<!-- $Id: scsi.sgml,v 220.127.116.11.4.5 1996-06-19 20:28:17 jkh Exp $ -->
<!-- The FreeBSD Documentation Project -->
<title>An introduction to SCSI and its use with FreeBSD</title>
<author>(c) 1995, Wilko Bulte, <firstname.lastname@example.org/
<date>Sun Sep 3 17:14:48 MET DST 1995</date>
Copyright 1995, Wilko C. Bulte, Arnhem, The Netherlands
This document attempts to describe the background of SCSI, its
(mis)use with FreeBSD and some common pitfalls.
<sect1><heading>What is SCSI?<label id="scsi"></heading>
<p><em>Copyright © 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
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 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
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.
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.
In SCSI-3 even faster bus types are introduced, along with a serial
SCSI bus that reduces the cabling overhead and allows a higher
maximum bus length.
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.
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.
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.
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.
<sect2><heading>Components of SCSI</heading>
<!-- <sect3><heading>A <it>smart</it> interface</heading>
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.
<!-- <sect3><heading>Do's and don't's on interconnections</heading>
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: 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.
<sect2><heading>SCSI bus types</heading>
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:
<item>FWD: Fast Wide Differential
<item>FND: Fast Narrow Differential
<item>SE: Single Ended
<item>FN: Fast Narrow
With a minor amount of imagination one can usually imagine what
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 data lines > 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.
<sect3><heading>Single ended buses</heading>
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
bus length 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
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.
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..).
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
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.
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.
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 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.
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
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 favor and stick to
this rule. It will save you endless grief, because wrong
termination has the potential to introduce highly mysterious
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
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.
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 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
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 that 'reset'
themselves after some time are sometimes used.
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
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 dip switch, or
something similar. Consult the documentation of your device for
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
data lines on the bus. For wide this increases to the number of
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 (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
tape changer. In this way, the host system can address each of
the parts of the tape unit as desired.
SCSI buses are linear. So, not shaped like Y-junctions, star
topologies, cobwebs or whatever else people might want to
You might notice that the terminator issue discussed earlier
becomes rather hairy if your bus is not linear..
The electrical characteristics, its noise margins and
ultimately the reliability of it all are tightly related to
linear bus rule.
<bf>Stick to the linear bus rule!</bf>
<sect2><heading>Using SCSI with FreeBSD</heading>
<sect3><heading>About translations, BIOSes and magic...</heading>
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
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.
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.
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?
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 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.
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
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 lead to
un-bootable systems or operating systems overwriting each
others partitions. Using fdisk you should be able to see
You might have heard some talk of 'lying' devices?
Older FreeBSD kernels used to report the geometry
of SCSI disks when booting. An example from one of my systems:
aha0 targ 0 lun 0: <MICROP 1588-15MB1057404HSP4>
sd0: 636MB (1303250 total sec), 1632 cyl, 15 head, 53 sec, bytes/sec 512
Newer kernels usually do not report this information.. e.g.
(bt0:0:0): "SEAGATE ST41651 7574" type 0 fixed SCSI 2
sd0(bt0:0:0): Direct-Access 1350MB (2766300 512 byte sectors)
Why has this changed?
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 <em>if the BIOS is never going to know
about this disk</em>, (e.g. it is not a booting disk) to supply a
fictitious geometry that is convenient.
<sect3><heading>SCSI subsystem design</heading>
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.
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 manageable problem.
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
/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 loosely based on the FreeBSD 2.0.5-Release kernel config
file LINT with some added comments (between ):
# 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!).
[For an Adaptec AHA274x, 284x etc controller]
controller ahc0 at isa? bio irq ? vector ahcintr # port??? iomem?
[For an Adaptec AHA174x controller]
controller ahb0 at isa? bio irq ? vector ahbintr
[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 ahb0
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 ahb0]
tape st1 at scbus0 target 6 [SCSI tape at target 6]
device cd0 at scbus? [the first ever CDROM found, no wiring]
The example above tells the kernel to look for a ahc (Adaptec 274x)
controller, then for an Adaptec 174x 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 on the corresponding bus.
Wired down devices get 'first shot' at the unit numbers
so the first non 'wired down' device, is allocated the unit number
one greater than the highest 'wired down' 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. Note that <em>wired down devices need not
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 <em>no</em> relationship with its target ID on
the SCSI bus.
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 <tt>/etc/fstab</tt> 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.
[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
device sd0 [support for 4 SCSI harddisks, sd0 up sd3]
device st0 [support for 2 SCSI tapes]
[for the cdrom]
device cd0 #Only need one of these, the code dynamically grows
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 'wired down'. If there are no 'wired down' devices
then counting starts at unit 0.
Use <tt>man 4 scsi</tt> to check for the latest info on the SCSI
subsystem. For more detailed info on host adapter drivers use eg
<tt>man 4 aha</tt> for info on the Adaptec 154x driver.
<sect3><heading>Tuning your SCSI kernel setup</heading>
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, 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 delay time in your kernel configuration file
using a line like:
options "SCSI_DELAY=15" #Be pessimistic about Joe SCSI device
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 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
<sect3><heading>Rogue SCSI devices</heading>
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 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:
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
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.
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.
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.
<sect3><heading>Busmaster host adapters</heading>
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
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
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:
options "TUNE_1542" #dynamic tune of bus DMA speed
Check the man pages for the host adapter that you use. Or better
still, use the ultimate documentation (read: driver source).
<sect2><heading>Tracking down problems</heading>
The following list is an attempt to give a guideline for the most
common SCSI problems and their solutions. It is by no means
Check for loose connectors and cables.
Check and double check the location and number of your terminators.
Check if your bus has at least one supplier of terminator power
(especially with external terminators.
Check if no double target IDs are used.
Check if all devices to be used are powered up.
Make a minimal bus config with as little devices as possible.
If possible, configure your host adapter to use slow bus speeds.
If you can compile a kernel, make one with the SCSIDEBUG 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
If it probes but just does not work, you can use the
<tt>scsi(8)</tt> command to dynamically set a debug level to
it in a running kernel (if SCSIDEBUG is defined).
This will give you COPIOUS debugging output with which to confuse
the gurus. see <tt>man 4 scsi</tt> for more exact information.
Also look at <tt>man 8 scsi</tt>.
<sect2><heading>Further reading<label id="scsi:further-reading"></heading>
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-
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:
<item>AT Attachment (ATA or IDE) [X3.221-1994] (<em>Approved</em>)
<item>ATA Extensions (ATA-2) [X3T10/948D Rev 2i]
<item>Enhanced Small Device Interface (ESDI) [X3.170-1990/X3.170a-1991] (<em>Approved</em>)
<item>Small Computer System Interface - 2 (SCSI-2) [X3.131-1994] (<em>Approved</em>)
<item>SCSI-2 Common Access Method Transport and SCSI Interface Module (CAM)
[X3T10/792D Rev 11]
Other publications that might provide you with additional information are:
<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
On Usenet the newsgroups <htmlurl
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 host adapter suppliers operate ftp sites
and/or BBS systems. They may be valuable sources of information
about the devices you own.