also be noted that National has had
	      five versions of the 16550 over the years and the newest parts
	      behave a bit differently than the classic NS16550AFN that is
	      considered the benchmark for functionality.  COMTEST appears to
	      turn a blind eye to the differences within the National product
	      line and reports no errors on the National parts (except for the
	      original 16550) even when there are official erratas that
	      describe bugs in the A, B and C revisions of the parts, so this
	      bias in COMTEST must be taken into account.</para>
	  </note>
	  
	  <para>It is important to understand that a simple count of
	    differences from COMTEST does not reveal a lot about what
	    differences are important and which are not.  For example, about
	    half of the differences reported in the two modems listed above
	    that have internal UARTs were caused by the clone UARTs not
	    supporting five- and six-bit character modes.  The real 16550,
	    16450, and 8250 UARTs all support these modes and COMTEST checks
	    the functionality of these modes so over fifty differences are
	    reported.  However, almost no modern modem supports five- or
	    six-bit characters, particularly those with error-correction and
	    compression capabilities.  This means that the differences related
	    to five- and six-bit character modes can be discounted.</para>
	      
	  <para>Many of the differences COMTEST reports have to do with
	    timing.  In many of the clone designs, when the host reads from
	    one port, the status bits in some other port may not update in the
	    same amount of time (some faster, some slower) as a
	    <emphasis>real</emphasis> NS16550AFN and COMTEST looks for these
	    differences.  This means that the number of differences can be
	    misleading in that one device may only have one or two differences
	    but they are extremely serious, and some other device that updates
	    the status registers faster or slower than the reference part
	    (that would probably never affect the operation of a properly
	    written driver) could have dozens of differences reported.</para>
	      
	  <para>COMTEST can be used as a screening tool to alert the
	    administrator to the presence of potentially incompatible
	    components that might cause problems or have to be handled as a
	    special case.</para>
	      
	  <para>If you run COMTEST on a 16550 that is in a modem or a modem is
	    attached to the serial port, you need to first issue a ATE0&amp;W
	    command to the modem so that the modem will not echo any of the
	    test characters.  If you forget to do this, COMTEST will report at
	    least this one difference:</para>

	  <screen>Error (6)...Timeout interrupt failed: IIR = c1  LSR = 61</screen>
	</sect4>

	<sect4>
	  <title>8250/16450/16550 Registers</title>
	  
	  <para>The 8250/16450/16550 UART occupies eight contiguous I/O port
	    addresses.  In the IBM PC, there are two defined locations for
	    these eight ports and they are known collectively as COM1 and
	    COM2.  The makers of PC-clones and add-on cards have created two
	    additional areas known as COM3 and COM4, but these extra COM ports
	    conflict with other hardware on some systems.  The most common
	    conflict is with video adapters that provide IBM 8514
	    emulation.</para>
	      
	  <para>COM1 is located from 0x3f8 to 0x3ff and normally uses IRQ 4
	    COM2 is located from 0x2f8 to 0x2ff and normally uses IRQ 3 COM3
	    is located from 0x3e8 to 0x3ef and has no standardized IRQ COM4 is
	    located from 0x2e8 to 0x2ef and has no standardized IRQ.</para>
	      
	  <para>A description of the I/O ports of the 8250/16450/16550 UART is
	    provided below.</para>

	  <informaltable frame="none">
	    <tgroup cols="3">
	      <thead>
		<row>
		  <entry>I/O Port</entry>
		  <entry>Access Allowed</entry>
		  <entry>Description</entry>
		</row>
	      </thead>
	      
	      <tbody>
		<row>
		  <entry>+0x00</entry>
		  <entry>write (DLAB==0)</entry>
		  <entry><para>Transmit Holding Register
		      (THR).</para><para>Information written to this port are
		      treated as data words and will be transmitted by the
		      UART.</para></entry>
		</row>
		
		<row>
		  <entry>+0x00</entry>
		  <entry>read (DLAB==0)</entry>
		  <entry><para>Receive Buffer Register (RBR).</para><para>Any
		      data words received by the UART form the serial link are
		      accessed by the host by reading this
		      port.</para></entry>
		</row>
		
		<row>
		  <entry>+0x00</entry>
		  <entry>write/read (DLAB==1)</entry>
		  <entry><para>Divisor Latch LSB (DLL)</para><para>This value
		      will be divided from the master input clock (in the IBM
		      PC, the master clock is 1.8432MHz) and the resulting
		      clock will determine the baud rate of the UART.  This
		      register holds bits 0 thru 7 of the
		      divisor.</para></entry>
		</row>
		
		<row>
		  <entry>+0x01</entry>
		  <entry>write/read (DLAB==1)</entry>
		  <entry><para>Divisor Latch MSB (DLH)</para><para>This value
		      will be divided from the master input clock (in the IBM
		      PC, the master clock is 1.8432MHz) and the resulting
		      clock will determine the baud rate of the UART.  This
		      register holds bits 8 thru 15 of the
		      divisor.</para></entry>
		</row>
		
		<row>
		  <entry>+0x01</entry>
		  <entry>write/read (DLAB==0)</entry>
		  <entrytbl cols="2">
		    <colspec colnum="1" colname="col1">
		    <colspec colnum="2" colname="col2">
		    <spanspec namest="col1" nameend="col2" spanname="1to2">

		    <tbody>
		      <row>
			<entry spanname="1to2"><para>Interrupt Enable Register
			    (IER)</para><para>The 8250/16450/16550 UART
			    classifies events into one of four categories.
			    Each category can be configured to generate an
			    interrupt when any of the events occurs.  The
			    8250/16450/16550 UART generates a single external
			    interrupt signal regardless of how many events in
			    the enabled categories have occurred.  It is up to
			    the host processor to respond to the interrupt and
			    then poll the enabled interrupt categories
			    (usually all categories have interrupts enabled)
			    to determine the true cause(s) of the
			    interrupt.</para></entry>
		      </row>
		      
		      <row>
			<entry>Bit 7</entry>
			<entry>Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 6</entry>
			<entry>Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 5</entry>
			<entry>Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 4</entry>
			<entry>Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 3</entry>
			<entry>Enable Modem Status Interrupt (EDSSI). Setting
			  this bit to "1" allows the UART to generate an
			  interrupt when a change occurs on one or more of the
			  status lines.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 2</entry>
			<entry>Enable Receiver Line Status Interrupt (ELSI)
			  Setting this bit to "1" causes the UART to generate
			  an interrupt when the an error (or a BREAK signal)
			  has been detected in the incoming data.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 1</entry>
			<entry>Enable Transmitter Holding Register Empty
			  Interrupt (ETBEI) Setting this bit to "1" causes the
			  UART to generate an interrupt when the UART has room
			  for one or more additional characters that are to be
			  transmitted.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 0</entry>
			<entry>Enable Received Data Available Interrupt
			  (ERBFI) Setting this bit to "1" causes the UART to
			  generate an interrupt when the UART has received
			  enough characters to exceed the trigger level of the
			  FIFO, or the FIFO timer has expired (stale data), or
			  a single character has been received when the FIFO
			  is disabled.</entry>
		      </row>
		    </tbody>
		  </entrytbl>
		</row>
		
		<row>
		  <entry>+0x02</entry>
		  <entry>write</entry>
		  <entrytbl cols="4">
		    <colspec colnum="1" colname="col1">
		    <colspec colnum="2" colname="col2">
		    <colspec colnum="3" colname="col3">
		    <colspec colnum="4" colname="col4">
		    <spanspec namest="col1" nameend="col4" spanname="1to4">
		    <spanspec namest="col2" nameend="col4" spanname="2to4">
		    
		    <tbody>
		      <row>
			<entry spanname="1to4">FIFO Control Register (FCR)
			  (This port does not exist on the 8250 and 16450
			  UART.)</entry>
		      </row>

		      <row>
			<entry>Bit 7</entry>
			<entry spanname="2to4">Receiver Trigger Bit #1</entry>
		      </row>
		      
		      <row>
			<entry>Bit 6</entry>
			<entry spanname="2to4"><para>Receiver Trigger Bit
			    #0</para><para>These two bits control at what
			    point the receiver is to generate an interrupt
			    when the FIFO is active.</para></entry>
		      </row>
		      
		      <row>
			<entry colname="col2">7</entry>
			<entry colname="col3">6</entry>
			<entry colname="col4">How many words are received
			  before an interrupt is generated</entry>
		      </row>
			  
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">0</entry>
			<entry colname="col4">1</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4">4</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">1</entry>
			<entry colname="col3">0</entry>
			<entry colname="col4">8</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">1</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4">14</entry>
		      </row>
		      
		      <row>
			<entry>Bit 5</entry>
			<entry spanname="2to4">Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 4</entry>
			<entry spanname="2to4">Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 3</entry>
			<entry spanname="2to4">DMA Mode Select.  If Bit 0 is
			  set to "1" (FIFOs enabled), setting this bit changes
			  the operation of the -RXRDY and -TXRDY signals from
			  Mode 0 to Mode 1.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 2</entry>
			<entry spanname="2to4">Transmit FIFO Reset.  When a
			  "1" is written to this bit, the contents of the FIFO
			  are discarded.  Any word currently being transmitted
			  will be sent intact.  This function is useful in
			  aborting transfers.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 1</entry>
			<entry spanname="2to4">Receiver FIFO Reset.  When a
			  "1" is written to this bit, the contents of the FIFO
			  are discarded.  Any word currently being assembled
			  in the shift register will be received
			  intact.</entry>
		      </row>

		      <row>
			<entry>Bit 0</entry>
			<entry spanname="2to4">16550 FIFO Enable.  When set,
			  both the transmit and receive FIFOs are enabled.
			  Any contents in the holding register, shift
			  registers or FIFOs are lost when FIFOs are enabled
			  or disabled.</entry>
		      </row>
		    </tbody>
		  </entrytbl>
		</row>
		
		<row>
		  <entry>+0x02</entry>
		  <entry>read</entry>
		  <entrytbl cols="6">
		    <colspec colnum="1" colname="col1">
		    <colspec colnum="2" colname="col2">
		    <colspec colnum="3" colname="col3">
		    <colspec colnum="4" colname="col4">
		    <colspec colnum="5" colname="col5">
		    <colspec colnum="6" colname="col6">
		    <spanspec namest="col1" nameend="col6" spanname="1to6">
		    <spanspec namest="col2" nameend="col6" spanname="2to6">

		    <tbody>
		      <row>
			<entry spanname="1to6">Interrupt Identification
			  Register</entry>
		      </row>

		      <row>
			<entry>Bit 7</entry>
			<entry spanname="2to6">FIFOs enabled.  On the
			  8250/16450 UART, this bit is zero.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 6</entry>
			<entry spanname="2to6">FIFOs enabled.  On the
			  8250/16450 UART, this bit is zero.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 5</entry>
			<entry spanname="2to6">Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 4</entry>
			<entry spanname="2to6">Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 3</entry>
			<entry spanname="2to6">Interrupt ID Bit #2.  On the
			  8250/16450 UART, this bit is zero.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 2</entry>
			<entry spanname="2to6">Interrupt ID Bit #1</entry>
		      </row>
		      
		      <row>
			<entry>Bit 1</entry>
			<entry spanname="2to6">Interrupt ID Bit #0.These three
			  bits combine to report the category of event that
			  caused the interrupt that is in progress.  These
			  categories have priorities, so if  multiple
			  categories of events occur at the same time, the
			  UART will report the more important events first and
			  the host must resolve the events in the order they
			  are reported.  All events that caused the current
			  interrupt must be resolved before any new interrupts
			  will be generated.  (This is a limitation of the PC
			  architecture.)</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">2</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4">0</entry>
			<entry colname="col5">Priority</entry>
			<entry colname="col6">Description</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4">1</entry>
			<entry colname="col5">First</entry>
			<entry colname="col6">Received Error (OE, PE, BI, or
			  FE)</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4">0</entry>
			<entry colname="col5">Second</entry>
			<entry colname="col6">Received Data Available</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">1</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4">0</entry>
			<entry colname="col5">Second</entry>
			<entry colname="col6">Trigger level identification
			  (Stale data in receive buffer)</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">0</entry>
			<entry colname="col4">1</entry>
			<entry colname="col5">Third</entry>
			<entry colname="col6">Transmitter has room for more
			  words (THRE)</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">0</entry>
			<entry colname="col4">0</entry>
			<entry colname="col5">Fourth</entry>
			<entry colname="col6">Modem Status Change (-CTS, -DSR,
			  -RI, or -DCD)</entry>
		      </row>
		      
		      <row>
			<entry>Bit 0</entry>
			<entry spanname="2to6">Interrupt Pending Bit.  If this
			  bit is set to "0", then at least one interrupt is
			  pending.</entry>
		      </row>
		    </tbody>
		  </entrytbl>
		</row>
		
		<row>
		  <entry>+0x03</entry>
		  <entry>write/read</entry>
		  <entrytbl cols="5">
		    <colspec colnum="1" colname="col1">
		    <colspec colnum="2" colname="col2">
		    <colspec colnum="3" colname="col3">
		    <colspec colnum="4" colname="col4">
		    <colspec colnum="5" colname="col5">
		    <spanspec namest="col1" nameend="col5" spanname="1to5">
		    <spanspec namest="col2" nameend="col5" spanname="2to5">
		    <spanspec namest="col4" nameend="col5" spanname="4to5">

		    <tbody>
		      <row>
			<entry spanname="1to5">Line Control Register
			  (LCR)</entry>
		      </row>
		      
		      <row>
			<entry>Bit 7</entry>
			<entry spanname="2to5">Divisor Latch Access Bit
			  (DLAB).  When set, access to the data
			  transmit/receive register (THR/RBR) and the
			  Interrupt Enable Register (IER) is disabled.  Any
			  access to these ports is now redirected to the
			  Divisor Latch Registers.  Setting this bit, loading
			  the Divisor Registers, and clearing DLAB should be
			  done with interrupts disabled.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 6</entry>
			<entry spanname="2to5">Set Break.  When set to "1",
			  the transmitter begins to transmit continuous
			  Spacing until this bit is set to "0".  This
			  overrides any bits of characters that are being
			  transmitted.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 5</entry>
			<entry spanname="2to5">Stick Parity.  When parity is
			  enabled, setting this bit causes parity to always be
			  "1" or "0", based on the value of Bit 4.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 4</entry>
			<entry spanname="2to5">Even Parity Select (EPS). When
			  parity is enabled and Bit 5 is "0", setting this bit
			  causes even parity to be transmitted and expected.
			  Otherwise, odd parity is used.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 3</entry>
			<entry spanname="2to5">Parity Enable (PEN).  When set
			  to "1", a parity bit is inserted between the last
			  bit of the data and the Stop Bit.  The UART will
			  also expect parity to be present in the received
			  data.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 2</entry>
			<entry spanname="2to5">Number of Stop Bits (STB). If
			  set to "1" and using 5-bit data words, 1.5 Stop Bits
			  are transmitted and expected in each data word.  For
			  6, 7 and 8-bit data words, 2 Stop Bits are
			  transmitted and expected.  When this bit is set to
			  "0", one Stop Bit is used on each data word.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 1</entry>
			<entry spanname="2to5">Word Length Select Bit #1
			  (WLSB1)</entry>
		      </row>
		      
		      <row>
			<entry>Bit 0</entry>
			<entry spanname="2to5">Word Length Select Bit #0
			  (WLSB0)</entry>
		      </row>
		      
		      <row>
			<entry colname="col2" spanname="2to5">Together these
			  bits specify the number of bits in each data
			  word.</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">1</entry>
			<entry colname="col3">0</entry>
			<entry colname="col4" spanname="4to5">Word
			  Length</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">0</entry>
			<entry colname="col4" spanname="4to5">5 Data
			  Bits</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">0</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4" spanname="4to5">6 Data
			  Bits</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">1</entry>
			<entry colname="col3">0</entry>
			<entry colname="col4" spanname="4to5">7 Data
			  Bits</entry>
		      </row>
		      
		      <row>
			<entry colname="col2">1</entry>
			<entry colname="col3">1</entry>
			<entry colname="col4" spanname="4to5">8 Data
			  Bits</entry>
		      </row>
		    </tbody>
		  </entrytbl>
		</row>
		
		<row>
		  <entry>+0x04</entry>
		  <entry>write/read</entry>
		  <entrytbl cols="2">
		    <colspec colnum="1" colname="col1">
		    <colspec colnum="2" colname="col2">
		    <spanspec namest="col1" nameend="col2" spanname="1to2">

		    <tbody>
		      <row>
			<entry spanname="1to2">Modem Control Register
			  (MCR)</entry>
		      </row>
		      
		      <row>
			<entry>Bit 7</entry>
			<entry>Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 6</entry>
			<entry>Reserved, always 0.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 5</entry>
			<entry>Reserved, always 0.</entry>
			  </row>
		      
		      <row>
			<entry>Bit 4</entry>
			<entry>Loop-Back Enable.  When set to "1", the UART
			  transmitter and receiver are internally connected
			  together to allow diagnostic operations.  In
			  addition, the UART modem control outputs are
			  connected to the UART modem  control inputs.  CTS is
			  connected to RTS, DTR is connected to DSR, OUT1 is
			  connected to RI, and OUT 2 is connected to
			  DCD.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 3</entry>
			<entry>OUT 2.  An auxiliary output that the host
			  processor may set high or low.  In the IBM PC serial
			  adapter (and most clones), OUT 2 is used to
			  tri-state (disable) the interrupt signal from the
			  8250/16450/16550 UART.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 2</entry>
			<entry>OUT 1.  An auxiliary output that the host
			  processor may set high or low.  This output is not
			  used on the IBM PC serial adapter.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 1</entry>
			<entry>Request to Send (RTS).  When set to "1", the
			  output of the UART -RTS line is Low
			  (Active).</entry>
		      </row>
		      
		      <row>
			<entry>Bit 0</entry>
			<entry>Data Terminal Ready (DTR).  When set to "1",
			  the output of the UART -DTR line is Low
			  (Active).</entry>
		      </row>
		    </tbody>
		  </entrytbl>
		</row>
		
		<row>
		  <entry>+0x05</entry>
		  <entry>write/read</entry>
		  <entrytbl cols="2">
		    <colspec colnum="1" colname="col1">
		    <colspec colnum="2" colname="col2">
		    <spanspec namest="col1" nameend="col2" spanname="1to2">

		    <tbody>
		      <row>
			<entry spanname="1to2">Line Status Register
			  (LSR)</entry>
		      </row>
		      
		      <row>
			<entry>Bit 7</entry>
			<entry>Error in Receiver FIFO.  On the 8250/16450
			  UART, this bit is zero.  This bit is set to "1" when
			  any of the bytes in the FIFO have one or more of the
			  following error conditions: PE, FE, or BI.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 6</entry>
			<entry>Transmitter Empty (TEMT).  When set to "1",
			  there are no words  remaining in the transmit FIFO
			  or the transmit shift register.  The transmitter is
			  completely idle.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 5</entry>
			<entry>Transmitter Holding Register Empty (THRE).
			  When set to "1", the FIFO (or holding register) now
			  has room for at least one additional word to
			  transmit.  The transmitter may still be transmitting
			  when this bit is set to "1".</entry>
		      </row>
		      
		      <row>
			<entry>Bit 4</entry>
			<entry>Break Interrupt (BI).  The receiver has
			  detected a Break signal.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 3</entry>
			<entry>Framing Error (FE).  A Start Bit was detected
			  but the Stop Bit did not appear at the expected
			  time.  The received word is probably
			  garbled.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 2</entry>
			<entry>Parity Error (PE).  The parity bit was
			  incorrect for the word received.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 1</entry>
			<entry>Overrun Error (OE).  A new word was received
			  and there was no room in the receive buffer.  The
			  newly-arrived word in the shift register is
			  discarded.  On 8250/16450 UARTs, the word in the
			  holding register is discarded and the newly- arrived
			  word is put in the holding register.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 0</entry>
			<entry>Data Ready (DR) One or more words are in the
			  receive FIFO that the host may read.  A word must be
			  completely received and moved from the shift
			  register into the FIFO (or holding register for
			  8250/16450 designs) before this bit is set.</entry>
		      </row>
		    </tbody>
		  </entrytbl>
		</row>
		
		<row>
		  <entry>+0x06</entry>
		  <entry>write/read</entry>
		  <entrytbl cols="2">
		    <colspec colnum="1" colname="col1">
		    <colspec colnum="2" colname="col2">
		    <spanspec namest="col1" nameend="col2" spanname="1to2">

		    <tbody>
		      <row>
			<entry spanname="1to2">Modem Status Register
			  (MSR)</entry>
		      </row>
		      
		      <row>
			<entry>Bit 7</entry>
			<entry>Data Carrier Detect (DCD).  Reflects the state
			  of the DCD line on the UART.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 6</entry>
			<entry>Ring Indicator (RI).  Reflects the state of the
			  RI line on the UART.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 5</entry>
			<entry>Data Set Ready (DSR).  Reflects the state of
			  the DSR line on the UART.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 4</entry>
			<entry>Clear To Send (CTS).  Reflects the state of the
			  CTS line on the UART.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 3</entry>
			<entry>Delta Data Carrier Detect (DDCD).  Set to "1"
			  if the -DCD line has changed state one more more
			  times since the last time the MSR was read by the
			  host.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 2</entry>
			<entry>Trailing Edge Ring Indicator (TERI).  Set to
			  "1" if the -RI line has had a low to high transition
			  since the last time the MSR was read by the
			  host.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 1</entry>
			<entry>Delta Data Set Ready (DDSR).  Set to "1" if the
			  -DSR line has changed state one more more times
			  since the last time the MSR was read by the
			  host.</entry>
		      </row>
		      
		      <row>
			<entry>Bit 0</entry>
			<entry>Delta Clear To Send (DCTS).  Set to "1" if the
			  -CTS line has changed state one more more times
			  since the last time the MSR was read by the
			  host.</entry>
		      </row>
		    </tbody>
		  </entrytbl>
		</row>
		
		<row>
		  <entry>+0x07</entry>
		  <entry>write/read</entry>
		  <entry>Scratch Register (SCR).  This register performs no
		    function in the UART.  Any value can be written by the
		    host to this location and read by the host later
		    on.</entry>
		</row>
	      </tbody>
	    </tgroup>
	  </informaltable>
	</sect4>

	<sect4>
	  <title>Beyond the 16550A UART</title>
	  
	  <para>Although National Semiconductor has not offered any components
	    compatible with the 16550 that provide additional features,
	    various other vendors have.  Some of these components are
	    described below.  It should be understood that to effectively
	    utilize these improvements, drivers may have to be provided by the
	    chip vendor since most of the popular operating systems do not
	    support features beyond those provided by the 16550.</para>
	      
	  <variablelist>
	    <varlistentry>
	      <term>ST16650</term>

	      <listitem>
		<para>By default this part is similar to the NS16550A, but an
		  extended 32-byte send and receive buffer can be optionally
		  enabled.  Made by Startech.</para>
	      </listitem>
	    </varlistentry>
	    
	    <varlistentry>
	      <term>TIL16660</term>
	      
	      <listitem>
		<para>By default this part behaves similar to the NS16550A,
		  but an extended 64-byte send and receive buffer can be
		  optionally enabled.  Made by Texas Instruments.</para>
	      </listitem>
	    </varlistentry>
	    
	    <varlistentry>
	      <term>Hayes ESP</term>
	      
	      <listitem>
		<para>This proprietary plug-in card contains a 2048-byte send
		  and receive buffer, and supports data rates to
		  230.4Kbit/sec.  Made by Hayes.</para>
	      </listitem>
	    </varlistentry>
	  </variablelist>
	  
	  <para>In addition to these <quote>dumb</quote> UARTs, many vendors
	    produce intelligent serial communication boards.  This type of
	    design usually provides a microprocessor that interfaces with
	    several UARTs, processes and buffers the data, and then alerts the
	    main PC processor when necessary.  Because the UARTs are not
	    directly accessed by the PC processor in this type of
	    communication system, it is not necessary for the vendor to use
	    UARTs that are compatible with the 8250, 16450, or the 16550 UART.
	    This leaves the designer free to components that may have better
	    performance characteristics.</para>
	</sect4>
      </sect3>
      
      <sect3 id="sio">
	<title>Configuring the <devicename>sio</devicename> driver</title>
	    
	<para>The <devicename>sio</devicename> driver provides support for
	  NS8250-, NS16450-, NS16550 and NS16550A-based EIA RS-232C (CCITT
	  V.24) communications interfaces.  Several multiport cards are
	  supported as well.  See the &man.sio.4;
	  manual page for detailed technical documentation.</para>
	    
	<sect4>
	  <title>Digi International (DigiBoard) PC/8</title>
	  
	  <para><emphasis>Contributed by &a.awebster;.  26 August
	      1995.</emphasis></para>
	      
	  <para>Here is a config snippet from a machine with a Digi
	    International PC/8 with 16550.  It has 8 modems connected to these
	    8 lines, and they work just great.  Do not forget to add
	    <literal>options COM_MULTIPORT</literal> or it will not work very
	    well!</para>
	      
	  <programlisting>
device          sio4    at isa? port 0x100 tty flags 0xb05
device          sio5    at isa? port 0x108 tty flags 0xb05
device          sio6    at isa? port 0x110 tty flags 0xb05
device          sio7    at isa? port 0x118 tty flags 0xb05
device          sio8    at isa? port 0x120 tty flags 0xb05
device          sio9    at isa? port 0x128 tty flags 0xb05
device          sio10   at isa? port 0x130 tty flags 0xb05
device          sio11   at isa? port 0x138 tty flags 0xb05 irq 9 vector siointr</programlisting>
	      
	  <para>The trick in setting this up is that the MSB of the flags
	    represent the last SIO port, in this case 11 so flags are
	    0xb05.</para>
	</sect4>

	<sect4>
	  <title>Boca 16</title>
	  
	  <para><emphasis>Contributed by &a.whiteside;.  26 August
	      1995.</emphasis></para>
	  
	  <para>The procedures to make a Boca 16 port board with FreeBSD are
	    pretty straightforward, but you will need a couple things to make
	    it work:</para>
	  
	  <orderedlist>
	    <listitem>
	      <para>You either need the kernel sources installed so you can
		recompile the necessary options or you will need someone else
		to compile it for you.  The 2.0.5 default kernel does
		<emphasis>not</emphasis> come with multiport support enabled
		and you will need to add a device entry for each port
		anyways.</para>
	    </listitem>
	    
	    <listitem>
	      <para>Two, you will need to know the interrupt and IO setting
		for your Boca Board so you can set these options properly in
		the kernel.</para>
	    </listitem>
	  </orderedlist>
	  
	  <para>One important note &mdash; the actual UART chips for the Boca
	    16 are in the connector box, not on the internal board itself.  So
	    if you have it unplugged, probes of those ports will fail.  I have
	    never tested booting with the box unplugged and plugging it back
	    in, and I suggest you do not either.</para>
	      
	  <para>If you do not already have a custom kernel configuration file
	    set up, refer to <link linkend="kernelconfig">Kernel
	      Configuration</link> for general procedures.  The following are
	    the specifics for the Boca 16 board and assume you are using the
	    kernel name MYKERNEL and editing with vi.</para>
	      
	  <procedure>
	    <step>
	      <para>Add the line 
		
		<programlisting>
options COM_MULTIPORT</programlisting>

		to the config file.</para>
	    </step>
	    
	    <step>
	      <para>Where the current <literal>device
		  sio<replaceable>n</replaceable></literal> lines are, you
		will need to add 16 more devices.  Only the last device
		includes the interrupt vector for the board.  (See the
		  &man.sio.4; manual page for detail as
		to why.)  The following example is for a Boca Board with an
		interrupt of 3, and a base IO address 100h.  The IO address
		for Each port is +8 hexadecimal from the previous port, thus
		the 100h, 108h, 110h...  addresses.</para>

	      <programlisting>
device sio1 at isa? port 0x100 tty flags 0x1005
device sio2 at isa? port 0x108 tty flags 0x1005
device sio3 at isa? port 0x110 tty flags 0x1005
device sio4 at isa? port 0x118 tty flags 0x1005
&hellip;
device sio15 at isa? port 0x170 tty flags 0x1005
device sio16 at isa? port 0x178 tty flags 0x1005 irq 3 vector siointr</programlisting>

	      <para>The flags entry <emphasis>must</emphasis> be changed from
		this example unless you are using the exact same sio
		assignments. Flags are set according to
		0x<replaceable>M</replaceable><replaceable>YY</replaceable>
		where <replaceable>M</replaceable> indicates the minor number
		of the master port (the last port on a Boca 16) and
		<replaceable>YY</replaceable> indicates if FIFO is enabled or
		disabled(enabled), IRQ sharing is used(yes) and if there is an
		AST/4 compatible IRQ control register(no).  In this example,
		<programlisting> flags 0x1005</programlisting> indicates that
		the master port is sio16.  If I added another board and
		assigned sio17 through sio28, the flags for all 16 ports on
		<emphasis>that</emphasis> board would be 0x1C05, where 1C
		indicates the minor number of the master port.  Do not change
		the 05 setting.</para>
		</step>
		  
	    <step>
	      <para>Save and complete the kernel configuration, recompile,
		install and reboot.  Presuming you have successfully installed
		the recompiled kernel and have it set to the correct address
		and IRQ, your boot message should indicate the successful
		probe of the Boca ports as follows: (obviously the sio
		numbers, IO and IRQ could be different)</para>
		    		    
	      <screen>sio1 at 0x100-0x107 flags 0x1005 on isa
sio1: type 16550A (multiport)
sio2 at 0x108-0x10f flags 0x1005 on isa
sio2: type 16550A (multiport)
sio3 at 0x110-0x117 flags 0x1005 on isa
sio3: type 16550A (multiport)
sio4 at 0x118-0x11f flags 0x1005 on isa
sio4: type 16550A (multiport)
sio5 at 0x120-0x127 flags 0x1005 on isa
sio5: type 16550A (multiport)
sio6 at 0x128-0x12f flags 0x1005 on isa
sio6: type 16550A (multiport)
sio7 at 0x130-0x137 flags 0x1005 on isa
sio7: type 16550A (multiport)
sio8 at 0x138-0x13f flags 0x1005 on isa
sio8: type 16550A (multiport)
sio9 at 0x140-0x147 flags 0x1005 on isa
sio9: type 16550A (multiport)
sio10 at 0x148-0x14f flags 0x1005 on isa
sio10: type 16550A (multiport)
sio11 at 0x150-0x157 flags 0x1005 on isa
sio11: type 16550A (multiport)
sio12 at 0x158-0x15f flags 0x1005 on isa
sio12: type 16550A (multiport)
sio13 at 0x160-0x167 flags 0x1005 on isa
sio13: type 16550A (multiport)
sio14 at 0x168-0x16f flags 0x1005 on isa
sio14: type 16550A (multiport)
sio15 at 0x170-0x177 flags 0x1005 on isa
sio15: type 16550A (multiport)
sio16 at 0x178-0x17f irq 3 flags 0x1005 on isa
sio16: type 16550A (multiport master)</screen>

	      <para>If the messages go by too fast to see,

		<screen>&prompt.root; <userinput>dmesg | more</userinput></screen>
		will show you the boot messages.</para>
	    </step>
	    
	    <step>
	      <para>Next, appropriate entries in <filename>/dev</filename> for
		the devices must be made using the
		<filename>/dev/MAKEDEV</filename> script. After becoming
		root:</para>
	      
	      <screen>&prompt.root; <userinput>cd /dev</userinput>
&prompt.root; <userinput>./MAKEDEV tty1</userinput>
&prompt.root; <userinput>./MAKEDEV cua1</userinput>
<emphasis>(everything in between)</emphasis>
&prompt.root; <userinput>./MAKEDEV ttyg</userinput>
&prompt.root; <userinput>./MAKEDEV cuag</userinput></screen>
		      
	      <para>If you do not want or need callout devices for some
		reason, you can dispense with making the
		<filename>cua*</filename> devices.</para>
	    </step>
	    
	    <step>
	      <para>If you want a quick and sloppy way to make sure the
		devices are working, you can simply plug a modem into each
		port and (as root)
		    
		<screen>&prompt.root; <userinput>echo at &gt; ttyd*</userinput></screen>
		for each device you have made.  You
		<emphasis>should</emphasis> see the RX lights flash for each
		working port.</para>
	    </step>
	  </procedure>
	</sect4>

	<sect4>
	  <title>Support for Cheap Multi-UART Cards</title>

	  <para><emphasis>Contributed by Helge Oldach
	    <email>hmo@sep.hamburg.com</email>, September
	      1999</emphasis></para>
	  
	  <para>Ever wondered about FreeBSD support for your 20$ multi-I/O
	    card with two (or more) COM ports, sharing IRQs? Here's
	    how:</para>
	  
	  <para>Usually the only option to support these kind of boards is to
	    use a distinct IRQ for each port.  For example, if your CPU board
	    has an on-board <devicename>COM1</devicename> port (aka
	    <devicename>sio0</devicename>&ndash;I/O address 0x3F8 and IRQ 4)
	    and you have an extension board with two UARTs, you will commonly
	    need to configure them as <devicename>COM2</devicename> (aka
	    <devicename>sio1</devicename>&ndash;I/O address 0x2F8 and IRQ 3),
	    and the third port (aka <devicename>sio2</devicename>) as I/O
	    0x3E8 and IRQ 5.  Obviously this is a waste of IRQ ressources, as
	    it should be basically possible to run both extension board ports
	    using a single IRQ with the <literal>COM_MULTIPORT</literal>
	    configuration described in the previous sections.</para>
	  
	  <para>Such cheap I/O boards commonly have a 4 by 3 jumper matrix for
	    the COM ports, similar to the following:</para>

<programlisting>            o  o  o  *
Port A               |
            o  *  o  *
Port B         |
            o  *  o  o
IRQ         2  3  4  5</programlisting>

	  <para>Shown here is port A wired for IRQ 5 and port B wired for IRQ
	    3.  The IRQ columns on your specific board may vary&mdash;other
	    boards may supply jumpers for IRQs 3, 4, 5, and 7 instead.</para>

	  <para>One could conclude that wiring both ports for IRQ 3 using a
	    handcrafted wire-made jumper covering all three connection points
	    in the IRQ 3 column would solve the issue, but no.  You cannot
	    duplicate IRQ 3 because the output drivers of each UART are wired
	    in a <quote>totem pole</quote> fashion, so if one of the UARTs
	    drives IRQ 3, the output signal will not be what you would expect.
	    Depending on the implementation of the extension board or your
	    motherboard, the IRQ 3 line will continuously stay up, or always
	    stay low.</para>

	  <para>You need to decouple the IRQ drivers for the two UARTs, so
	    that the IRQ line of the board only goes up if (and only if) one
	    of the UARTs asserts a IRQ, and stays low otherwise.  The solution
	    was proposed by Jrg Wunsch
	    <email>j@ida.interface-business.de</email>: To solder up a
	    wired-or consisting of two diodes (Germanium or Schottky-types
	    strongly preferred) and a 1 kOhm resistor.  Here is the schematic,
	    starting from the 4 by 3 jumper field above:</para>

<programlisting>                          Diode
                +---------->|-------+
               /                    |
            o  *  o  o              |     1 kOhm
Port A                              +----|######|-------+
            o  *  o  o              |                   |
Port B          `-------------------+                 ==+==
            o  *  o  o              |                 Ground
                \                   |
                 +--------->|-------+
IRQ         2  3  4  5    Diode</programlisting>

	  <para>The cathodes of the diodes are connected to a common point,
	    together with a 1 kOhm pull-down resistor.  It is essential to
	    connect the resistor to ground to avoid floating of the IRQ line
	    on the bus.</para>

	  <para>Now we are ready to configure a kernel.  Staying with this
	    example, we would configure:</para>

	  <programlisting># standard on-board COM1 port
device          sio0    at isa? port "IO_COM1" tty flags 0x10
# patched-up multi-I/O extension board
options         COM_MULTIPORT
device          sio1    at isa? port "IO_COM2" tty flags 0x205
device          sio2    at isa? port "IO_COM3" tty flags 0x205 irq 3</programlisting>

	  <para>Note that the <literal>flags</literal> setting for
	    <devicename>sio1</devicename> and <devicename>sio2</devicename> is
	    truely essential; refer to
	      &man.sio.4; for details. (Generally, the <literal>2</literal> in
	    the "flags" attribute refers to <devicename>sio</devicename>2
	    which holds the IRQ, and you surely want a <literal>5</literal>
	    low nibble.)  With kernel verbose mode turned on this should yield
	    something similar to this:</para>

	  <screen>sio0: irq maps: 0x1 0x11 0x1 0x1
sio0 at 0x3f8-0x3ff irq 4 flags 0x10 on isa
sio0: type 16550A
sio1: irq maps: 0x1 0x9 0x1 0x1
sio1 at 0x2f8-0x2ff flags 0x205 on isa
sio1: type 16550A (multiport)
sio2: irq maps: 0x1 0x9 0x1 0x1
sio2 at 0x3e8-0x3ef irq 3 flags 0x205 on isa
sio2: type 16550A (multiport master)</screen>

	  <para>Though <filename>/sys/i386/isa/sio.c</filename> is somewhat
	    cryptic with its use of the <quote>irq maps</quote> array above,
	    the basic idea is that you observe <literal>0x1</literal> in the
	    first, third, and fourth place.  This means that the corresponding
	    IRQ was set upon output and cleared after, which is just what we
	    would expect. If your kernel does not display this behaviour, most
	    likely there is something wrong with your wiring.</para>
	</sect4>
      </sect3>
      
      <sect3 id="cy">
	<title>Configuring the <devicename>cy</devicename> driver</title>
	    
	<para><emphasis>Contributed by &a.alex;.  6 June
	    1996.</emphasis></para>
	    
	<para>The Cyclades multiport cards are based on the
	  <devicename>cy</devicename> driver instead of the usual
	  <devicename>sio</devicename> driver used by other multiport cards.
	  Configuration is a simple matter of:</para>

	<procedure>
	  <step>
	    <para>Add the <devicename>cy</devicename> device to your <link
		linkend="kernelconfig-config">kernel configuration</link>
	      (note that your irq and iomem settings may differ).</para>
		    
	    <programlisting>
device cy0 at isa? tty irq 10 iomem 0xd4000 iosiz 0x2000 vector cyintr</programlisting>
	  </step>
		
	  <step>
	    <para><link linkend="kernelconfig-building">Rebuild and
		install</link>  the new kernel.</para>
	  </step>
	  
	  <step>
	    <para>Make the <link linkend="kernelconfig-nodes">device
		nodes</link> by typing (the following example assumes an
	      8-port board):</para>
		  
	    <screen>&prompt.root; <userinput>cd /dev</userinput>
&prompt.root; <userinput>for i in 0 1 2 3 4 5 6 7;do ./MAKEDEV cuac$i ttyc$i;done</userinput></screen>
	  </step>
	  
	  <step>
	    <para>If appropriate, add <link linkend="dialup">dialup</link>
	      entries to <link linkend="dialup-ttys">/etc/ttys</link> by
	      duplicating serial device (<literal>ttyd</literal>) entries and
	      using <literal>ttyc</literal> in place of
	      <literal>ttyd</literal>.  For example:</para>
	    
	    <programlisting>
ttyc0   "/usr/libexec/getty std.38400"  unknown on insecure
ttyc1   "/usr/libexec/getty std.38400"  unknown on insecure
ttyc2   "/usr/libexec/getty std.38400"  unknown on insecure
&hellip;
ttyc7   "/usr/libexec/getty std.38400"  unknown on insecure</programlisting>
	  </step>
		
	  <step>
	    <para>Reboot with the new kernel.</para>
	  </step>
	</procedure>
      </sect3>

      <sect3>
	<title>Configuring the <devicename>si</devicename> driver</title>

	<para><emphasis>Contributed by &a.nsayer;. 25 March
	    1998.</emphasis></para>
     
	<para>The Specialix SI/XIO and SX multiport cards use the
	  <devicename>si</devicename> driver. A single machine can
	  have up to 4 host cards. The following host cards
	  are supported:</para>

	<itemizedlist>
	  <listitem><para>ISA SI/XIO host card (2 versions)</para></listitem>
	  <listitem><para>EISA SI/XIO host card</para></listitem>
	  <listitem><para>PCI SI/XIO host card</para></listitem>
	  <listitem><para>ISA SX host card</para></listitem>
	  <listitem><para>PCI SX host card</para></listitem>
	</itemizedlist>

	<para>Although the SX and SI/XIO host cards look markedly different,
	  their functionality are basically the same. The host cards do not
	  use I/O locations, but instead require a 32K chunk of memory. The
	  factory configuration for ISA cards places this at
	  <literal>0xd0000-0xd7fff</literal>.
	  They also require an IRQ. PCI cards will, of course, autoconfigure
	  themselves.</para>

	<para>You can attach up to 4 external modules to each host card. The
	  external modules contain either 4 or 8 serial ports. They come in
	  the following varieties:</para>

	<itemizedlist>
	  <listitem><para>SI 4 or 8 port modules. Up to 57600 bps on each port
	      supported.</para></listitem>
	  
	  <listitem><para>XIO 8 port modules. Up to 115200 bps on each port
	      supported. One type of XIO module has 7 serial and 1 parallel
	      port.</para></listitem>
	  
	  <listitem><para>SXDC 8 port modules. Up to 921600 bps on each port
	      supported. Like XIO, a module is available with one parallel
	      port as well.</para></listitem>
	</itemizedlist>

	<para>To configure an ISA host card, add the following line to your
	  <link linkend="kernelconfig-config">kernel configuration
	    file</link>, changing the numbers as appropriate:</para>
     
	<programlisting>
device si0 at isa? tty iomem 0xd0000 irq 11</programlisting>

	<para>Valid IRQ numbers are 9, 10, 11, 12 and 15 for SX ISA host cards
	  and 11, 12 and 15 for SI/XIO ISA host cards.</para>
     
	<para>To configure an EISA or PCI host card, use this line:</para>
     
	<programlisting>
device si0</programlisting>

	<para>After adding the configuration entry, <link
	    linkend="kernelconfig-building"> rebuild and install</link>  your
	  new kernel.</para>
     
	<para>After rebooting with the new kernel, you need to make the <link
	    linkend="kernelconfig-nodes"> device nodes</link> in /dev. The
	  <filename>MAKEDEV</filename> script will take care of this for you.
	  Count how many total ports you have and type:</para>
     
	<screen>&prompt.root; <userinput>cd /dev</userinput>
&prompt.root; <userinput>./MAKEDEV ttyA<replaceable>nn</replaceable> cuaA<replaceable>nn</replaceable></userinput></screen>
     
	<para>(where <replaceable>nn</replaceable> is the number of
	  ports)</para>
     
	<para>If you want login prompts to appear on these ports, you will
	  need to add lines like this to <link
	    linkend="dialup"><filename>/etc/ttys</filename></link>:</para>

	<programlisting>
ttyA01  "/usr/libexec/getty std.9600"   vt100   on insecure
	</programlisting>
     
	<para>Change the terminal type as appropriate. For modems,
	  <userinput>dialup</userinput> or <userinput>unknown</userinput> is
	  fine.</para>
      </sect3>
    </sect2>
    
    <sect2>
      <title>* Parallel ports</title>
      
      <para></para>
    </sect2>
    
    <sect2>
      <title>* Modems</title>
      
      <para></para>
    </sect2>
    
    <sect2>
      <title>* Network cards</title>
      
      <para></para>
    </sect2>
    
    <sect2>
      <title>* Keyboards</title>
      
      <para></para>
    </sect2>
    
    <sect2>
      <title>Mice</title>
	
      <para><emphasis>Contributed by Joel Sutton
  	  <email>jsutton@bbcon.com.au</email> January 2000</emphasis></para>

      <para>FreeBSD supports a variety of different mice via the PS/2, serial
	and USB ports.  Most users choose to use the mouse daemon to handle
	their mouse because it allows interaction in both X and on the system
	console.  For more information on the mouse daemon refer to
	  &man.moused.8;.  The examples throughout this section assume that
	the mouse daemon is being used.</para>
      
      <note>
        <para>This section contains the names of specific products that the
	  author has confirmed will work with FreeBSD. Other similar devices
	  not listed may also be supported.</para>
      </note>

      <sect3>
        <title>PS/2</title>
	
        <sect4>
          <title>System Configuration</title>

	  <para>To ensure that your PS/2 mouse functions correctly with the
	    mouse daemon you will need to include the following text in
	    <filename>/etc/rc.conf</filename></para>

          <programlisting>moused_enable="YES"
moused_type="ps/2"
moused_port="/dev/psm0"</programlisting>
        </sect4>

        <sect4>
          <title>Known Compatible Devices</title>
	  
          <itemizedlist>
            <listitem>
              <para>Logitech First Mouse - Three Button</para>
            </listitem>
	    
            <listitem>
              <para>Microsoft Serial - PS/2 Compatible Mouse</para>
            </listitem>
          </itemizedlist>
        </sect4>
      </sect3>

      <sect3>
        <title>Serial</title>

        <sect4>
          <title>System Configuration</title>

	  <para>To ensure that your serial mouse functions correctly with the
	    mouse daemon you will need to include the following text in
	    <filename>/etc/rc.conf</filename>.  This example assumes that the
	    mouse is connected to <devicename>COM1:</devicename> and can be
	    automatically recognised by the mouse daemon.</para>

          <programlisting>moused_enable="YES"
moused_type="auto"
moused_port="/dev/cuaa0"</programlisting>

          <para>See the &man.moused.8; manual page for a detailed description
	    of how to configure the mouse daemon to work with specific types
	    of serial mice.</para>
        </sect4>

        <sect4>
          <title>Known Compatible Devices</title>
	  
          <itemizedlist>
            <listitem>
              <para>Generic Microsoft Compatible Mice</para>
            </listitem>

            <listitem>
              <para>Logitech First Mouse - Three Button</para>
            </listitem>

            <listitem>
              <para>Microsoft Serial - PS/2 Compatible Mouse</para>
            </listitem>
          </itemizedlist>
        </sect4>
      </sect3>

      <sect3>
        <title>USB</title>
	
        <sect4>
          <title>System Configuration</title>
	  
          <para>The USB device drivers are a relatively new addition to
	    FreeBSD and have not yet been included in the GENERIC kernel.  The
	    following prodecure is an example of how to setup the relevant
	    drivers on a typical system.</para>

          <procedure>
            <step>
       	      <para>Add the <devicename>ums</devicename> device to the usb
		section of your <link linkend="kernelconfig-config">kernel
		  configuration</link>.  For example:
       	      </para>
       		
	      <programlisting>controller usb0 controller uhci0 device ums0</programlisting>
	    </step>
       		
	    <step>
	      <para><link linkend="kernelconfig-building">Rebuild and
		  install</link> the new kernel.</para>
	    </step>
	    
       	    <step>
       	      <para>Make the <link linkend="kernelconfig-nodes">device
		  node</link> by typing:</para>

	      <screen>&prompt.root; <userinput>cd /dev</userinput>
&prompt.root; <userinput>sh MAKEDEV ums0</userinput></screen>
	    </step>

	    <step>
	      <para>Include the following text in
		<filename>/etc/rc.conf</filename> to ensure correct operation
		of the mouse daemon:</para>
	      
	      <programlisting>moused_enable="YES"
moused_type="auto"
moused_port="/dev/ums0"</programlisting>
            </step>
	    
            <step>
              <para>Reboot the system.</para>
	      <screen>&prompt.root; <userinput>shutdown -r now</userinput></screen>
            </step>
          </procedure>
        </sect4>

        <sect4>
          <title>Known Compatible Devices</title>

          <itemizedlist>
            <listitem>
              <para>Logitech TrackMan - Marble Wheel</para>
            </listitem>
          </itemizedlist>
        </sect4>
      </sect3>
    </sect2>

<![ %not.published; [

    <sect2>
      <title>* Other</title>
      
      <para></para>
    </sect2>
  </sect1>

]]>
  
  <sect1 id="hw-storage">
    <title>Storage Devices</title>
        
    <sect2 id="esdi">
      <title>Using ESDI hard disks</title>
      
      <para><emphasis>Copyright &copy; 1995, &a.wilko;.  24
	  September 1995.</emphasis></para>
	  
      <para>ESDI is an acronym that means Enhanced Small Device Interface. It
	is loosely based on the good old ST506/412 interface originally
	devised by Seagate Technology, the makers of the first affordable
	5.25" winchester disk.</para>
	  
      <para>The acronym says Enhanced, and rightly so.  In the first place the
	speed of the interface is higher, 10 or 15 Mbits/second instead of the
	5 Mbits/second of ST412 interfaced drives.  Secondly some higher level
	commands are added, making the ESDI interface somewhat 'smarter' to
	the operating system driver writers.  It is by no means as smart as
	SCSI by the way.  ESDI is standardized by ANSI.</para>
	  
      <para>Capacities of the drives are boosted by putting more sectors on
	each track.  Typical is 35 sectors per track, high capacity drives I
	have seen were up to 54 sectors/track.</para>
      
      <para>Although ESDI has been largely obsoleted by IDE and SCSI
	interfaces, the availability of free or cheap surplus drives makes
	them  ideal for low (or now) budget systems.</para>
      
      <sect3>
	<title>Concepts of ESDI</title>

	<sect4>
	  <title>Physical connections</title>
	  
	  <para>The ESDI interface uses two cables connected to each drive.
	    One cable is a 34 pin flat cable edge connector that carries the
	    command and status signals from the controller to the drive and
	    vice-versa.  The command cable is daisy chained between all the
	    drives.  So, it forms a bus onto which all drives are
	    connected.</para>
	      
	  <para>The second cable is a 20 pin flat cable edge connector that
	    carries the data to and from the drive.  This cable is radially
	    connected, so each drive has its own direct connection to the
	    controller.</para>
	      
	  <para>To the best of my knowledge PC ESDI controllers are limited to
	    using a maximum of 2 drives per controller.  This is compatibility
	    feature(?) left over from the WD1003 standard that reserves only a
	    single bit for device addressing.</para>
	</sect4>

	<sect4>
	  <title>Device addressing</title>
	  
	  <para>On each command cable a maximum of 7 devices and 1 controller
	    can be present.  To enable the controller to uniquely  identify
	    which drive it addresses, each ESDI device is equipped with
	    jumpers or switches to select the devices address.</para>
	      
	  <para>On PC type controllers the first drive is set to address 0,
	    the second disk to address 1.  <emphasis>Always make
	      sure</emphasis> you set each disk to an unique address! So, on a
	    PC with its two drives/controller maximum the first drive is drive
	    0, the second is drive 1.</para>
	</sect4>

	<sect4>
	  <title>Termination</title>
	  
	  <para>The daisy chained command cable (the 34 pin cable remember?)
	    needs to be terminated at the last drive on the chain.  For this
	    purpose ESDI drives come with a termination resistor network that
	    can be removed or disabled by a jumper when it is not used.</para>
	  
	  <para>So, one and <emphasis>only</emphasis> one drive, the one at
	    the farthest end of the command cable has its terminator
	    installed/enabled.  The controller automatically terminates the
	    other end of the cable.  Please note that this implies that the
	    controller must be  at one end of the cable and
	    <emphasis>not</emphasis> in the middle.</para>
	</sect4>
      </sect3>
      
      <sect3>
	<title>Using ESDI disks with FreeBSD</title>

	<para>Why is ESDI such a pain to get working in the first
	  place?</para>

	<para>People who tried ESDI disks with FreeBSD are known to have
	  developed a profound sense of frustration.  A combination of factors
	  works against you to produce effects that are hard to understand
	  when you have never seen them before.</para>
	    
	<para>This has also led to the popular legend ESDI and FreeBSD is a
	  plain NO-GO.  The following sections try to list all the pitfalls
	  and  solutions.</para>
	    
	<sect4>
	  <title>ESDI speed variants</title>
	  
	  <para>As briefly mentioned before, ESDI comes in two speed flavors.
	    The older drives and controllers use a 10 Mbits/second data
	    transfer rate.  Newer stuff uses 15 Mbits/second.</para>
	      
	  <para>It is not hard to imagine that 15 Mbits/second drive cause
	    problems on controllers laid out for 10 Mbits/second.  As always,
	    consult your controller <emphasis>and</emphasis> drive
	    documentation to see if things match.</para>
	</sect4>

	<sect4>
	  <title>Stay on track</title>
	  
	  <para>Mainstream ESDI drives use 34 to 36 sectors per track. Most
	    (older) controllers cannot handle more than this  number of
	    sectors.  Newer, higher capacity, drives use higher numbers of
	    sectors per track.  For instance, I own a 670 Mb drive that has 54
	    sectors per track.</para>
	      
	  <para>In my case, the controller could not handle this number of
	    sectors.  It proved to work well except that it only used 35
	    sectors on each track.  This meant losing a lot of disk
	    space.</para>
	      
	  <para>Once again, check the documentation of your hardware for more
	    info.  Going out-of-spec like in the example might or might not
	    work.  Give it a try or get another more capable
	    controller.</para>
	</sect4>

	<sect4>
	  <title>Hard or soft sectoring</title>
	  
	  <para>Most ESDI drives allow hard or soft sectoring to be selected
	    using a jumper.  Hard sectoring means that the drive will produce
	    a sector pulse on the start of each new sector. The controller
	    uses this pulse to tell when it should start to write or
	    read.</para>
	  
	  <para>Hard sectoring allows a selection of sector size (normally
	    256, 512 or 1024 bytes per formatted sector).  FreeBSD uses 512
	    byte sectors.  The number of sectors per track also varies while
	    still using the same number of bytes per formatted sector.  The
	    number of <emphasis>unformatted</emphasis> bytes per sector
	    varies, dependent on your controller it needs more or less
	    overhead  bytes to work correctly.  Pushing more sectors on a
	    track  of course gives you more usable space, but might give
	    problems if your controller needs more bytes than the  drive
	    offers.</para>
	      
	  <para>In case of soft sectoring, the controller itself determines
	    where to start/stop reading or writing.  For ESDI hard sectoring
	    is the default (at least on everything I came across).  I never
	    felt the urge to try soft sectoring.</para>
	      
	  <para>In general, experiment with sector settings before you install
	    FreeBSD because you need to re-run the low-level format after each
	    change.</para>
	</sect4>

	<sect4>
	  <title>Low level formatting</title>
	  
	  <para>ESDI drives need to be low level formatted before they are
	    usable.  A reformat is needed whenever you figgle with the number
	    of sectors/track jumpers or the physical orientation of the drive
	    (horizontal, vertical).  So, first think, then format.  The format
	    time must not be underestimated, for big disks it can take
	    hours.</para>
	      
	  <para>After a low level format, a surface scan is done to find and
	    flag bad sectors.  Most disks have a manufacturer bad block list
	    listed on a piece of paper or adhesive sticker.  In addition, on
	    most disks the list is also written onto the disk.  Please use the
	    manufacturer's list.  It is much easier to remap a defect now than
	    after FreeBSD is installed.</para>
	      
	  <para>Stay away from low-level formatters that mark all sectors of a
	    track as bad as soon as they find one bad sector.  Not only does
	    this waste space, it also and more importantly causes you grief
	    with bad144 (see the section on bad144).</para>
	</sect4>

	<sect4>
	  <title>Translations</title>
	  
	  <para>Translations, although not exclusively a ESDI-only problem,
	    might give you real trouble.  Translations come in multiple
	    flavors.  Most of them  have in common that they attempt to work
	    around the limitations posed upon disk geometries by the original
	    IBM PC/AT design (thanks IBM!).</para>
	      
	  <para>First of all there is the (in)famous 1024 cylinder limit. For
	    a system to be able to boot, the stuff (whatever operating system)
	    must be in the first 1024 cylinders of a disk.  Only 10 bits are
	    available to encode the cylinder number.  For the number of
	    sectors the limit is 64 (0-63).  When you combine the 1024
	    cylinder limit with the 16 head limit (also a design feature) you
	    max out at fairly limited  disk sizes.</para>
	      
	  <para>To work around this problem, the manufacturers of ESDI PC
	    controllers added a BIOS prom extension on their boards.  This
	    BIOS extension handles disk I/O for booting (and for some
	    operating systems <emphasis>all</emphasis> disk I/O) by using
	    translation.  For instance, a big drive might be presented to the
	    system as having 32 heads and 64 sectors/track.  The result is
	    that the number of cylinders is reduced to something below 1024
	    and is therefore usable by the system without problems.  It is
	    noteworthy to know that FreeBSD does not use the BIOS after its
	    kernel has started.  More on this later.</para>
	      
	  <para>A second reason for translations is the fact that most older
	    system BIOSes could only handle drives with 17 sectors per track
	    (the old ST412 standard).  Newer system BIOSes usually have a
	    user-defined drive type (in most cases this is drive type
	    47).</para>

	  <warning>
	    <para>Whatever you do to translations after reading this document,
	      keep in mind that if you have multiple operating systems on the
	      same disk, all must use the same translation</para>
	  </warning>
	  
	  <para>While on the subject of translations, I have seen one
	    controller type (but there are probably more like this) offer the
	    option to logically split a drive in multiple partitions as a BIOS
	    option.  I had select 1 drive == 1 partition because this
	    controller wrote this info onto the disk.  On power-up it read the
	    info and presented itself to the system based on the info from the
	    disk.</para>
	</sect4>
	    
	<sect4>
	  <title>Spare sectoring</title>
	  
	  <para>Most ESDI controllers offer the possibility to remap bad
	    sectors.  During/after the low-level format of the disk bad
	    sectors are marked as such, and a replacement sector is put in
	    place (logically of course) of the bad one.</para>
	      
	  <para>In most cases the remapping is done by using N-1 sectors on
	    each track for actual data storage, and sector N itself is the
	    spare sector.  N is the total number of sectors physically
	    available on the track.  The idea behind this is that the
	    operating system sees a 'perfect' disk without bad sectors.  In
	    the case of FreeBSD this concept is not usable.</para>
	      
	  <para>The problem is that the translation from
	    <emphasis>bad</emphasis> to <emphasis>good</emphasis> is performed
	    by the BIOS of the ESDI controller.  FreeBSD, being a true 32 bit
	    operating system, does not use the BIOS after it has been booted.
	    Instead, it has device drivers that talk directly to the
	    hardware.</para>
	      
	  <para><emphasis>So: don't use spare sectoring, bad block remapping
	      or whatever it may be called by the controller manufacturer when
	      you want to use the disk for FreeBSD.</emphasis></para>
	</sect4>

	<sect4>
	  <title>Bad block handling</title>
	  
	  <para>The preceding section leaves us with a problem.  The
	    controller's bad block handling is not usable and still FreeBSD's
	    filesystems assume perfect media without any flaws. To solve this
	    problem, FreeBSD use the <command>bad144</command> tool.  Bad144
	    (named after a Digital Equipment standard for bad block handling)
	    scans a FreeBSD slice for bad blocks.  Having found these bad
	    blocks, it writes a table with the offending block numbers to the
	    end of the FreeBSD slice.</para>
	      
	  <para>When the disk is in operation, the disk accesses are checked
	    against the table read from the disk.  Whenever a block number is
	    requested that is in the <command>bad144</command> list, a
	    replacement block (also from the end of the FreeBSD slice) is
	    used.  In this way, the <command>bad144</command> replacement
	    scheme presents 'perfect' media to the FreeBSD filesystems.</para>
	      
	  <para>There are a number of potential pitfalls associated with the
	    use of <command>bad144</command>.  First of all, the slice cannot
	    have more than 126 bad sectors.  If your drive has a high number
	    of bad sectors, you might need to divide it into multiple FreeBSD
	    slices each containing less than 126 bad sectors.  Stay away from
	    low-level format programs that mark <emphasis>every</emphasis>
	    sector of a track as bad when  they find a flaw on the track.  As
	    you can imagine, the  126 limit is quickly reached when the
	    low-level format is done this way.</para>
	      
	  <para>Second, if the slice contains the root filesystem, the slice
	    should be within the 1024 cylinder BIOS limit.  During the boot
	    process the bad144 list is read using the BIOS and this only
	    succeeds when the list is within the 1024 cylinder limit.</para>

	  <note>
	    <para>The restriction is not that only the root
	      <emphasis>filesystem</emphasis> must be within the 1024 cylinder
	      limit, but rather the entire <emphasis>slice</emphasis> that
	      contains the root filesystem.</para>
	  </note>
	</sect4>

	<sect4>
	  <title>Kernel configuration</title>
	  
	  <para>ESDI disks are handled by the same <literal>wd</literal>driver
	    as IDE and ST412 MFM disks.  The <literal>wd</literal> driver
	    should work for all WD1003 compatible interfaces.</para>
	      
	  <para>Most hardware is jumperable for one of two different I/O
	    address ranges and IRQ lines.  This allows you to have  two wd
	    type controllers in one system.</para>
	  
	  <para>When your hardware allows non-standard strappings, you can use
	    these with FreeBSD as long as you enter the  correct info into the
	    kernel config file.  An example from the kernel config file (they
	    live in <filename>/sys/i386/conf</filename> BTW).</para>
	      
	  <programlisting>
# First WD compatible controller
controller      wdc0    at isa? port "IO_WD1" bio irq 14 vector wdintr
disk            wd0     at wdc0 drive 0
disk            wd1     at wdc0 drive 1
# Second WD compatible controller
controller      wdc1    at isa? port "IO_WD2" bio irq 15 vector wdintr
disk            wd2     at wdc1 drive 0
disk            wd3     at wdc1 drive 1</programlisting>
	</sect4>
      </sect3>
      
      <sect3>
	<title>Particulars on ESDI hardware</title>

	<sect4>
	  <title>Adaptec 2320 controllers</title>
	  
	  <para>I successfully installed FreeBSD onto a ESDI disk controlled
	    by a ACB-2320.  No other operating system was present on the
	    disk.</para>
	      
	  <para>To do so I low level formatted the disk using
	    <command>NEFMT.EXE</command> (<command>ftp</command>able from
	    <hostid role="fqdn">www.adaptec.com</hostid>) and answered NO to
	    the question whether the disk should be formatted with a spare
	    sector on each track.  The BIOS on the ACD-2320 was disabled.  I
	    used the <literal>free configurable</literal> option in the system
	    BIOS to allow the BIOS to boot it.</para>
	      
	  <para>Before using <command>NEFMT.EXE</command> I tried to format
	    the disk using the ACB-2320 BIOS builtin formatter.  This proved
	    to be a show stopper, because it did not give me an option to
	    disable spare sectoring.  With spare sectoring enabled the FreeBSD
	    installation process broke down on the <command>bad144</command>
	    run.</para>
	      
	  <para>Please check carefully which
	    ACB-232<replaceable>xy</replaceable> variant you have. The
	    <replaceable>x</replaceable> is either <literal>0</literal> or
	    <literal>2</literal>, indicating a controller without or with a
	    floppy controller on board.</para>
	      
	  <para>The <literal>y</literal> is more interesting.  It can either
	    be a blank, a <literal>A-8</literal> or a <literal>D</literal>.  A
	    blank indicates a plain 10 Mbits/second controller.  An
	    <literal>A-8</literal> indicates a 15 Mbits/second controller
	    capable of handling 52 sectors/track.  A <literal>D</literal>
	    means a 15 Mbits/second controller that can also handle drives
	    with &gt; 36 sectors/track (also 52 ?).</para>
	      
	  <para>All variations should be capable of using 1:1 interleaving.
	    Use 1:1, FreeBSD is fast enough to handle it.</para>
	</sect4>

	<sect4>
	  <title>Western Digital WD1007 controllers</title>
	  
	  <para>I successfully installed FreeBSD onto a ESDI disk controlled
	    by a WD1007 controller.  To be precise, it was a WD1007-WA2.
	    Other variations of the WD1007 do exist.</para>
	      
	  <para>To get it to work, I had to disable the sector translation and
	    the WD1007's onboard BIOS.  This implied I could not use the
	    low-level formatter built into this BIOS.  Instead, I grabbed
	    <command>WDFMT.EXE</command> from <hostid
	      role="fqdn">www.wdc.com</hostid> Running this formatted my drive
	    just fine.</para>
	</sect4>

	<sect4>
	  <title>Ultrastor U14F controllers</title>
	  
	  <para>According to multiple reports from the net, Ultrastor ESDI
	    boards work OK with FreeBSD.  I lack any further info on
	    particular settings.</para>
	</sect4>
      </sect3>
      
      <sect3 id="esdi-further-reading">
	<title>Further reading</title>
	    
	<para>If you intend to do some serious ESDI hacking, you might want to
	  have the official standard at hand:</para>
	    
	<para>The latest ANSI X3T10 committee document is: Enhanced Small
	  Device Interface (ESDI) [X3.170-1990/X3.170a-1991]    [X3T10/792D
	  Rev 11]</para>
		
	<para>On Usenet the newsgroup <ulink
	    url="news:comp.periphs">comp.periphs</ulink> is a noteworthy place
	  to look  for more info.</para>
	    
	<para>The World Wide Web (WWW) also proves to be a very handy info
	  source: For info on Adaptec ESDI controllers see <ulink
	    url="http://www.adaptec.com/">http://www.adaptec.com/</ulink>. For
	  info on Western Digital controllers see <ulink
	    url="http://www.wdc.com/">http://www.wdc.com/</ulink>.</para>
      </sect3>
      
      <sect3>
	<title>Thanks to...</title>

	<para>Andrew Gordon for sending me an Adaptec 2320 controller and ESDI
	  disk for testing.</para>
      </sect3>
    </sect2>
    
    <sect2 id="scsi">
      <title>What is SCSI?</title>
      
      <para><emphasis>Copyright &copy; 1995, &a.wilko;.  July 6,
	  1996.</emphasis></para>
	  
      <para>SCSI is an acronym for Small Computer Systems Interface.  It is an
	ANSI standard that has become one of the leading I/O buses in the
	computer industry.  The foundation of the SCSI standard was laid by
	Shugart Associates (the same guys that gave the world the first mini
	floppy disks) when they introduced the SASI bus (Shugart Associates
	Standard Interface).</para>
	  
      <para>After some time an industry effort was started to come to a more
	strict standard allowing devices from different vendors to work
	together.  This effort was recognized in the ANSI SCSI-1 standard.
	The SCSI-1 standard (approx 1985) is rapidly becoming obsolete.  The
	current standard is SCSI-2 (see <link
	  linkend="scsi-further-reading">Further reading</link>), with SCSI-3
	on the drawing boards.</para>
	  
      <para>In addition to a physical interconnection standard, SCSI defines a
	logical (command set) standard to which disk devices must adhere.
	This standard is called the Common Command Set (CCS) and was developed
	more or less in parallel with ANSI SCSI-1. SCSI-2 includes the
	(revised) CCS as part of the standard itself. The commands are
	dependent on the type of device at hand.  It does not make much sense
	of course to define a Write command for a scanner.</para>
	  
      <para>The SCSI bus is a parallel bus, which comes in a number of
	variants.  The oldest and most used is an 8 bit wide bus, with
	single-ended signals, carried on 50 wires.  (If you do not know what
	single-ended means, do not worry, that is what this document is all
	about.)  Modern designs also use 16 bit wide buses, with differential
	signals.  This allows transfer speeds of 20Mbytes/second, on cables
	lengths of up to 25 meters.  SCSI-2 allows a maximum bus width of 32
	bits, using an additional cable. Quickly emerging are Ultra SCSI (also
	called Fast-20) and Ultra2 (also called Fast-40).  Fast-20 is 20
	million transfers per second (20 Mbytes/sec on a 8 bit bus), Fast-40
	is 40 million transfers per second (40 Mbytes/sec on a 8 bit bus).
	Most hard drives sold today are single-ended Ultra SCSI (8 or 16
	bits).</para>
	  
      <para>Of course the SCSI bus not only has data lines, but also a number
	of control signals.  A very elaborate protocol is part of the standard
	to allow multiple devices to share the bus in an efficient manner.  In
	SCSI-2, the data is always checked using a separate parity line.  In
	pre-SCSI-2 designs parity was optional.</para>
	  
      <para>In SCSI-3 even faster bus types are introduced, along with a
	serial SCSI busses that reduces the cabling overhead and allows a
	higher maximum bus length.  You might see names like SSA and
	Fiberchannel in this context.  None of the serial buses are currently
	in widespread use (especially not in the typical FreeBSD environment).
	For this reason the serial bus types are not discussed any
	further.</para>
	  
      <para>As you could have guessed from the description above, SCSI devices
	are intelligent.  They have to be to adhere to the SCSI standard
	(which is over 2 inches thick BTW).  So, for a hard disk drive for
	instance you do not specify a head/cylinder/sector to address a
	particular block, but simply the number of the block you want.
	Elaborate caching schemes, automatic bad block replacement etc are all
	made possible by this 'intelligent device' approach.</para>
	  
      <para>On a SCSI bus, each possible pair of devices can communicate.
	Whether their function allows this is another matter, but the standard
	does not restrict it.  To avoid signal contention, the 2 devices have
	to arbitrate for the bus before using it.</para>
	  
      <para>The philosophy of SCSI is to have a standard that allows
	older-standard devices to work with newer-standard ones.  So, an old
	SCSI-1 device should normally work on a SCSI-2 bus.  I say Normally,
	because it is not absolutely sure that the implementation of an old
	device follows the (old) standard closely enough to be acceptable on a
	new bus.  Modern devices are usually more well-behaved, because the
	standardization has become more strict and is better adhered to by the
	device manufacturers.</para>
	  
      <para>Generally speaking, the chances of getting a working set of
	devices on a single bus is better when all the devices are SCSI-2 or
	newer.  This implies that you do not have to dump all your old stuff
	when you get that shiny 2GB disk: I own a system on which a pre-SCSI-1
	disk, a SCSI-2 QIC tape unit, a SCSI-1 helical scan tape unit and 2
	SCSI-1 disks work together quite happily.  From a performance
	standpoint you might want to separate your older and newer (=faster)
	devices however.</para>
      
      <sect3>
	<title>Components of SCSI</title>

	<para>As said before, SCSI devices are smart.  The idea is to put the
	  knowledge about intimate hardware details onto the SCSI device
	  itself.  In this way, the host system does not have to worry about
	  things like how many heads are hard disks has, or how many tracks
	  there are on a specific tape device.  If you are curious, the
	  standard specifies commands with which you can query your devices on
	  their hardware particulars.  FreeBSD uses this capability during
	  boot to check out what devices are connected and whether they need
	  any special treatment.</para>
	    
	<para>The advantage of intelligent devices is obvious: the device
	  drivers on the host can be made in a much more generic fashion,
	  there is no longer a need to change (and qualify!) drivers for every
	  odd new device that is introduced.</para>
	    
	<para>For cabling and connectors there is a golden rule: get good
	  stuff.  With bus speeds going up all the time you will save yourself
	  a lot of grief by using good material.</para>
	    
	<para>So, gold plated connectors, shielded cabling, sturdy connector
	  hoods with strain reliefs etc are the way to go. Second golden rule:
	  do no use cables longer than necessary.  I once spent 3 days hunting
	  down a problem with a flaky machine only to discover that shortening
	  the SCSI bus by 1 meter solved the problem.  And the original bus
	  length was well within the SCSI specification.</para>
      </sect3>
	  
      <sect3>
	<title>SCSI bus types</title>

	<para>From an electrical point of view, there are two incompatible bus
	  types: single-ended and differential.  This means that there are two
	  different main groups of SCSI devices and controllers, which cannot
	  be mixed on the same bus.  It is possible however to use special
	  converter hardware to transform a single-ended bus into a
	  differential one (and vice versa).  The differences between the bus
	  types are explained in the next sections.</para>
	    
	<para>In lots of SCSI related documentation there is a sort of jargon
	  in use to abbreviate the different bus types.  A small list:</para>
	    
	<itemizedlist>
	  <listitem>
	    <para>FWD:	Fast Wide Differential</para>
	  </listitem>
	  
	  <listitem>
	    <para>FND:	Fast Narrow Differential</para>
	  </listitem>
	  
	  <listitem>
	    <para>SE:	Single Ended</para>
	  </listitem>
	  
	  <listitem>
	    <para>FN:	Fast Narrow</para>
	  </listitem>
	  
	  <listitem>
	    <para>etc.</para>
	  </listitem>
	</itemizedlist>


	<para>With a minor amount of imagination one can usually imagine what
	  is meant.</para>
	    
	<para>Wide is a bit ambiguous, it can indicate 16 or 32 bit buses. As
	  far as I know, the 32 bit variant is not (yet) in use, so wide
	  normally means 16 bit.</para>
	    
	<para>Fast means that the timing on the bus is somewhat different, so
	  that on a narrow (8 bit) bus 10 Mbytes/sec are possible instead of 5
	  Mbytes/sec for 'slow' SCSI.  As discussed before, bus speeds of 20
	  and 40 million transfers/second are also emerging  (Fast-20 == Ultra
	  SCSI and Fast-40 == Ultra2 SCSI).</para>

	<note>
	  <para>The data lines &gt; 8 are only used for data transfers and
	    device addressing.  The transfers of commands and status messages
	    etc are only performed on the lowest 8 data lines. The standard
	    allows narrow devices to operate on a wide bus. The usable bus
	    width is negotiated between the devices.  You have to watch your
	    device addressing closely when mixing wide and narrow.</para>
	</note>

	<sect4>
	  <title>Single ended buses</title>
	  
	  <para>A single-ended SCSI bus uses signals that are either 5 Volts
	    or 0 Volts (indeed, TTL levels) and are relative to a COMMON
	    ground reference.  A singled ended 8 bit SCSI bus has
	    approximately 25 ground lines, who are all tied to a single `rail'
	    on all devices.  A standard single ended bus has a maximum length
	    of 6 meters.  If the same bus is used with fast-SCSI devices, the
	    maximum length allowed drops to 3 meters.  Fast-SCSI means that
	    instead of 5Mbytes/sec the bus allows 10Mbytes/sec
	    transfers.</para>
	      
	  <para>Fast-20 (Ultra SCSI) and Fast-40 allow for 20 and 40 million
	    transfers/second respectively.  So, F20 is 20 Mbytes/second on a 8
	    bit bus, 40 Mbytes/second on a 16 bit bus etc.  For F20 the max
	    bus length is 1.5 meters, for F40 it becomes 0.75 meters.  Be
	    aware that F20 is pushing  the limits quite a bit, so you will
	    quickly find out if your SCSI bus is electrically sound.</para>

	  <note>
	    <para>If some devices on your bus use 'fast' to communicate your
	      bus must adhere to the length restrictions for fast
	      buses!</para>
	  </note>
	  
	  <para>It is obvious that with the newer fast-SCSI devices the bus
	    length can become a real bottleneck.  This is why the differential
	    SCSI bus was introduced in the SCSI-2 standard.</para>
	      
	  <para>For connector pinning and connector types please refer to the
	    SCSI-2 standard (see <link linkend="scsi-further-reading">Further
	      reading</link>) itself, connectors etc are listed there in
	    painstaking detail.</para>
	  
	  <para>Beware of devices using non-standard cabling.  For instance
	    Apple uses a 25pin D-type connecter (like the one on serial ports
	    and parallel printers).  Considering that the official SCSI bus
	    needs 50 pins you can imagine the use of this connector needs some
	    'creative cabling'.  The reduction of the number of ground wires
	    they used is a bad idea, you better stick to 50 pins cabling  in
	    accordance with the SCSI standard.  For Fast-20 and 40 do not even
	    think about buses like this.</para>
	</sect4>

	<sect4>
	  <title>Differential buses</title>
	  
	  <para>A differential SCSI bus has a maximum length of 25 meters.
	    Quite a difference from the 3 meters for a single-ended fast-SCSI
	    bus.  The idea behind differential signals is that each bus signal
	    has its own return wire.  So, each signal is carried on a
	    (preferably twisted) pair of wires.  The voltage difference
	    between these two wires determines whether the signal is asserted
	    or de-asserted.  To a certain extent the voltage difference
	    between ground and the signal wire pair is not relevant (do not
	    try 10 kVolts though).</para>
	      
	  <para>It is beyond the scope of this document to explain why this
	    differential idea is so much better.  Just accept that
	    electrically seen the use of differential signals gives a much
	    better noise margin.  You will normally find differential buses in
	    use for inter-cabinet connections.  Because of the lower cost
	    single ended is mostly used for shorter buses like inside
	    cabinets.</para>
	      
	  <para>There is nothing that stops you from using differential stuff
	    with FreeBSD, as long as you use a controller that has device
	    driver support in FreeBSD.  As an example, Adaptec marketed the
	    AHA1740 as a single ended board, whereas the AHA1744 was
	    differential.  The software interface to the host is identical for
	    both.</para>
	</sect4>

	<sect4>
	  <title>Terminators</title>
	  
	  <para>Terminators in SCSI terminology are resistor networks that are
	    used to get a correct impedance matching.  Impedance matching is
	    important to get clean signals on the bus, without reflections or
	    ringing.  If you once made a long distance telephone call on a bad
	    line you probably know what reflections are.  With 20Mbytes/sec
	    traveling over your SCSI bus, you do not want signals echoing
	    back.</para>
	      
	  <para>Terminators come in various incarnations, with more or less
	    sophisticated designs.  Of course, there are internal and external
	    variants.  Many SCSI devices come with a number of sockets in
	    which a number of resistor networks can (must be!) installed.  If
	    you remove terminators from a device, carefully store them.  You
	    will need them when you ever decide to reconfigure your SCSI bus.
	    There is enough variation in even these simple tiny things to make
	    finding the exact replacement a frustrating business.  There are
	    also SCSI devices that have a single jumper to enable or disable a
	    built-in terminator. There are special terminators you can stick
	    onto a flat cable bus.  Others look like external connectors, or a
	    connector hood without a cable.  So, lots of choice as you can
	    see.</para>
	      
	  <para>There is much debate going on if and when you should switch
	    from simple resistor (passive) terminators to active terminators.
	    Active terminators contain slightly more elaborate circuit to give
	    cleaner bus signals.  The general consensus seems to be that the
	    usefulness of active termination increases when you have long
	    buses and/or fast devices.  If you ever have problems with your
	    SCSI buses you might consider trying an active terminator.  Try to
	    borrow one first, they reputedly are quite expensive.</para>
	      
	  <para>Please keep in mind that terminators for differential and
	    single-ended buses are not identical.  You should <emphasis>not
	      mix</emphasis> the two variants.</para>
	      
	  <para>OK, and now where should you install your terminators? This is
	    by far the most misunderstood part of SCSI.  And it is by far the
	    simplest.  The rule is: <emphasis>every single line on the SCSI
	      bus has 2 (two) terminators, one at each end of the
	      bus.</emphasis> So, two and not one or three or whatever.  Do
	    yourself a favor and stick to this rule.  It will save you endless
	    grief, because wrong termination has the potential to introduce
	    highly mysterious bugs.  (Note the <quote>potential</quote> here;
	    the nastiest part is that it may or may not work.)</para>
	      
	  <para>A common pitfall is to have an internal (flat) cable in a
	    machine and also an external cable attached to the controller. It
	    seems almost everybody forgets to remove the terminators from the
	    controller.  The terminator must now be on the last external
	    device, and not on the controller! In general, every
	    reconfiguration of a SCSI bus must pay attention to this.</para>

	  <note>
	    <para>Termination is to be done on a per-line basis.  This means
	      if you have both narrow and wide buses connected to the same
	      host adapter, you need to enable termination on the higher 8
	      bits of the bus on the adapter (as well as the last devices on
	      each bus, of course).</para>
	      </note>
	      
	  <para>What I did myself is remove all terminators from my SCSI
	    devices and controllers.  I own a couple of external terminators,
	    for both the Centronics-type external cabling and for the internal
	    flat cable connectors.  This makes reconfiguration much
	    easier.</para>
	      
	  <para>On modern devices, sometimes integrated terminators are used.
	    These things are special purpose integrated circuits that can be
	    dis/en-abled with a control pin.  It is not necessary to
	    physically remove them from a device.  You may find them on newer
	    host adapters, sometimes they are software configurable, using
	    some sort of setup tool.  Some will even auto-detect the cables
	    attached to the connectors and automatically set up the
	    termination as necessary.  At any rate, consult your
	    documentation!</para>
	</sect4>

	<sect4>
	  <title>Terminator power</title>
	  
	  <para>The terminators discussed in the previous chapter need power
	    to operate properly.  On the SCSI bus, a line is dedicated to this
	    purpose.  So, simple huh?</para>
	      
	  <para>Not so.  Each device can provide its own terminator power to
	    the terminator sockets it has on-device.  But if you have external
	    terminators, or when the device supplying the terminator power to
	    the SCSI bus line is switched off you are in trouble.</para>
	      
	  <para>The idea is that initiators (these are devices that initiate
	    actions on the bus, a discussion follows) must supply terminator
	    power.  All SCSI devices are allowed (but not required) to supply
	    terminator power.</para>
	      
	  <para>To allow for un-powered devices on a bus, the terminator power
	    must be supplied to the bus via a diode.  This prevents the
	    backflow of current to un-powered devices.</para>
	      
	  <para>To prevent all kinds of nastiness, the terminator power is
	    usually fused.  As you can imagine, fuses might blow.  This can,
	    but does not have to, lead to a non functional bus.  If multiple
	    devices supply terminator power, a single blown fuse will not put
	    you out of business.  A single supplier with a blown fuse
	    certainly will.  Clever external terminators sometimes have a  LED
	    indication that shows whether terminator power is present.</para>
	      
	  <para>In newer designs auto-restoring fuses that 'reset' themselves
	    after some time are sometimes used.</para>
	</sect4>

	<sect4>
	  <title>Device addressing</title>
	  
	  <para>Because the SCSI bus is, ehh, a bus there must be a way to
	    distinguish or address the different devices connected to
	    it.</para>
	      
	  <para>This is done by means of the SCSI or target ID.  Each device
	    has a unique target ID.  You can select the ID to which a device
	    must respond using a set of jumpers, or a dip switch, or something
	    similar.  Some SCSI host adapters let you change the target ID
	    from the boot menu.  (Yet some others will not let you change the
	    ID from 7.)  Consult the documentation of your device for more
	    information.</para>
	  
	  <para>Beware of multiple devices configured to use the same ID.
	    Chaos normally reigns in this case.  A pitfall is that one of the
	    devices sharing the same ID sometimes even manages to answer to
	    I/O requests!</para>
	      
	  <para>For an 8 bit bus, a maximum of 8 targets is possible.  The
	    maximum is 8 because the selection is done bitwise using the 8
	    data lines on the bus.  For wide buses this increases to the
	    number of data lines (usually 16).</para>

	  <note>
	    <para>A narrow SCSI device can not communicate with a SCSI device
	      with a target ID larger than 7.  This means it is generally not
	      a good idea to move your SCSI host adapter's target ID to
	      something higher than 7 (or your CD-ROM will stop
	      working).</para>
	  </note>
	  
	  <para>The higher the SCSI target ID, the higher the priority the
	    devices has.  When it comes to arbitration between devices that
	    want to use the bus at the same time, the device that has the
	    highest SCSI ID will win.  This also means that the SCSI host
	    adapter usually uses target ID 7.  Note however that the lower 8
	    IDs have higher priorities than the higher 8 IDs on a wide-SCSI
	    bus.  Thus, the order of target IDs is: [7 6 .. 1 0 15 14 .. 9 8]
	    on a wide-SCSI system.  (If you you are wondering why the lower 8
	    have higher priority, read the previous paragraph for a
	    hint.)</para>
	      
	  <para>For a further subdivision, the standard allows for Logical
	    Units or LUNs for short.  A single target ID may have multiple
	    LUNs.  For example, a tape device including a tape changer may
	    have LUN 0 for the tape device itself, and LUN 1 for the tape
	    changer.  In this way, the host system can address each of the
	    functional units of the tape changer as desired.</para>
	</sect4>
	    
	<sect4>
	  <title>Bus layout</title>
	  
	  <para>SCSI buses are linear.  So, not shaped like Y-junctions, star
	    topologies, rings, cobwebs or whatever else people might want to
	    invent.  One of the most common mistakes is for people with
	    wide-SCSI host adapters to connect devices on all three connecters
	    (external connector, internal wide connector, internal narrow
	    connector).  Don't do that.  It may appear to work if you are
	    really lucky, but I can almost guarantee that your system will
	    stop functioning at the most unfortunate moment (this is also
	    known as <quote>Murphy's law</quote>).</para>
	      
	  <para>You might notice that the terminator issue discussed earlier
	    becomes rather hairy if your bus is not linear.  Also, if you have
	    more connectors than devices on your internal SCSI cable, make
	    sure you attach devices on connectors on both ends instead of
	    using the connectors in the middle and let one or both ends
	    dangle.  This will screw up the termination of the bus.</para>
	      
	  <para>The electrical characteristics, its noise margins and
	    ultimately the reliability of it all are tightly related to linear
	    bus rule.</para>
	  
	  <para><emphasis>Stick to the linear bus rule!</emphasis></para>
	</sect4>
      </sect3>
      
      <sect3>
	<title>Using SCSI with FreeBSD</title>

	<sect4>
	  <title>About translations, BIOSes and magic...</title>
	  
	  <para>As stated before, you should first make sure that you have a
	    electrically sound bus.</para>
	      
	  <para>When you want to use a SCSI disk on your PC as boot disk, you
	    must aware of some quirks related to PC BIOSes.  The PC BIOS in
	    its first incarnation used a low level physical interface to the
	    hard disk.  So, you had to tell the BIOS (using a setup tool or a
	    BIOS built-in setup) how your disk physically looked like.  This
	    involved stating number of heads, number of cylinders, number of
	    sectors per track, obscure things like precompensation and reduced
	    write current cylinder etc.</para>
	      
	  <para>One might be inclined to think that since SCSI disks are smart
	    you can forget about this.  Alas, the arcane setup issue is still
	    present today.  The system BIOS needs to know how to access your
	    SCSI disk with the head/cyl/sector method in order to load the
	    FreeBSD kernel during boot.</para>
	      
	  <para>The SCSI host adapter or SCSI controller you have put in your
	    AT/EISA/PCI/whatever bus to connect your disk therefore has its
	    own on-board BIOS.  During system startup, the SCSI BIOS takes
	    over the hard disk interface routines from the system BIOS.  To
	    fool the system BIOS, the system setup is normally set to No hard
	    disk present.  Obvious, isn't it?</para>
	      
	  <para>The SCSI BIOS itself presents to the system a so called
	    <emphasis>translated</emphasis> drive.  This means that a fake
	    drive table is constructed that allows the PC to boot the drive.
	    This translation is often (but not always) done using a pseudo
	    drive with 64 heads and 32 sectors per track.  By varying the
	    number of cylinders, the SCSI BIOS adapts to the actual drive
	    size.  It is useful to note that 32 * 64 / 2 = the size of your
	    drive in megabytes.  The division by 2 is to get from disk blocks
	    that are normally 512 bytes in size to Kbytes.</para>
	      
	  <para>Right.  All is well now?! No, it is not.  The system BIOS has
	    another quirk you might run into.  The number of cylinders of a
	    bootable hard disk cannot be greater than 1024.  Using the
	    translation above, this is a show-stopper for disks greater than 1
	    GB.  With disk capacities going up all the time this is causing
	    problems.</para>
	      
	  <para>Fortunately, the solution is simple: just use another
	    translation, e.g. with 128 heads instead of 32.  In most cases new
	    SCSI BIOS versions are available to upgrade older SCSI host
	    adapters.  Some newer adapters have an option, in the form of a
	    jumper or software setup selection, to switch the translation the
	    SCSI BIOS uses.</para>
	      
	  <para>It is very important that <emphasis>all</emphasis> operating
	    systems on the disk use the <emphasis>same translation</emphasis>
	    to get the right idea about where to find the relevant partitions.
	    So, when installing FreeBSD you must answer any questions about
	    heads/cylinders etc using the translated values your host adapter
	    uses.</para>
	      
	  <para>Failing to observe the translation issue might lead to
	    un-bootable systems or operating systems overwriting each others
	    partitions.  Using fdisk you should be able to see all
	    partitions.</para>
	      
	  <para>You might have heard some talk of <quote>lying</quote> devices?
	    Older FreeBSD kernels used to report the geometry of SCSI disks
	    when booting.  An example from one of my systems:</para>
	      
	  <screen>aha0 targ 0 lun 0: &lt;MICROP 1588-15MB1057404HSP4&gt;
sd0: 636MB (1303250 total sec), 1632 cyl, 15 head, 53 sec, bytes/sec 512</screen>

	  <para>Newer kernels usually do not report this information.
	    e.g.</para>

	  <screen>(bt0:0:0): "SEAGATE ST41651 7574" type 0 fixed SCSI 2
sd0(bt0:0:0): Direct-Access 1350MB (2766300 512 byte sectors)</screen>
		
	  <para>Why has this changed?</para>
	      
	  <para>This info is retrieved from the SCSI disk itself.  Newer disks
	    often use a technique called zone bit recording.  The idea is that
	    on the outer cylinders of the drive there is more space so more
	    sectors per track can be put on them.  This results in disks that
	    have more tracks on outer cylinders than on the inner cylinders
	    and, last but not least, have more capacity.  You can imagine that
	    the value reported by the drive when inquiring about the geometry
	    now becomes suspect at best, and nearly always misleading.  When
	    asked for a geometry , it is nearly always better to supply the
	    geometry used by the BIOS, or <emphasis>if the BIOS is never going
	      to know about this disk</emphasis>, (e.g. it is not a booting
	    disk) to supply a fictitious geometry that is convenient.</para>
	</sect4>

	<sect4>
	  <title>SCSI subsystem design</title>
	  
	  <para>FreeBSD uses a layered SCSI subsystem.  For each different
	    controller card a device driver is written.  This driver knows all
	    the intimate details about the hardware it controls.  The driver
	    has a interface to the upper layers of the SCSI subsystem through
	    which it receives its commands and reports back any status.</para>
	      
	  <para>On top of the card drivers there are a number of more generic
	    drivers for a class of devices.  More specific: a driver for tape
	    devices (abbreviation: st), magnetic disks (sd), CD-ROMs (cd) etc.
	    In case you are wondering where you can find this stuff, it all
	    lives in <filename>/sys/scsi</filename>.  See the man pages in
	    section 4 for more details.</para>
	      
	  <para>The multi level design allows a decoupling of low-level bit
	    banging and more high level stuff.  Adding support for another
	    piece of hardware is a much more manageable problem.</para>
	</sect4>

	<sect4>
	  <title>Kernel configuration</title>
	  
	  <para>Dependent on your hardware, the kernel configuration file must
	    contain one or more lines describing your host adapter(s).  This
	    includes I/O addresses, interrupts etc. Consult the man page for
	    your adapter driver to get more info. Apart from that, check out
	    <filename>/sys/i386/conf/LINT</filename> for an overview of a
	    kernel config file.  <filename>LINT</filename> contains every
	    possible option you can dream of.  It does
	    <emphasis>not</emphasis> imply <filename>LINT</filename> will
	    actually get you to a working kernel at all.</para>
	  
	  <para>Although it is probably stating the obvious: the kernel config
	    file should reflect your actual hardware setup.  So, interrupts,
	    I/O addresses etc must match the kernel config file.  During
	    system boot messages will be displayed to indicate whether the
	    configured hardware was actually found.</para>

	  <note>
	    <para>Note that most of the EISA/PCI drivers (namely
	      <devicename>ahb</devicename>, <devicename>ahc</devicename>,
	      <devicename>ncr</devicename> and <devicename>amd</devicename>
	      will automatically obtain the correct parameters from the host
	      adapters themselves at boot time; thus, you just need to write,
	      for instance, <literal>controller ahc0</literal>.</para>
	  </note>
	  
	  <para>An example loosely based on the FreeBSD 2.2.5-Release kernel
	    config  file <filename>LINT</filename> with some added comments
	    (between []):</para>
	      
	  <programlisting>
# SCSI host adapters: `aha', `ahb', `aic', `bt', `nca'
#
# aha: Adaptec 154x
# ahb: Adaptec 174x
# ahc: Adaptec 274x/284x/294x
# aic: Adaptec 152x and sound cards using the Adaptec AIC-6360 (slow!)
# amd: AMD 53c974 based SCSI cards (e.g., Tekram DC-390 and 390T)
# bt: Most Buslogic controllers
# nca: ProAudioSpectrum cards using the NCR 5380 or Trantor T130
# ncr: NCR/Symbios 53c810/815/825/875 etc based SCSI cards
# uha: UltraStore 14F and 34F
# sea: Seagate ST01/02 8 bit controller (slow!)
# wds: Western Digital WD7000 controller (no scatter/gather!).
#

[For an Adaptec AHA274x/284x/294x/394x etc controller]
controller	ahc0

[For an NCR/Symbios 53c875 based controller]
controller	ncr0

[For an Ultrastor adapter]
controller	uha0	at isa? port "IO_UHA0" bio irq ? drq 5 vector uhaintr

# Map SCSI buses to specific SCSI adapters
controller	scbus0	at ahc0
controller	scbus2 at ncr0
controller	scbus1  at uha0

# The actual SCSI devices
disk sd0 at scbus0 target 0 unit 0	[SCSI disk 0 is at scbus 0, LUN 0]
disk sd1 at scbus0 target 1             [implicit LUN 0 if omitted]
disk sd2 at scbus1 target 3             [SCSI disk on the uha0]
disk sd3 at scbus2 target 4             [SCSI disk on the ncr0]
tape st1 at scbus0 target 6             [SCSI tape at target 6]
device cd0 at scbus?                    [the first ever CD-ROM found, no wiring]</programlisting>
	      
	  <para>The example above tells the kernel to look for a ahc (Adaptec
	    274x) controller, then for an NCR/Symbios board, and so on.  The
	    lines following the controller specifications  tell the kernel to
	    configure specific devices but <emphasis>only</emphasis> attach
	    them when they match the target ID and LUN specified on the
	    corresponding bus.</para>
	      
	  <para>Wired down devices get <quote>first shot</quote> at the unit
	    numbers so the first non <quote>wired down</quote> device, is
	    allocated the unit number  one greater than the highest
	    <quote>wired down</quote> unit number for that kind of device.  So,
	    if you had a SCSI tape at target ID 2 it would be configured as
	    st2, as the tape at target ID 6 is wired down to unit number
	    1.</para>

	  <note>
	    <para>Wired down devices need not be found to get their unit
	      number.  The unit number for a wired down device is reserved for
	      that device, even if it is turned off at boot time.  This allows
	      the device to be turned on and brought on-line at a later time,
	      without rebooting.  Notice that a device's unit number has
	      <emphasis>no</emphasis> relationship with its target ID on  the
	      SCSI bus.</para>
	  </note>
	  
	  <para>Below is another example of a kernel config file as used by
	    FreeBSD version &lt; 2.0.5.  The difference with the first example
	    is that devices are not <quote>wired down</quote>.  <quote>Wired
	    down</quote> means that you specify which SCSI target belongs to
	    which device.</para>
	      
	  <para>A kernel built to the config file below will attach  the first
	    SCSI disk it finds to sd0, the second disk to sd1 etc. If you ever
	    removed or added a disk, all other devices of the same type (disk
	    in this case) would 'move around'.  This implies you have to
	    change <filename>/etc/fstab</filename> each time.</para>
	      
	  <para>Although the old style still works, you  are
	    <emphasis>strongly</emphasis> recommended to use this new feature.
	    It will save you a lot of grief whenever you shift your hardware
	    around on the SCSI buses.  So, when you re-use your old trusty
	    config file after upgrading from a pre-FreeBSD2.0.5.R system check
	    this out.</para>
	  
	  <programlisting>
[driver for Adaptec 174x]
controller      ahb0 at isa? bio irq 11 vector ahbintr

[for Adaptec 154x]
controller      aha0    at isa? port "IO_AHA0" bio irq 11 drq 5 vector ahaintr

[for Seagate ST01/02]
controller      sea0    at isa? bio irq 5 iomem 0xc8000 iosiz 0x2000 vector seaintr

controller      scbus0

device          sd0     [support for 4 SCSI harddisks, sd0 up sd3]
device          st0	[support for 2 SCSI tapes]

[for the CD-ROM]
device          cd0     #Only need one of these, the code dynamically grows</programlisting>
	      
	  <para>Both examples support SCSI disks.  If during boot more devices
	    of a specific type (e.g. sd disks) are found than are configured
	    in the booting kernel, the system will simply allocate more
	    devices, incrementing the unit number starting at the last number
	    <quote>wired down</quote>.  If there are no <quote>wired
	    down</quote> devices then counting starts at unit 0.</para>
	      
	  <para>Use <command>man 4 scsi</command> to check for the latest info
	    on the SCSI subsystem.  For more detailed info on host adapter
	    drivers use eg <command>man 4 ahc</command> for info on the
	    Adaptec 294x driver.</para>
	</sect4>

	<sect4>
	  <title>Tuning your SCSI kernel setup</title>
	  
	  <para>Experience has shown that some devices are slow to respond to
	    INQUIRY commands after a SCSI bus reset (which happens at boot
	    time).  An INQUIRY command is sent by the kernel on boot to see
	    what kind of device (disk, tape, CD-ROM etc) is connected to a
	    specific target ID.  This process is called device probing by the
	    way.</para>
	      
	  <para>To work around the 'slow response' problem, FreeBSD allows a
	    tunable delay time before the SCSI devices are probed following a
	    SCSI bus reset.  You can set this delay time in your kernel
	    configuration file using a line like:</para>
	      
	  <programlisting>
options         SCSI_DELAY=15         #Be pessimistic about Joe SCSI device</programlisting>

	  <para>This line sets the delay time to 15 seconds.  On my own system
	    I had to use 3 seconds minimum to get my trusty old CD-ROM drive
	    to be recognized.  Start with a high value (say 30 seconds or so)
	    when you have problems  with device recognition.  If this helps,
	    tune it back until it just stays working.</para>
	</sect4>

	<sect4 id="scsi-rogue-devices">
	  <title>Rogue SCSI devices</title>
	      
	  <para>Although the SCSI standard tries to be complete and concise,
	    it is a complex standard and implementing things correctly is no
	    easy task.  Some vendors do a better job then others.</para>
	      
	  <para>This is exactly where the <quote>rogue</quote> devices come
	    into view. Rogues are devices that are recognized by the FreeBSD
	    kernel as behaving slightly (...) non-standard.  Rogue devices are
	    reported by the kernel when booting.  An example for two of my
	    cartridge tape units:</para>
	  
	  <screen>Feb 25 21:03:34 yedi /kernel: ahb0 targ 5 lun 0: &lt;TANDBERG TDC 3600       -06:&gt;
Feb 25 21:03:34 yedi /kernel: st0: Tandberg tdc3600 is a known rogue

Mar 29 21:16:37 yedi /kernel: aha0 targ 5 lun 0: &lt;ARCHIVE VIPER 150  21247-005&gt;
Mar 29 21:16:37 yedi /kernel: st1: Archive  Viper 150 is a known rogue </screen>
	      
	  <para>For instance, there are devices that respond to all LUNs on a
	    certain target ID, even if they are actually only one device.  It
	    is easy to see that the kernel might be fooled into believing that
	    there are 8 LUNs at that particular target ID. The confusion this
	    causes is left as an exercise to the reader.</para>
	      
	  <para>The SCSI subsystem of FreeBSD recognizes devices with bad
	    habits by looking at the INQUIRY response they send when probed.
	    Because the INQUIRY response also includes the version number of
	    the device  firmware, it is even possible that for different
	    firmware versions different workarounds are used. See e.g.
	    <filename>/sys/scsi/st.c</filename> and
	    <filename>/sys/scsi/scsiconf.c</filename> for more info on how
	    this is done.</para>
	      
	  <para>This scheme works fine, but keep in mind that it of course
	    only works for devices that are known to be weird.  If you are the
	    first to connect your bogus Mumbletech SCSI CD-ROM you might be
	    the one that has to define which workaround is needed.</para>
	      
	  <para>After you got your Mumbletech working, please send the
	    required workaround to the FreeBSD development team for inclusion
	    in the next release of FreeBSD.  Other Mumbletech owners will be
	    grateful  to you.</para>
	</sect4>

	<sect4>
	  <title>Multiple LUN devices</title>
	  
	  <para>In some cases you come across devices that use multiple
	    logical units (LUNs) on a single SCSI ID.  In most cases FreeBSD
	    only probes devices for LUN 0.  An example are so called bridge
	    boards that connect 2 non-SCSI harddisks to a SCSI bus (e.g. an
	    Emulex MD21 found in old Sun systems).</para>
	      
	  <para>This means that any devices with LUNs != 0 are not normally
	    found during device probe on system boot.  To work around this
	    problem you must add an appropriate entry in /sys/scsi/scsiconf.c
	    and rebuild your kernel.</para>
	      
	  <para>Look for a struct that is initialized like below:</para>

	  <programlisting>
{
        T_DIRECT, T_FIXED, "MAXTOR", "XT-4170S", "B5A",
        "mx1", SC_ONE_LU
}</programlisting>
	      
	  <para>For you Mumbletech BRIDGE2000 that has more than one LUN, acts
	    as a SCSI disk and has firmware revision 123 you would add
	    something like:</para>
	      
	  <programlisting>
{
        T_DIRECT, T_FIXED, "MUMBLETECH", "BRIDGE2000", "123",
        "sd", SC_MORE_LUS
}</programlisting>
	      
	  <para>The kernel on boot scans the inquiry data it receives against
	    the table and acts accordingly.  See the source for more
	    info.</para>
	</sect4>

	<sect4>
	  <title>Tagged command queueing</title>
	  
	  <para>Modern SCSI devices, particularly magnetic disks,
	    support what is called tagged command queuing (TCQ).</para>
	  
	  <para>In a nutshell, TCQ allows the device to have multiple I/O
	    requests outstanding at the same time.  Because the device is
	    intelligent, it can optimise its operations (like head
	    positioning) based on its own request queue.  On  SCSI devices
	    like RAID (Redundant Array of Independent Disks) arrays the TCQ
	    function is indispensable to take advantage of the device's
	    inherent parallelism.</para>
	      
	  <para>Each I/O request is uniquely identified by a <quote>tag</quote>
	    (hence the name tagged command queuing) and this tag is used by
	    FreeBSD to see which I/O in the device drivers queue is reported
	    as complete by the device.</para>
	      
	  <para>It should be noted however that TCQ requires device driver
	    support and that some devices implemented it <quote>not quite
	    right</quote> in their firmware.  This problem bit me once, and it
	    leads to highly mysterious problems.  In such cases, try to
	    disable TCQ.</para>
	</sect4>

	<sect4>
	  <title>Busmaster host adapters</title>
	  
	  <para>Most, but not all, SCSI host adapters are bus mastering
	    controllers.  This means that they can do I/O on their own without
	    putting load onto the host CPU for data movement.</para>
	      
	  <para>This is of course an advantage for a multitasking operating
	    system like FreeBSD.  It must be noted however that there might be
	    some rough edges.</para>
	      
	  <para>For instance an Adaptec 1542 controller can be set to use
	    different transfer speeds on the host bus (ISA or AT in this
	    case).  The controller is settable to different rates because not
	    all motherboards can handle the higher speeds.  Problems like
	    hangups, bad data etc might be the result of using a higher data
	    transfer rate then your motherboard can stomach.</para>
	      
	  <para>The solution is of course obvious: switch to a lower data
	    transfer rate and try if that works better.</para>
	      
	  <para>In the case of a Adaptec 1542, there is an option that can be
	    put into the kernel config file to allow dynamic determination of
	    the right, read: fastest feasible, transfer rate.  This option is
	    disabled by default:</para>
	      
	  <programlisting>
options        "TUNE_1542"             #dynamic tune of bus DMA speed</programlisting>
	      
	  <para>Check the man pages for the host adapter that you use.  Or
	    better still, use the ultimate documentation (read: driver
	    source).</para>
	</sect4>
      </sect3>
      
      <sect3>
	<title>Tracking down problems</title>

	<para>The following list is an attempt to give a guideline for the
	  most common SCSI problems and their solutions.  It is by no means
	  complete.</para>
	    
	<itemizedlist>
	  <listitem>
	    <para>Check for loose connectors and cables.</para>
	  </listitem>
	  
	  <listitem>
	    <para>Check and double check the location and number of your
	      terminators.</para>
	  </listitem>
	  
	  <listitem>
	    <para>Check if your bus has at least one supplier of terminator
	      power (especially with external terminators.</para>
	  </listitem>
	  
	  <listitem>
	    <para>Check if no double target IDs are used.</para>
	  </listitem>
	  
	  <listitem>
	    <para>Check if all devices to be used are powered up.</para>
	  </listitem>
	  
	  <listitem>
	    <para>Make a minimal bus config with as little devices as
	      possible.</para>
	  </listitem>
	  
	  <listitem>
	    <para>If possible, configure your host adapter to use slow bus
	      speeds.</para>
	  </listitem>
	  
	  <listitem>
	    <para>Disable tagged command queuing to make things as simple as
	      possible (for a NCR hostadapter based system see man
	      ncrcontrol)</para>
	  </listitem>
	  
	  <listitem>
	    <para>If you can compile a kernel, make one with the
	      <literal>SCSIDEBUG</literal> option, and try accessing the
	      device with debugging turned on for that device.  If your device
	      does not even probe at startup, you may have to define the
	      address of the device that is failing, and the desired debug
	      level in <filename>/sys/scsi/scsidebug.h</filename>. If it
	      probes but just does not work, you can use the
		    &man.scsi.8; command to dynamically set a debug level to
	      it in a running kernel (if <literal>SCSIDEBUG</literal> is
	      defined).  This will give you <emphasis>copious</emphasis>
	      debugging output with which to confuse the gurus. See
	      <command>man 4 scsi</command> for more exact information.  Also
	      look at <command>man 8 scsi</command>.</para>
	  </listitem>
	</itemizedlist>
      </sect3>
      
      <sect3 id="scsi-further-reading">
	<title>Further reading</title>

	<para>If you intend to do some serious SCSI hacking, you might want to
	  have the official standard at hand:</para>
	    
	<para>Approved American National Standards can be purchased from
	  ANSI at
	  
	  <address>
	    <otheraddr>13th Floor</otheraddr>
	    <street>11 West 42nd Street</street>
	    <city>New York</city>
	    <state>NY</state> <postcode>10036</postcode>
	    Sales Dept: <phone>(212) 642-4900</phone>
	  </address>
	</para>

	<para>You can also buy many ANSI
	  standards and most committee draft documents from Global
	  Engineering Documents,

	  <address>
	    <street>15 Inverness Way East</street>
	    <city>Englewood</city>
	    <state>CO</state>, <postcode>80112-5704</postcode>
	    Phone: <phone>(800) 854-7179</phone>
	    Outside USA and Canada: <phone>(303) 792-2181</phone>
	    Fax: <fax>(303) 792- 2192</fax>
	  </address>
	</para>

	<para>Many X3T10 draft documents are available electronically on the
	  SCSI BBS (719-574-0424) and on the <hostid
	    role="fqdn">ncrinfo.ncr.com</hostid> anonymous ftp site.</para>
	    
	<para>Latest X3T10 committee documents are:</para>

	<itemizedlist>
	  <listitem>
	    <para>AT Attachment (ATA or IDE) [X3.221-1994]
	      (<emphasis>Approved</emphasis>)</para>
	  </listitem>
	  
	  <listitem>
	    <para>ATA Extensions (ATA-2) [X3T10/948D Rev 2i]</para>
	  </listitem>
	  
	  <listitem>
	    <para>Enhanced Small Device Interface (ESDI)
	      [X3.170-1990/X3.170a-1991]
	      (<emphasis>Approved</emphasis>)</para>
	  </listitem>
	  
	  <listitem>
	    <para>Small Computer System Interface &mdash; 2 (SCSI-2)
	      [X3.131-1994] (<emphasis>Approved</emphasis>)</para>
	  </listitem>
	  
	  <listitem>
	    <para>SCSI-2 Common Access Method Transport and SCSI Interface
	      Module (CAM)  [X3T10/792D Rev 11]</para>
	  </listitem>
	</itemizedlist>

	<para>Other publications that might provide you with additional
	  information are:</para>

	<itemizedlist>
	  <listitem>
	    <para><quote>SCSI: Understanding the Small Computer System
	      Interface</quote>, written by NCR  Corporation.  Available from:
	      Prentice Hall, Englewood Cliffs, NJ, 07632 Phone: (201) 767-5937
	      ISBN 0-13-796855-8</para>
	  </listitem>
	  
	  <listitem>
	    <para><quote>Basics of SCSI</quote>, a SCSI tutorial written by
	      Ancot Corporation Contact Ancot for availability information at:
	      Phone: (415) 322-5322  Fax: (415) 322-0455</para>
	  </listitem>
	  
	  <listitem>
	    <para><quote>SCSI Interconnection Guide Book</quote>, an AMP
	      publication (dated 4/93, Catalog  65237) that lists the various
	      SCSI connectors and suggests cabling schemes.  Available from
	      AMP at (800) 522-6752 or (717) 564-0100</para>
	  </listitem>
	  
	  <listitem>
	    <para><quote>Fast Track to SCSI</quote>, A Product Guide written by
	      Fujitsu.  Available from: Prentice Hall, Englewood Cliffs, NJ,
	      07632 Phone: (201) 767-5937 ISBN 0-13-307000-X</para>
	  </listitem>
	  
	  <listitem>
	    <para><quote>The SCSI Bench Reference</quote>, <quote>The SCSI
	      Encyclopedia</quote>, and the <quote>SCSI Tutor</quote>, ENDL
	      Publications, 14426 Black Walnut Court, Saratoga CA, 95070
	      Phone: (408) 867-6642</para>
	  </listitem>
	  
	  <listitem>
	    <para><quote>Zadian SCSI Navigator</quote> (quick ref. book) and
	      <quote>Discover the Power of SCSI</quote>  (First book along with
	      a one-hour video and tutorial book), Zadian Software, Suite 214,
	      1210 S. Bascom Ave., San Jose, CA 92128, (408) 293-0800</para>
	  </listitem>
	</itemizedlist>

	<para>On Usenet the newsgroups <ulink
	    url="news:comp.periphs.scsi">comp.periphs.scsi</ulink> and <ulink
	    url="news:comp.periphs">comp.periphs</ulink> are noteworthy places
	  to look for more info.  You can also find the SCSI-Faq there, which
	  is posted periodically.</para>

	<para>Most major SCSI device and host adapter suppliers operate ftp
	  sites and/or BBS systems.  They may be valuable sources of
	  information about the devices you own.</para>
      </sect3>
    </sect2>
    
    <sect2 id="hw-storage-controllers">
      <title>* Disk/tape controllers</title>
      
      <sect3>
	<title>* SCSI</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>* IDE</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>* Floppy</title>

	<para></para>
      </sect3>
    </sect2>
    
    <sect2>
      <title>Hard drives</title>
      
      <sect3>
	<title>SCSI hard drives</title>

	<para><emphasis>Contributed by &a.asami;.  17 February
	    1998.</emphasis></para>
	    
	<para>As mentioned in the <link linkend="scsi">SCSI</link> section,
	  virtually all SCSI hard drives sold today are SCSI-2 compliant and
	  thus will work fine as long as you connect them to a supported SCSI
	  host adapter.  Most problems people encounter are either due to
	  badly designed cabling (cable too long, star topology, etc.),
	  insufficient termination, or defective parts. Please refer to the
	  <link linkend="scsi">SCSI</link> section first if your SCSI hard
	  drive is not working.  However, there are a couple of things you may
	  want to take into account before you purchase SCSI hard drives for
	  your system.</para>

	<sect4>
	  <title>Rotational speed</title>
	  
	  <para>Rotational speeds of SCSI drives sold today range from around
	    4,500RPM to 10,000RPM.  Most of them are either 5,400RPM or
	    7,200RPM.  Even though the 7,200RPM drives can generally transfer
	    data faster, they run considerably hotter than their 5,400RPM
	    counterparts.  A large fraction of today's disk drive malfunctions
	    are heat-related.  If you do not have very good cooling in your PC
	    case, you may want to stick with 5,400RPM or slower drives.</para>
	      
	  <para>Note that newer drives, with higher areal recording densities,
	    can deliver much more bits per rotation than older ones.  Today's
	    top-of-line 5,400RPM drives can sustain a throughput comparable to
	    7,200RPM drives of one or two model generations ago.  The number
	    to find on the spec sheet for bandwidth is <quote>internal data
	    (or transfer) rate</quote>.  It is usually in megabits/sec so
	    divide it by 8 and you'll get the rough approximation of how much
	    megabytes/sec you can get out of the drive.</para>
	      
	  <para>(If you are a speed maniac and want a 10,000RPM drive for your
	    cute little peecee, be my guest; however, those drives become
	    extremely hot.  Don't even think about it if you don't have a fan
	    blowing air <emphasis>directly at</emphasis> the drive or a
	    properly ventilated disk enclosure.)</para>
	      
	  <para>Obviously, the latest 10,000RPM drives and 7,200RPM drives can
	    deliver more data than the latest 5,400RPM drives, so if absolute
	    bandwidth is the necessity for your applications, you have little
	    choice but to get the faster drives.  Also, if you need low
	    latency, faster drives are better; not only do they usually have
	    lower average seek times, but also the rotational delay is one
	    place where slow-spinning drives can never beat a faster one.
	    (The average rotational latency is half the time it takes to
	    rotate the drive once; thus, it's 3 milliseconds for 10,000RPM
	    drives, 4.2ms for 7,200RPM drives and 5.6ms for 5,400RPM drives.)
	    Latency is seek time plus rotational delay. Make sure you
	    understand whether you need low latency or more accesses per
	    second, though; in the latter case (e.g., news servers), it may
	    not be optimal to purchase one big fast drive.  You can achieve
	    similar or even better results by using the ccd (concatenated
	    disk) driver to create a striped disk array out of multiple slower
	    drives for comparable overall cost.</para>
	      
	  <para>Make sure you have adequate air flow around the drive,
	    especially if you are going to use a fast-spinning drive.  You
	    generally need at least 1/2" (1.25cm) of spacing above and below a
	    drive.  Understand how the air flows through your PC case.  Most
	    cases have the power supply suck the air out of the back.  See
	    where the air flows in, and put the drive where it will have the
	    largest volume of cool air flowing around it. You may need to seal
	    some unwanted holes or add a new fan for effective cooling.</para>
	      
	  <para>Another consideration is noise.  Many 7,200 or faster drives
	    generate a high-pitched whine which is quite unpleasant to most
	    people.  That, plus the extra fans often required for cooling, may
	    make 7,200 or faster drives unsuitable for some office and home
	    environments.</para>
	</sect4>

	<sect4>
	  <title>Form factor</title>
	  
	  <para>Most SCSI drives sold today are of 3.5" form factor.  They
	    come in two different heights; 1.6" (<quote>half-height</quote>) or
	    1" (<quote>low-profile</quote>).  The half-height drive is the same
	    height as a CD-ROM drive.  However, don't forget the spacing rule
	    mentioned in the previous section.  If you have three standard
	    3.5" drive bays, you will not be able to put three half-height
	    drives in there (without frying them, that is).</para>
	</sect4>

	<sect4>
	  <title>Interface</title>
	  
	  <para>The majority of SCSI hard drives sold today are Ultra or
	    Ultra-wide SCSI.  The maximum bandwidth of Ultra SCSI is 20MB/sec,
	    and Ultra-wide SCSI is 40MB/sec.  There is no difference in max
	    cable length between Ultra and Ultra-wide; however, the more
	    devices you have on the same bus, the sooner you will start having
	    bus integrity problems.  Unless you have a well-designed disk
	    enclosure, it is not easy to make more than 5 or 6 Ultra SCSI
	    drives work on a single bus.</para>
	      
	  <para>On the other hand, if you need to connect many drives, going
	    for Fast-wide SCSI may not be a bad idea.  That will have the same
	    max bandwidth as Ultra (narrow) SCSI, while electronically it's
	    much easier to get it <quote>right</quote>.  My advice would be: if
	    you want to connect many disks, get wide SCSI drives; they usually
	    cost a little more but it may save you down the road.  (Besides,
	    if you can't afford the cost difference, you shouldn't be building
	    a disk array.)</para>
	      
	  <para>There are two variant of wide SCSI drives; 68-pin and 80-pin
	    SCA (Single Connector Attach).  The SCA drives don't have a
	    separate 4-pin power connector, and also read the SCSI ID settings
	    through the 80-pin connector.  If you are really serious about
	    building a large storage system, get SCA drives and a good SCA
	    enclosure (dual power supply with at least one extra fan).  They
	    are more electronically sound than 68-pin counterparts because
	    there is no <quote>stub</quote> of the SCSI bus inside the disk
	    canister as in arrays built from 68-pin drives.  They are easier
	    to install too (you just need to screw the drive in the canister,
	    instead of trying to squeeze in your fingers in a tight place to
	    hook up all the little cables (like the SCSI ID and disk activity
	    LED lines).</para>
	</sect4>
      </sect3>
      
      <sect3>
	<title>* IDE hard drives</title>

	<para></para>
      </sect3>
    </sect2>
    
    <sect2>
      <title>Tape drives</title>
      
      <para><emphasis>Contributed by &a.jmb;.  2 July
	  1996.</emphasis></para>
      
      <sect3>
	<title>General tape access commands</title>

	<para>&man.mt.1; provides generic access to the tape drives.  Some of
	  the more common commands are <command>rewind</command>,
	  <command>erase</command>, and <command>status</command>.  See the
	    &man.mt.1; manual page for a detailed description.</para>
      </sect3>
      
      <sect3>
	<title>Controller Interfaces</title>

	<para>There are several different interfaces that support tape drives.
	  The interfaces are SCSI, IDE, Floppy and Parallel Port. A wide
	  variety of tape drives are available for these interfaces.
	  Controllers are discussed in <link
	    linkend="hw-storage-controllers">Disk/tape
	    controllers</link>.</para>
      </sect3>
      
      <sect3>
	<title>SCSI drives</title>

	<para>The &man.st.4; driver provides support for 8mm (Exabyte), 4mm
	  (DAT: Digital Audio Tape), QIC (Quarter-Inch Cartridge), DLT
	  (Digital Linear Tape), QIC Minicartridge and 9-track (remember the
	  big reels that you see spinning in Hollywood computer rooms) tape
	  drives.  See the &man.st.4; manual page for a detailed
	  description.</para>

	<para>The drives listed below are currently being used by members of
	  the FreeBSD community.  They are not the only drives that will work
	  with FreeBSD.  They just happen to be the ones that we use.</para>

	<sect4>
	  <title>4mm (DAT: Digital Audio Tape)</title>
	  
	  <para><link linkend="hw-storage-python-28454">Archive Python
	      28454</link></para>

	  <para><link linkend="hw-storage-python-04687">Archive Python
	      04687</link></para>
	  
	  <para><link linkend="hw-storage-hp1533a">HP C1533A</link></para>
	      
	  <para><link linkend="hw-storage-hp1534a">HP C1534A</link></para>
	      
	  <para><link linkend="hw-storage-hp35450a">HP 35450A</link></para>
	      
	  <para><link linkend="hw-storage-hp35470a">HP 35470A</link></para>
	      
	  <para><link linkend="hw-storage-hp35480a">HP 35480A</link></para>
	      
	  <para><link linkend="hw-storage-sdt5000">SDT-5000</link></para>
	      
	  <para><link linkend="hw-storage-wangtek6200">Wangtek
	      6200</link></para>
	</sect4>

	<sect4>
	  <title>8mm (Exabyte)</title>
	  
	  <para><link linkend="hw-storage-exb8200">EXB-8200</link></para>
	  
	  <para><link linkend="hw-storage-exb8500">EXB-8500</link></para>
	  
	  <para><link linkend="hw-storage-exb8505">EXB-8505</link></para>
	</sect4>

	<sect4>
	  <title>QIC (Quarter-Inch Cartridge)</title>
	  
	  <para><link linkend="hw-storage-anaconda">Archive Ananconda
	      2750</link></para>
	      
	  <para><link linkend="hw-storage-viper60">Archive Viper
	      60</link></para>
	      
	  <para><link linkend="hw-storage-viper150">Archive Viper
	      150</link></para>
	      
	  <para><link linkend="hw-storage-viper2525">Archive Viper
	      2525</link></para>
	      
	  <para><link linkend="hw-storage-tandberg3600">Tandberg TDC
	      3600</link></para>

	  <para><link linkend="hw-storage-tandberg3620">Tandberg TDC
	      3620</link></para>
	    
	  <para><link linkend="hw-storage-tandberg3800">Tandberg TDC
	      3800</link></para>
	      
	  <para><link linkend="hw-storage-tandberg4222">Tandberg TDC
	      4222</link></para>
	      
	  <para><link linkend="hw-storage-wangtek5525es">Wangtek
	      5525ES</link></para>
	</sect4>

	<sect4>
	  <title>DLT (Digital Linear Tape)</title>
	  
	  <para><link linkend="hw-storage-dectz87">Digital TZ87</link></para>
	</sect4>

	<sect4>
	  <title>Mini-Cartridge</title>
	  
	  <para><link linkend="hw-storage-ctms3200">Conner CTMS
	      3200</link></para>
	      
	  <para><link linkend="hw-storage-exb2501">Exabyte 2501</link></para>
	</sect4>

	<sect4>
	  <title>Autoloaders/Changers</title>
	  
	  <para><link linkend="hw-storage-hp1553a">Hewlett-Packard HP C1553A
	      Autoloading DDS2</link></para>
	</sect4>
      </sect3>
      
      <sect3>
	<title>* IDE drives</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>Floppy drives</title>

	<para><link linkend="hw-storage-conner420r">Conner 420R</link></para>
      </sect3>
      
      <sect3>
	<title>* Parallel port drives</title>

	<para></para>
      </sect3>
      
      <sect3>
	<title>Detailed Information</title>

	<sect4 id="hw-storage-anaconda">
	  <title>Archive Anaconda 2750</title>
	  
	  <para>The boot message identifier for this drive is <literal>ARCHIVE
	      ANCDA 2750 28077 -003 type 1 removable SCSI 2</literal></para>
	  
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 1.35GB when using QIC-1350 tapes.  This
	    drive will read and write QIC-150 (DC6150), QIC-250 (DC6250), and
	    QIC-525 (DC6525) tapes as well.</para>
	      
	  <para>Data transfer rate is 350kB/s using
		&man.dump.8;.  Rates of 530kB/s have been reported when using
	    <link linkend="backups-programs-amanda">Amanda</link></para>

	  <para>Production of this drive has been discontinued.</para>
	  
	  <para>The SCSI bus connector on this tape drive is reversed from
	    that on most other SCSI devices.  Make sure that you have enough
	    SCSI cable to twist the cable one-half turn before and after the
	    Archive Anaconda tape drive, or turn your other SCSI devices
	    upside-down.</para>
	  
	  <para>Two kernel code changes are required to use this drive. This
	    drive will not work as delivered.</para>
	      
	  <para>If you have a SCSI-2 controller, short jumper 6. Otherwise,
	    the drive behaves are a SCSI-1 device.  When operating as a SCSI-1
	    device, this drive, <quote>locks</quote> the SCSI bus during some
	    tape operations, including: fsf, rewind, and rewoffl.</para>
	      
	  <para>If you are using the NCR SCSI controllers, patch the file
	    <filename>/usr/src/sys/pci/ncr.c</filename> (as shown below).
	    Build and install a new kernel.</para>
	      
	  <programlisting>
*** 4831,4835 ****
                };
        
!               if (np-&gt;latetime&gt;4) {
                        /*
                        **      Although we tried to wake it up,
--- 4831,4836 ----
                };

!               if (np-&gt;latetime&gt;1200) {
                        /*
                        **      Although we tried to wake it up,</programlisting>
	      
	  <para>Reported by: &a.jmb;</para>
	</sect4>

	<sect4 id="hw-storage-python-28454">
	  <title>Archive Python 28454</title>
	  
	  <para>The boot message identifier for this drive is <literal>ARCHIVE
	      Python 28454-XXX4ASB</literal> <literal>type 1 removable SCSI
	      2</literal> <literal>density code 0x8c, 512-byte
	      blocks</literal></para>
	      
	  <para>This is a DDS-1 tape drive.</para>
	      
	  <para>Native capacity is 2.5GB on 90m tapes.</para>
	  
	  <para>Data transfer rate is XXX.</para>
	  
	  <para>This drive was repackaged by Sun Microsystems as model
	    595-3067.</para>
	      
	  <para>Reported by: Bob Bishop <email>rb@gid.co.uk</email></para>
	  
	  <para>Throughput is in the 1.5 MByte/sec range, however this will
	    drop if the disks and tape drive are on the same SCSI
	    controller.</para>

	  <para>Reported by: Robert E. Seastrom
	    <email>rs@seastrom.com</email></para>
	</sect4>

	<sect4 id="hw-storage-python-04687">
	  <title>Archive Python 04687</title>

	  <para>The boot message identifier for this drive is <literal>ARCHIVE 
	      Python 04687-XXX 6580</literal> <literal>Removable Sequential
	      Access SCSI-2 device</literal></para>

	  <para>This is a DAT-DDS-2 drive.</para>

	  <para>Native capacity is 4GB when using 120m tapes.</para>

	  <para>This drive supports hardware data compression.  Switch 4
	    controls MRS (Media Recognition System).  MRS tapes have stripes
	    on the transparent leader.  Switch 4 <emphasis>off</emphasis>
	    enables MRS, <emphasis>on</emphasis> disables MRS.</para>

	  <para>Parity is controlled by switch 5.  Switch 5
	    <emphasis>on</emphasis> to enable parity control.  Compression is
	    enabled with Switch 6 <emphasis>off</emphasis>.  It is possible to 
	    override compression with the <literal>SCSI MODE SELECT</literal>
	    command (see &man.mt.1;).</para>

	  <para>Data transfer rate is 800kB/s.</para>
	</sect4>

	<sect4 id="hw-storage-viper60">
	  <title>Archive Viper 60</title>
	  
	  <para>The boot message identifier for this drive is <literal>ARCHIVE
	      VIPER 60 21116 -007</literal> <literal>type 1 removable SCSI
	      1</literal></para>
	      
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 60MB.</para>
	  
	  <para>Data transfer rate is XXX.</para>
	  
	  <para>Production of this drive has been discontinued.</para>
	  
	  <para>Reported by: Philippe Regnauld
	    <email>regnauld@hsc.fr</email></para>
	</sect4>

	<sect4 id="hw-storage-viper150">
	  <title>Archive Viper 150</title>
	  
	  <para>The boot message identifier for this drive is <literal>ARCHIVE
	      VIPER 150 21531 -004</literal> <literal>Archive Viper 150 is a
	      known rogue</literal> <literal>type 1 removable SCSI
	      1</literal>.  A multitude of firmware revisions exist for this
	    drive.  Your drive may report different numbers (e.g
	    <literal>21247 -005</literal>.</para>
	      
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 150/250MB.  Both 150MB (DC6150) and 250MB
	    (DC6250) tapes have the recording format.  The 250MB tapes are
	    approximately 67% longer than the 150MB tapes.  This drive can
	    read 120MB tapes as well.  It can not write 120MB tapes.</para>
	      
	  <para>Data transfer rate is 100kB/s</para>
	  
	  <para>This drive reads and writes DC6150 (150MB) and DC6250 (250MB)
	    tapes.</para>
	      
	  <para>This drives quirks are known and pre-compiled into the scsi
	    tape device driver (&man.st.4;).</para>
	      
	  <para>Under FreeBSD 2.2-current, use <command>mt blocksize
	      512</command> to set the blocksize.  (The particular drive had
	    firmware revision 21247 -005.  Other firmware revisions may behave
	    differently) Previous versions of FreeBSD did not have this
	    problem.</para>
	      
	  <para>Production of this drive has been discontinued.</para>
	  
	  <para>Reported by: Pedro A M Vazquez
	    <email>vazquez@IQM.Unicamp.BR</email></para>
	  
	  <para>Mike Smith
	    <email>msmith@atrad.adelaide.edu.au</email></para>
	</sect4>
	    
	<sect4 id="hw-storage-viper2525">
	  <title>Archive Viper 2525</title>
	  
	  <para>The boot message identifier for this drive is <literal>ARCHIVE
	      VIPER 2525 25462 -011</literal> <literal>type 1 removable SCSI
	      1</literal></para>
	  
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 525MB.</para>
	  
	  <para>Data transfer rate is 180kB/s at 90 inches/sec.</para>
	  
	  <para>The drive reads QIC-525, QIC-150, QIC-120 and QIC-24 tapes.
	    Writes QIC-525, QIC-150, and QIC-120.</para>
	      
	  <para>Firmware revisions prior to <literal>25462 -011</literal> are
	    bug ridden and will not function properly.</para>
	  
	  <para>Production of this drive has been discontinued.</para>
	</sect4>

	<sect4 id="hw-storage-conner420r">
	  <title>Conner 420R</title>
	  
	  <para>The boot message identifier for this drive is <literal>Conner
	      tape</literal>.</para>
	      
	  <para>This is a floppy controller, minicartridge tape drive.</para>
	      
	  <para>Native capacity is XXXX</para>
	  
	  <para>Data transfer rate is XXX</para>
	  
	  <para>The drive uses QIC-80 tape cartridges.</para>
	  
	  <para>Reported by: Mark Hannon
	    <email>mark@seeware.DIALix.oz.au</email></para>
	</sect4>

	<sect4 id="hw-storage-ctms3200">
	  <title>Conner CTMS 3200</title>
	      
	  <para>The boot message identifier for this drive is <literal>CONNER
	      CTMS 3200 7.00</literal> <literal>type 1 removable SCSI
	      2</literal>.</para>
	      
	  <para>This is a minicartridge tape drive.</para>
	  
	  <para>Native capacity is XXXX</para>
	  
	  <para>Data transfer rate is XXX</para>
	  
	  <para>The drive uses QIC-3080 tape cartridges.</para>
	  
	  <para>Reported by: Thomas S. Traylor
	    <email>tst@titan.cs.mci.com</email></para>
	</sect4>

	<sect4 id="hw-storage-dectz87">
	  <title><ulink
	      url="http://www.digital.com/info/Customer-Update/931206004.txt.html">DEC TZ87</ulink></title>
	  
	  <para>The boot message identifier for this drive is <literal>DEC
	      TZ87 (C) DEC 9206</literal> <literal>type 1 removable SCSI
	      2</literal> <literal>density code 0x19</literal></para>
	      
	  <para>This is a DLT tape drive.</para>
	  
	  <para>Native capacity is 10GB.</para>
	  
	  <para>This drive supports hardware data compression.</para>
	  
	  <para>Data transfer rate is 1.2MB/s.</para>
	  
	  <para>This drive is identical to the Quantum DLT2000.  The drive
	    firmware can be set to emulate several well-known drives,
	    including an Exabyte 8mm drive.</para>
	      
	  <para>Reported by: &a.wilko;</para>
	</sect4>

	<sect4 id="hw-storage-exb2501">
	  <title><ulink
	      url="http://www.Exabyte.COM:80/Products/Minicartridge/2501/Rfeatures.html">Exabyte EXB-2501</ulink></title>
	      
	  <para>The boot message identifier for this drive is <literal>EXABYTE
	      EXB-2501</literal></para>
	      
	  <para>This is a mini-cartridge tape drive.</para>
	      
	  <para>Native capacity is 1GB when using MC3000XL
	    minicartridges.</para>
	      
	  <para>Data transfer rate is XXX</para>
	  
	  <para>This drive can read and write DC2300 (550MB), DC2750 (750MB),
	    MC3000 (750MB), and MC3000XL (1GB) minicartridges.</para>
	      
	  <para>WARNING: This drive does not meet the SCSI-2 specifications.
	    The drive locks up completely in response to a SCSI MODE_SELECT
	    command unless there is a formatted tape in the drive.  Before
	    using this drive, set the tape blocksize with</para>
	      
	  <screen>&prompt.root; <userinput>mt -f /dev/st0ctl.0 blocksize 1024</userinput></screen>
	      
	  <para>Before using a minicartridge for the first time, the
	    minicartridge must be formated.  FreeBSD 2.1.0-RELEASE and
	    earlier:</para>
	      
	  <screen>&prompt.root; <userinput>/sbin/scsi -f /dev/rst0.ctl -s 600 -c "4 0 0 0 0 0"</userinput></screen>
	      
	  <para>(Alternatively, fetch a copy of the
	    <command>scsiformat</command> shell script from FreeBSD
	    2.1.5/2.2.) FreeBSD 2.1.5 and later:</para>
	      
	  <screen>&prompt.root; <userinput>/sbin/scsiformat -q -w /dev/rst0.ctl</userinput></screen>
	      
	  <para>Right now, this drive cannot really be recommended for
	    FreeBSD.</para>
	      
	  <para>Reported by: Bob Beaulieu
	    <email>ez@eztravel.com</email></para>
	</sect4>

	<sect4 id="hw-storage-exb8200">
	  <title>Exabyte EXB-8200</title>
	  
	  <para>The boot message identifier for this drive is <literal>EXABYTE
	      EXB-8200 252X</literal> <literal>type 1 removable SCSI
	      1</literal></para>
	      
	  <para>This is an 8mm tape drive.</para>
	  
	  <para>Native capacity is 2.3GB.</para>
	  
	  <para>Data transfer rate is 270kB/s.</para>
	  
	  <para>This drive is fairly slow in responding to the SCSI bus during
	    boot.  A custom kernel may be required (set SCSI_DELAY to 10
	    seconds).</para>
	  
	  <para>There are a large number of firmware configurations for this
	    drive, some have been customized to a particular vendor's
	    hardware.  The firmware can be changed via EPROM
	    replacement.</para>
	      
	  <para>Production of this drive has been discontinued.</para>
	  
	  <para>Reported by: Mike Smith
	    <email>msmith@atrad.adelaide.edu.au</email></para>
	</sect4>

	<sect4 id="hw-storage-exb8500">
	  <title>Exabyte EXB-8500</title>
	  
	  <para>The boot message identifier for this drive is <literal>EXABYTE
	      EXB-8500-85Qanx0 0415</literal> <literal>type 1 removable SCSI
	      2</literal></para>
	      
	  <para>This is an 8mm tape drive.</para>
	  
	  <para>Native capacity is 5GB.</para>
	  
	  <para>Data transfer rate is 300kB/s.</para>
	      
	  <para>Reported by: Greg Lehey <email>grog@lemis.de</email></para>
	</sect4>

	<sect4 id="hw-storage-exb8505">
	  <title><ulink
	      url="http://www.Exabyte.COM:80/Products/8mm/8505XL/Rfeatures.html">Exabyte EXB-8505</ulink></title>
	      
	      <para>The boot message identifier for this drive is
	    <literal>EXABYTE EXB-85058SQANXR1 05B0</literal> <literal>type 1
	      removable SCSI 2</literal></para>
	      
	  <para>This is an 8mm tape drive which supports compression, and is
	    upward compatible with the EXB-5200 and EXB-8500.</para>
	  
	  <para>Native capacity is 5GB.</para>
	  
	  <para>The drive supports hardware data compression.</para>
	  
	  <para>Data transfer rate is 300kB/s.</para>
	  
	  <para>Reported by: Glen Foster
	    <email>gfoster@gfoster.com</email></para>
	</sect4>

	<sect4 id="hw-storage-hp1533a">
	  <title>Hewlett-Packard HP C1533A</title>
	  
	  <para>The boot message identifier for this drive is <literal>HP
	      C1533A 9503</literal> <literal>type 1 removable SCSI
	      2</literal>.</para>
	      
	  <para>This is a DDS-2 tape drive.  DDS-2 means hardware data
	    compression and narrower tracks for increased data
	    capacity.</para>
	  
	  <para>Native capacity is 4GB when using 120m tapes.  This drive
	    supports hardware data compression.</para>
	      
	  <para>Data transfer rate is 510kB/s.</para>
	  
	  <para>This drive is used in Hewlett-Packard's SureStore 6000eU and
	    6000i tape drives and C1533A DDS-2 DAT drive.</para>
	  
	  <para>The drive has a block of 8 dip switches.  The proper settings
	    for FreeBSD are: 1 ON; 2 ON; 3 OFF; 4 ON; 5 ON; 6 ON; 7 ON; 8
	    ON.</para>
	  
	  <informaltable frame="none">
	    <tgroup cols="3">
	      <thead>
		<row>
		  <entry>switch 1</entry>
		  <entry>switch 2</entry>
		  <entry>Result</entry>
		</row>
	      </thead>
	      
	      <tbody>
		<row>
		  <entry>On</entry>
		  <entry>On</entry>
		  <entry>Compression enabled at power-on, with host
		    control</entry>
		</row>
		
		<row>
		  <entry>On</entry>
		  <entry>Off</entry>
		  <entry>Compression enabled at power-on, no host
		    control</entry>
		</row>
		
		<row>
		  <entry>Off</entry>
		  <entry>On</entry>
		  <entry>Compression disabled at power-on, with host
		    control</entry>
		</row>
		
		<row>
		  <entry>Off</entry>
		  <entry>Off</entry>
		  <entry>Compression disabled at power-on, no host
		    control</entry>
		</row>
	      </tbody>
	    </tgroup>
	  </informaltable>
	  
	  <para>Switch 3 controls MRS (Media Recognition System).  MRS tapes
	    have stripes on the transparent leader.  These identify the tape
	    as DDS (Digital Data Storage) grade media.  Tapes that do not have
	    the stripes will be treated as write-protected.  Switch 3 OFF
	    enables MRS.  Switch 3 ON disables MRS.</para>
	      
	      <para>See <ulink url="http://www.hp.com/tape/c_intro.html">HP
	      SureStore Tape Products</ulink> and <ulink
	      url="http://www.impediment.com/hp/hp_technical.html">Hewlett-Packard
	      Disk and Tape Technical Information</ulink> for more information
	    on configuring this drive.</para>
	      
	  <para><emphasis>Warning:</emphasis> Quality control on these drives
	    varies greatly.  One FreeBSD core-team member has returned 2 of
	    these drives.  Neither lasted more than 5 months.</para>
	      
	  <para>Reported by: &a.se;</para>
	</sect4>

	<sect4 id="hw-storage-hp1534a">
	  <title>Hewlett-Packard HP 1534A</title>
	  
	  <para>The boot message identifier for this drive is <literal>HP
	      HP35470A T503</literal> <literal>type 1 removable SCSI
	      2</literal> <literal>Sequential-Access density code 0x13,
	      variable blocks</literal>.</para>
	      
	  <para>This is a DDS-1 tape drive.  DDS-1 is the original DAT tape
	    format.</para>
	      
	  <para>Native capacity is 2GB when using 90m tapes.</para>
	      
	  <para>Data transfer rate is 183kB/s.</para>
	  
	  <para>The same mechanism is used in Hewlett-Packard's SureStore
	    <ulink url="http://www.dmo.hp.com/tape/sst2000.htm">2000i</ulink>
	    tape drive, C35470A DDS format DAT drive, C1534A DDS format DAT
	    drive and HP C1536A DDS format DAT drive.</para>
	      
	  <para>The HP C1534A DDS format DAT drive has two indicator lights,
	    one green and one amber.  The green one indicates tape action:
	    slow flash during load, steady when loaded, fast flash during
	    read/write operations.  The amber one indicates warnings: slow
	    flash when cleaning is required or tape is nearing the end of its
	    useful life, steady indicates an hard fault.  (factory service
	    required?)</para>
	      
	  <para>Reported by Gary Crutcher
	    <email>gcrutchr@nightflight.com</email></para>
	</sect4>

	<sect4 id="hw-storage-hp1553a">
	  <title>Hewlett-Packard HP C1553A Autoloading DDS2</title>
	  
	  <para>The boot message identifier for this drive is "".</para>
	  
	  <para>This is a DDS-2 tape drive with a tape changer.  DDS-2 means
	    hardware data compression and narrower tracks for increased data
	    capacity.</para>
	      
	  <para>Native capacity is 24GB when using 120m tapes.  This drive
	    supports hardware data compression.</para>
	      
	  <para>Data transfer rate is 510kB/s (native).</para>
	      
	  <para>This drive is used in Hewlett-Packard's SureStore <ulink
	      url="http://www.dmo.hp.com/tape/sst12000.htm">12000e</ulink>
	    tape drive.</para>
	      
	  <para>The drive has two selectors on the rear panel.  The selector
	    closer to the fan is SCSI id.  The other selector should be set to
	    7.</para>
	      
	  <para>There are four internal switches.  These should be set: 1 ON;
	    2 ON; 3 ON; 4 OFF.</para>
	      
	  <para>At present the kernel drivers do not automatically change
	    tapes at the end of a volume.  This shell script can be used to
	    change tapes:</para>
	      
	  <programlisting>
#!/bin/sh
PATH="/sbin:/usr/sbin:/bin:/usr/bin"; export PATH

usage()
{
        echo "Usage: dds_changer [123456ne] raw-device-name
        echo "1..6 = Select cartridge"
        echo "next cartridge"
        echo "eject magazine"
        exit 2
}

if [ $# -ne 2 ] ; then
        usage
fi

cdb3=0
cdb4=0
cdb5=0

case $1 in
        [123456])
                cdb3=$1
                cdb4=1
                ;;
        n)
                ;;
        e)
                cdb5=0x80
                ;;
        ?)
                usage
                ;;
esac

scsi -f $2 -s 100 -c "1b 0 0 $cdb3 $cdb4 $cdb5"</programlisting>
	</sect4>

	<sect4 id="hw-storage-hp35450a">
	  <title>Hewlett-Packard HP 35450A</title>
	  
	  <para>The boot message identifier for this drive is <literal>HP
	      HP35450A -A C620</literal> <literal>type 1 removable SCSI
	      2</literal> <literal>Sequential-Access density code
	      0x13</literal></para>
	      
	  <para>This is a DDS-1 tape drive.  DDS-1 is the original DAT tape
	    format.</para>
	      
	  <para>Native capacity is 1.2GB.</para>
	      
	  <para>Data transfer rate is 160kB/s.</para>
	  
	  <para>Reported by: mark thompson
	    <email>mark.a.thompson@pobox.com</email></para>
	</sect4>

	<sect4 id="hw-storage-hp35470a">
	  <title>Hewlett-Packard HP 35470A</title>
	  
	  <para>The boot message identifier for this drive is <literal>HP
	      HP35470A 9 09</literal> <literal>type 1 removable SCSI
	      2</literal></para>
	      
	  <para>This is a DDS-1 tape drive.  DDS-1 is the original DAT tape
	    format.</para>
	      
	  <para>Native capacity is 2GB when using 90m tapes.</para>
	      
	  <para>Data transfer rate is 183kB/s.</para>
	  
	  <para>The same mechanism is used in Hewlett-Packard's SureStore
	    <ulink url="http://www.dmo.hp.com/tape/sst2000.htm">2000i</ulink>
	    tape drive, C35470A DDS format DAT drive, C1534A DDS format DAT
	    drive, and HP C1536A DDS format DAT drive.</para>
	      
	  <para><emphasis>Warning:</emphasis> Quality control on these drives
	    varies greatly.  One FreeBSD core-team member has returned 5 of
	    these drives.  None lasted more than 9 months.</para>
	      
	  <para>Reported by: David Dawes
	    <email>dawes@rf900.physics.usyd.edu.au</email> (9 09)</para>
	      
	</sect4>

	<sect4 id="hw-storage-hp35480a">
	  <title>Hewlett-Packard HP 35480A</title>
	  
	  <para>The boot message identifier for this drive is <literal>HP
	      HP35480A 1009</literal> <literal>type 1 removable SCSI
	      2</literal> <literal>Sequential-Access density code
	      0x13</literal>.</para>
	      
	  <para>This is a DDS-DC tape drive.  DDS-DC is DDS-1 with hardware
	    data compression.  DDS-1 is the original DAT tape format.</para>
	      
	  <para>Native capacity is 2GB when using 90m tapes.  It cannot handle
	    120m tapes.  This drive supports hardware data compression.
	    Please refer to the section on <link
	      linkend="hw-storage-hp1533a">HP C1533A</link> for the proper
	    switch settings.</para>
	      
	  <para>Data transfer rate is 183kB/s.</para>
	  
	  <para>This drive is used in Hewlett-Packard's SureStore <ulink
	      url="http://www.dmo.hp.com/tape/sst5000.htm">5000eU</ulink> and
	    <ulink url="http://www.dmo.hp.com/tape/sst5000.htm">5000i</ulink>
	    tape drives and C35480A DDS format DAT drive..</para>
	      
	  <para>This drive will occasionally hang during a tape eject
	    operation (<command>mt offline</command>). Pressing the front
	    panel button will eject the tape and bring the tape drive back to
	    life.</para>
	      
	  <para>WARNING: HP 35480-03110 only.  On at least two occasions this
	    tape drive when used with FreeBSD 2.1.0, an IBM Server 320 and an
	    2940W SCSI controller resulted in all SCSI disk partitions being
	    lost.  The problem has not be analyzed or resolved at this
	    time.</para>
	</sect4>

	<sect4 id="hw-storage-sdt5000">
	  <title><ulink
	      url="http://www.sel.sony.com/SEL/ccpg/storage/tape/t5000.html">Sony SDT-5000</ulink></title>
	      
	  <para>There are at least two significantly different models: one is
	    a DDS-1 and the other DDS-2.  The DDS-1 version is
	    <literal>SDT-5000 3.02</literal>.  The DDS-2 version is
	    <literal>SONY SDT-5000 327M</literal>. The DDS-2 version has a 1MB
	    cache.  This cache is able to keep the tape streaming in almost
	    any circumstances.</para>
	      
	  <para>The boot message identifier for this drive is <literal>SONY
	      SDT-5000 3.02</literal> <literal>type 1 removable SCSI
	      2</literal> <literal>Sequential-Access density code
	      0x13</literal></para>
	      
	  <para>Native capacity is 4GB when using 120m tapes.  This drive
	    supports hardware data compression.</para>
	      
	  <para>Data transfer rate is depends upon the model or the drive. The
	    rate is 630kB/s for the <literal>SONY SDT-5000 327M</literal>
	    while compressing the data.  For the <literal>SONY SDT-5000
	      3.02</literal>, the data transfer rate is 225kB/s.</para>
	      
	  <para>In order to get this drive to stream, set the blocksize to 512
	    bytes (<command>mt blocksize 512</command>) reported by Kenneth
	    Merry ken@ulc199.residence.gatech.edu</para>
	      
	  <para><literal>SONY SDT-5000 327M</literal> information reported by
	    Charles Henrich henrich@msu.edu</para>
	      
	  <para>Reported by: &a.jmz;</para>
	</sect4>

	<sect4 id="hw-storage-tandberg3600">
	  <title>Tandberg TDC 3600</title>
	  
	  <para>The boot message identifier for this drive is
	    <literal>TANDBERG TDC 3600 =08:</literal> <literal>type 1
	      removable SCSI 2</literal></para>
	      
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 150/250MB.</para>
	  
	  <para>This drive has quirks which are known and work around code is
	    present in the scsi tape device driver (&man.st.4;).
	    Upgrading the firmware to XXX version will fix the quirks and
	    provide SCSI 2 capabilities.</para>
	      
	  <para>Data transfer rate is 80kB/s.</para>
	  
	  <para>IBM and Emerald units will not work.  Replacing the firmware
	    EPROM of these units will solve the problem.</para>
	      
	  <para>Reported by: Michael Smith
	    <email>msmith@atrad.adelaide.edu.au</email></para>
	</sect4>

	<sect4 id="hw-storage-tandberg3620">
	  <title>Tandberg TDC 3620</title>
	  
	  <para>This is very similar to the <link
	      linkend="hw-storage-tandberg3600">Tandberg TDC 3600</link>
	    drive.</para>
	      
	  <para>Reported by: &a.joerg;</para>
	</sect4>

	<sect4 id="hw-storage-tandberg3800">
	  <title>Tandberg TDC 3800</title>

	  <para>The boot message identifier for this drive is
	    <literal>TANDBERG TDC 3800 =04Y</literal> <literal>Removable
	      Sequential Access SCSI-2 device</literal></para>
	      
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 525MB.</para>
	  
	  <para>Reported by: &a.jhs;</para>
	</sect4>

	<sect4 id="hw-storage-tandberg4222">
	  <title>Tandberg TDC 4222</title>
	  
	  <para>The boot message identifier for this drive is
	    <literal>TANDBERG TDC 4222 =07</literal> <literal>type 1 removable
	      SCSI 2</literal></para>
	      
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 2.5GB.  The drive will read all cartridges
	    from the 60 MB (DC600A) upwards, and write 150 MB (DC6150)
	    upwards.  Hardware compression is optionally supported for the 2.5
	    GB cartridges.</para>
	      
	  <para>This drives quirks are known and pre-compiled into the scsi
	    tape device driver (&man.st.4;) beginning with FreeBSD
	    2.2-current.  For previous versions of FreeBSD, use
	    <command>mt</command> to read one block from the tape, rewind the
	    tape, and then execute the backup program (<command>mt fsr 1; mt
	      rewind; dump ...</command>)</para>
	      
	  <para>Data transfer rate is 600kB/s (vendor claim with compression),
	    350 KB/s can even be reached in start/stop mode. The rate
	    decreases for smaller cartridges.</para>
	  
	  <para>Reported by: &a.joerg;</para>
	</sect4>

	<sect4 id="hw-storage-wangtek5525es">
	  <title>Wangtek 5525ES</title>
	  
	  <para>The boot message identifier for this drive is <literal>WANGTEK
	      5525ES SCSI REV7 3R1</literal> <literal>type 1 removable SCSI
	      1</literal> <literal>density code 0x11, 1024-byte
	      blocks</literal></para>
	      
	  <para>This is a QIC tape drive.</para>
	  
	  <para>Native capacity is 525MB.</para>
	  
	  <para>Data transfer rate is 180kB/s.</para>
	  
	  <para>The drive reads 60, 120, 150, and 525MB tapes.  The drive will
	    not write 60MB (DC600 cartridge) tapes.  In order to overwrite 120
	    and 150 tapes reliably, first erase (<command>mt erase</command>)
	    the tape.  120 and 150 tapes used a wider track (fewer tracks per
	    tape) than 525MB tapes. The <quote>extra</quote> width of the
	    previous tracks is not overwritten, as a result the new data lies
	    in a band surrounded on both sides by the previous data unless the
	    tape have been erased.</para>
	      
	  <para>This drives quirks are known and pre-compiled into the scsi
	    tape device driver (&man.st.4;).</para>
	  
	  <para>Other firmware revisions that are known to work are:
	    M75D</para>
	  
	  <para>Reported by: Marc van Kempen <email>marc@bowtie.nl</email>
	    <literal>REV73R1</literal> Andrew Gordon
	    <email>Andrew.Gordon@net-tel.co.uk</email>
	    <literal>M75D</literal></para>
	</sect4>

	<sect4 id="hw-storage-wangtek6200">
	  <title>Wangtek 6200</title>
	  
	  <para>The boot message identifier for this drive is <literal>WANGTEK
	      6200-HS 4B18</literal> <literal>type 1 removable SCSI
	      2</literal> <literal>Sequential-Access density code
	      0x13</literal></para>
	  
	  <para>This is a DDS-1 tape drive.</para>
	  
	  <para>Native capacity is 2GB using 90m tapes.</para>
	  
	  <para>Data transfer rate is 150kB/s.</para>
	  
	  <para>Reported by: Tony Kimball <email>alk@Think.COM</email></para>
	</sect4>
      </sect3>
      
      <sect3>
	<title>* Problem drives</title>

	<para></para>
      </sect3>
    </sect2>
    
    <sect2>
      <title>CD-ROM drives</title>
      
      <para><emphasis>Contributed by &a.obrien;.  23 November
	      1997.</emphasis></para>
      
      <para>As mentioned in <link linkend="hw-jordans-picks-cdrom">Jordan's
	  Picks</link> Generally speaking those in <emphasis>The FreeBSD
	  Project</emphasis> prefer SCSI CDROM drives over IDE CDROM drives.
	However not all SCSI CDROM drives are equal.  Some feel the quality of
	some SCSI CDROM drives have been deteriorating to that of IDE CDROM
	drives.  Toshiba used to be the favored stand-by, but many on the SCSI
	mailing list have found displeasure with the 12x speed XM-5701TA as
	its volume (when playing audio CDROMs) is not controllable by the
	various audio player software.</para>
	  
      <para>Another area where SCSI CDROM manufacturers are cutting corners is
	adherence to the <link linkend="scsi-further-reading">SCSI
	  specification</link>. Many SCSI CDROMs will respond to <link
	  linkend="scsi-rogue-devices">multiple LUNs</link> for its target
	address.  Known violators include the 6x Teac CD-56S 1.0D.</para>
    </sect2>
    
    <sect2>
      <title>* Other</title>
      
      <para></para>
    </sect2>
  </sect1>
  
  <sect1 id="hw-other">
    <title>* Other</title>
    
    
    <sect2>
      <title>* PCMCIA</title>
      
      <para></para>
    </sect2>
  </sect1>
</appendix>

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