path: root/handbook/scsi.sgml

<!-- $Id: scsi.sgml,v 1.1.1.1.4.1 1995-09-17 11:19:39 davidg Exp $ -->
<!-- The FreeBSD Documentation Project -->

<!--
      <title>An introduction to SCSI and its use with FreeBSD</title>
      <author>(c) 1995, Wilko Bulte, <tt/wilko@yedi.iaf.nl/
      <date>Sun Sep  3 17:14:48 MET DST 1995</date>
     Copyright 1995, Wilko C. Bulte, Arnhem, The Netherlands 

      <abstract>
        This document attempts to describe the background of SCSI, its
        (mis)use with FreeBSD and some common pitfalls.
      </abstract>
      
-->
    <sect><heading>SCSI<label id="scsi"></heading>

      <p><em>&copy; 1995, &a.wilko;.<newline>3 September 1995.</em>

        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).

        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 recognised in the ANSI SCSI-1 standard.  The SCSI-1
        standard (approx 1985) is now more or less obsolete.  The current
        standard is SCSI-2 (see <ref id="scsi:further-reading" name="Further
        reading">), with SCSI-3 on the drawing boards.

        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.

        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 don't know what
        single-ended means, don't worry, that is what this document is all
        about.)  Modern designs also use 16 bit wides buses, with
        differential signals.  This allows transfer speeds of
        20Mbytes/second, on cables lengths of up to 25 meters. SCSI-2
        allows a maximum buswidth of 32 bits, using an additional cable.

        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 seperate
        parity line. In pre-SCSI-2 designs parity was optional.

	In SCSI-3 even faster bustypes are introduced, along with a serial
	SCSI bus that reduces the cabling overhead and allows a higher
	maximum buslength. 
	
        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 badblock replacement etc
	are all made possible by this 'intelligent device' approach.

        On a SCSI bus, each possible pair of devices can communicate. If
        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.

        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.  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 standardisation has become more strict and is better
        adhered to by the device manufacturers.  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 does not
        imply that you 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.

    <sect1><heading>Components of SCSI</heading>
      <p>
<!--      <sect2><heading>A <it>smart</it> interface</heading>
        <p> -->
          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.

          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.

<!--      <sect2><heading>Do's and don't's on interconnections</heading>
        <p> -->
          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.

          So, gold plated connectors, shielded cabling, sturdy connector
          hoods with strain reliefs etc are the way to go. Second golden
          rule: don't 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 with 1 meter solved the problem.  And the
          original bus length was well within the SCSI specification.

      <sect2><heading>SCSI bus types</heading>
        <p>
          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.

          In lots of SCSI related documentation there is a sort of jargon
          in use to abbreviate the different bus types. A small list:

          <itemize>
            <item>FWD:	Fast Wide Differential
            <item>FND:	Fast Narrow Differential
            <item>SE:	Single Ended
            <item>FN:	Fast Narrow
            <item>etc.
          </itemize>

          With a minor amount of imagination one can usually imagine what
          is meant.
          
          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.

          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. More on this later.

	  It should be noted that the datalines &gt; 8 are only used for
	  datatransfers and device addressing. The transfers of commands
	  and status messages etc are only performed on the lowest 8
          datalines. The standard allows narrow devices to operate on
	  a wide bus. The usable buswidth is negotiated 
          between the devices. You have to watch your device addressing
          closely when mixing wide and narrow.

        <sect3><heading>Single ended buses</heading>
          <p>
            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. 

	    Please note that this means that
            if some devices on your bus use 'fast' to communicate your
            bus must adhere to the length restrictions for fast buses!

            It is obvious that with the newer fast-SCSI devices the
            buslength can become a real bottleneck. This is why the
            differential SCSI bus was introduced in the SCSI-2 standard.

            For connector pinning and connector types please refer to the
            SCSI-2 standard (see <ref id="scsi:further-reading" name="Further
            reading">) itself, connectors etc are listed there in
            painstaking detail.

	    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.

        <sect3><heading>Differential buses</heading>
          <p>
            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 it's 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 (don't try 10 kVolts though..).
            
            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.

	    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
	    AH1740 as a single ended board, whereas the AH1744 was differential.
	    The software interface to the host is identical for both.

        <sect2><heading>Terminators</heading>
          <p>
            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 travelling over your SCSI bus, you
            don't want signals echoing back.

            Terminators come in various incarnations, with more or less
            sophisticated designs.  Of course, there are internal and
            external variants.  Almost every SCSI device comes with a
            number of sockets in which a number of resistor networks can
            (must be!) installed.  If you remove terminators from a device,
            carefully stock 'm. 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 builtin terminator.
            There are special terminators you can stick onto a flatcable
            bus.  Others look like external connectors, so a connector hood
            without a cable.  So, lots of choice as you can see.

            There is much debate going on if and when you should switch
            from simple resistor (passive) terminators to active
            terminators. Active terminators contain more or less elaborate
            circuits to give more clean bus signals. The general consensus
            seems to be that the usefullnes 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.

            Please keep in mind that terminators for differential and
            single-ended buses are not identical. You should <bf>not
            mix</bf> the two variants.

            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: <bf>every SCSI bus has 2 (two)
            terminators, one at each end of the bus.</bf> So, two and not
            one or three or whatever. Do yourself a favour and stick to
            this rule. It will save you endless grief, because wrong
            termination has the potential to introduce highly mysterious
            bugs.

            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.

            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.
        
        <sect2><heading>Terminator power</heading>
          <p>
            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?

            Not so. Each device can provide it's 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.

            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.

            To allow for switched-off devices on a bus, the terminator
            power must be supplied to the bus via a diode. This prevents
            the backflow of current to switched-off devices.

            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.

            In newer designs auto-restoring fuses are used who 'reset'
            themselves after some time.

            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 even are software
            configurable, using some sort of setup tool. Consult you
	    documentation!

        <sect2><heading>Device addressing</heading>
          <p>
            Because the SCSI bus is, ehh, a bus there must be a way to
            distinguish or address the different devices connected to it.

            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 dipswitch, or
            something similar. Consult the documentation of your device for
            more information.

            Beware of multiple devices configured to use the same ID. Chaos
            normally reigns in this case.

            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
            datalines on the bus.  For wide this increases to the number of
            datalines.

            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
            hostadapter usually uses target ID 7 (for narrow buses).

            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
            tapechanger. In this way, the host system can address each of
            the parts of the tape unit as desired.

        <sect2><heading>Bus layout</heading>
          <p>
            SCSI buses are linear. So, not shaped like Y-junctions, star
            topologies, cobwebbs or whatever else people might want to
            invent.

            You might notice that the terminator issue discussed earlier
            becomes rather hairy if your bus is not linear..

            The electrical characteristics, it's noise margins and
            ultimately the reliability of it all are tightly related to
            linear bus rule.

            <bf>Stick to the linear bus rule!</bf>

    <sect1><heading>Using SCSI with FreeBSD</heading>
      <p>
      <sect2><heading>About translations, BIOSes and magic...</heading>
        <p>
          As stated before, you should first make sure that you have a
          electrically sound bus.

          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
          it's first incarnation used a low level physical interface to the
          harddisk. So, you had to tell the BIOS (using a setup tool or a
          BIOS builtin 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.

          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.

          The SCSI host adapter or SCSI controller you have put in your
          AT/EISA/PCI/whatever bus to connect your disk therefore has it's
          own onboard BIOS. During system startup, the SCSI BIOS takes over
          the harddisk interface routines from the system BIOS. To fool the
          system BIOS, the system setup is normally set to No harddisk
          present. Obvious, isn't it?

          The SCSI BIOS itself presents to the system a so called
          <bf>translated</bf> 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 32 heads and 64 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.

          Right.. All is well now?! No, it isn't. The system BIOS has
          another quirk you might run into. The number of cylinders of a
          bootable harddisk cannot be greater than 1024. Using the
          translation above, this is a showstopper for disks greater than
          1 Gb. With disk capacities going up all the time this is causing
          problems.

          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.

          It is very important that <bf>all</bf> operating systems on the disk use
          the <bf>same translation</bf> 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.

	  Failing to observe the translation issue might be un-bootable systems or
	  operating systems overwriting eachothers partitions. Using fdisk
	  you should be able to see all partitions.

          As promised earlier: what is this talk about 'lying' devices? As
          you might already know, the FreeBSD kernel reports the geometry
          of SCSI disks when booting. An example from one of my systems:

          <verb>
Feb  9 19:33:46 yedi /386bsd: aha0 targ 0 lun 0: <MICROP  1588-15MB1057404HSP4>
Feb  9 19:33:46 yedi /386bsd: sd0: 636MB (1303250 total sec), 1632 cyl, 15 head,
 53 sec, bytes/sec 512
          </verb>

          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 fake.

      <sect2><heading>SCSI subsystem design</heading>
        <p>
          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 it's commands and
          reports back any status.

          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), cdroms (cd)
          etc. In case you are wondering where you can find this stuff, it
          all lives in <tt>/sys/scsi</tt>. See the man pages in section 4
	  for more details.

          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 managable problem.
        
      <sect2><heading>Kernel configuration</heading>
        <p>
          Dependent on your hardware, the kernel configuration file must
          contain one or more lines describing your hostadapter(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
	  /sys/i386/conf/LINT for an overview of a kernel config file.
	  LINT contains every possible option you can dream of. It
	  does <em>not</em> imply LINT will actually get you to a
	  working kernel at all.

	  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.

          An example based on the FreeBSD 2.0.5-Release kernel config 
	  file LINT with some added comments (between &lsqb;&rsqb;):

	  <verb>
		
# 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!)
# bt: Most Buslogic controllers
# nca: ProAudioSpectrum cards using the NCR 5380 or Trantor T130
# uha: UltraStore 14F and 34F
# sea: Seagate ST01/02 8 bit controller (slow!)
# wds: Western Digital WD7000 controller (no scatter/gather!).
#
# Note that the order is important in order for Buslogic cards to be
# probed correctly.
#

&lsqb;For a Bustek controller&rsqb;
controller	bt0	at isa? port "IO_BT0" bio irq ? vector btintr

&lsqb;For an Adaptec AHA274x, 284x etc controller&rsqb;
controller	ahc0	at isa? bio irq ? vector ahcintr # port??? iomem?

&lsqb;For an Adaptec AHA174x controller&rsqb;
controller	ahb0	at isa? bio irq ? vector ahbintr

&lsqb;For an Adaptec AHA154x controller&rsqb;
controller	aha0	at isa? port "IO_AHA0" bio irq ? drq 5 vector ahaintr

&lsqb;For an Ultrastor adapter&rsqb;
controller	uha0	at isa? port "IO_UHA0" bio irq ? drq 5 vector uhaintr

controller	scbus0	#base SCSI code

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 scbus0 target 3
disk sd3 at scbus0 target 4
tape st1 at scbus0 target 6		[SCSI tape at target 6]
device cd0 at scbus?			[the first ever CDROM found, no wiring]

	  </verb>

	  The example above tells the kernel to look for a bt (Bustek)
	  controller, then for an Adaptec 274x, 284x etc board, and 
	  so on. The lines following the controller specifications 
	  tell the kernel to configure specific devices but 
	  <em>only</em> attach them when they match the target ID and
	  LUN specified. 

	  So, if you had a SCSI tape at target ID 2 it would not be
	  configured, but it will attach when it is at target ID 6.

	  Below is another example of a kernel config file as used by
	  FreeBSD version < 2.0.5. The difference with the first example is
          that devices are not 'wired down'. 'Wired down' means
          that you specify which SCSI target belongs to which device.

	  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 /etc/fstab each time.

	  Although the old style still works, you 
	  are <em>strongly</em> 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.

          <verb>
controller      ahb0    at isa? bio irq 11 vector ahbintr	&lsqb;driver for Adaptec 174x&rsqb;
controller      aha0    at isa? port "IO_AHA0" bio irq 11 drq 5 vector ahaintr &lsqb;for Adaptec 154x&rsqb;
controller      sea0    at isa? bio irq 5 iomem 0xc8000 iosiz 0x2000 vector seaintr &lsqb;for Seagate
ST01/02&rsqb;
controller      scbus0

device          sd0	&lsqb;support for 4 SCSI harddisks, sd0 up sd3&rsqb;
device          sd1
device          sd2
device          sd3

device          st0	&lsqb;support for 2 SCSI tapes&rsqb;
device          st1

device          cd0     #Only need one of these, the code dynamically grows &lsqb;for the cdrom&rsqb;
          </verb>

          Both examples support 4 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 complain.  You
          will have to build and boot a new kernel (after adapting the kernel
          configuration file) before you can use all of the devices. It
          does not hurt to have 'extra' devices in the kernel, the example
          above will work fine when you have only 2 SCSI disks.

          Use <tt>man 4 scsi</tt> to check for the latest info on the SCSI
          subsystem. For more detailed info on hostadapter drivers use eg
          <tt>man 4 aha</tt> for info on the Adaptec 154x driver.

      <sect2><heading>Tuning your SCSI kernel setup</heading>
        <p>
          Experience has shown that some devices are slow to respond to INQUIRY 
	  commands after a SCSI bus reset. An INQUIRY command is sent by the kernel
	  on boot to see what kind of device (disk, tape, cdrom etc) is connected
	  to a specific target ID. This process is called device probing by the way.

	  To work around this problem, FreeBSD allows a tunable delay time before
	  the SCSI devices are probed following a SCSI bus reset. You can set this
          delaytime in your kernel configuration file using a line like:

	  <verb>
options         "SCSI_DELAY=15"         #Be pessimistic about Joe SCSI device
	  </verb>
	  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 CDROM drive to be recognised.
	  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.

      <sect2><heading>Rogue SCSI devices</heading>
        <p>  
	  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. 

	  This is exactly where the 'rogue' devices come into view. Rogues are
	  devices that are recognised 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:
	
	 <verb>
Feb 25 21:03:34 yedi /386bsd: ahb0 targ 5 lun 0: <TANDBERG TDC 3600       -06:>
Feb 25 21:03:34 yedi /386bsd: st0: Tandberg tdc3600 is a known rogue

Mar 29 21:16:37 yedi /386bsd: aha0 targ 5 lun 0: <ARCHIVE VIPER 150  21247-005>
Mar 29 21:16:37 yedi /386bsd: st1: Archive  Viper 150 is a known rogue
	 </verb>

	  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 user.

	  The SCSI subsystem of FreeBSD recognises 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.

	  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 cdrom you might be the one
	  that has to define which workaround is needed.

      <sect2><heading>Busmaster host adapters</heading>
        <p>
	  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.

	  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.

	  For instance an Adaptec 1542 controller can be set to use different
	  transferspeeds 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.

	  The solution is of course obvious: switch to a lower data transfer rate
	  and try if that works better. 

	  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:

	  <verb>
options        "TUNE_1542"             #dynamic tune of bus DMA speed
	  </verb>
	  
	  Check the man pages for the host adapter that you use. Or better
	  still, use the ultimate documentation (read: driver source).

    <sect1><heading>Tracking down problems</heading>
      <p>
        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.

        <itemize>
          <item>
            Check for loose connectors and cables.
          <item>
            Check and doublecheck the location and number of your terminators.
          <item>
            Check if your bus has at least one supplier of terminator power
            (especially with external terminators.
          <item>
            Check if no double target IDs are used.
          <item>
            Check if at least one device provides terminator power to the bus.
          <item>
            Check if all devices to be used are powered up. 
          <item>
            Make a minimal bus config with as little devices as possible.
          <item>
            If possible, configure your hostadapter to use slow bus speeds.
        </itemize>

    <sect1><heading>Further reading<label id="scsi:further-reading"></heading>
      <p>
        If you intend to do some serious SCSI hacking, you might want to
        have the official standard at hand:

        Approved American National Standards can be purchased from ANSI at
        11 West 42nd Street, 13th Floor, New York, NY 10036, Sales Dept:
        (212) 642-4900.  You can also buy many ANSI standards and most
        committee draft documents from Global Engineering Documents, 15
        Inverness Way East, Englewood, CO 80112-5704, Phone: (800)
        854-7179, Outside USA and Canada: (303) 792-2181, FAX: (303) 792-
        2192.

        Many X3T10 draft documents are available electronically on the SCSI
        BBS (719-574-0424) and on the ncrinfo.ncr.com anonymous ftp site.

        Latest X3T10 committee documents are:
        <itemize>
<item>AT Attachment (ATA or IDE) &lsqb;X3.221-1994&rsqb; (<em>Approved</em>)
<item>ATA Extensions (ATA-2) &lsqb;X3T10/948D Rev 2i&rsqb;
<item>Enhanced Small Device Interface (ESDI) &lsqb;X3.170-1990/X3.170a-1991&rsqb;   (<em>Approved</em>)
<item>Small Computer System Interface - 2 (SCSI-2) &lsqb;X3.131-1994&rsqb; (<em>Approved</em>)
<item>SCSI-2 Common Access Method Transport and SCSI Interface Module (CAM) 
                                   &lsqb;X3T10/792D Rev 11&rsqb;
        </itemize>
        Other publications that might provide you with additional information are:
<itemize>
<item>"SCSI: Understanding the Small Computer System Interface", written by NCR 
Corporation.  Available from: Prentice Hall, Englewood Cliffs, NJ, 07632
Phone: (201) 767-5937 ISBN 0-13-796855-8

<item>"Basics of SCSI", a SCSI tutorial written by Ancot Corporation
Contact Ancot for availability information at:
Phone: (415) 322-5322  Fax: (415) 322-0455

<item>"SCSI Interconnection Guide Book", 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

<item>"Fast Track to SCSI", A Product Guide written by Fujitsu.
Available from: Prentice Hall, Englewood Cliffs, NJ, 07632
Phone: (201) 767-5937 ISBN 0-13-307000-X

<item>"The SCSI Bench Reference", "The SCSI Encyclopedia", and the "SCSI Tutor",
ENDL Publications, 14426 Black Walnut Court, Saratoga CA, 95070
Phone: (408) 867-6642
        
<item>"Zadian SCSI Navigator" (quick ref. book) and "Discover the Power of SCSI" 
(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
        </itemize>

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

        Most major SCSI device and hostadapter suppliers operate ftp sites
        and/or BBS systems. They may be valuable sources of information
        about the devices you own.