.H2 "Managing hardware"
.H2 "* Adding and reconfiguring disks"
.H2 "SCSI"
\*h
.XS \n%
\*(SN \*h
.XE
.P
\fICopyright (c) 1995, Wilko Bulte 
\fC<wilko@yedi.iaf.nl>\fR.
.br
3 September 1995.\fP
.P
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).
.P
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 ``Further         reading''), with SCSI-3 on the drawing boards.
.P
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.
.P
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 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.
.P
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.
.P
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. 
.P
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.
.P
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.
.P
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 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.
.P
.H2 "Components of SCSI"
\*h
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.XE
.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.
.P
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.
.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.
.P
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.
.P
.H2 "SCSI bus types"
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.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.
.P
In lots of SCSI related documentation there is a sort of jargon
in use to abbreviate the different bus types. A small list:
.P
.LI
FWD:	Fast Wide Differential
.LI
FND:	Fast Narrow Differential
.LI
SE:	Single Ended
.LI
FN:	Fast Narrow
.LI
etc.
.P
With a minor amount of imagination one can usually imagine what
is meant.
.P
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.
.P
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.
.P
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.
.P
.H2 "Single ended buses"
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.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. 
.P
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!
.P
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.
.P
For connector pinning and connector types please refer to the
SCSI-2 standard (see ``Further             reading'') itself, connectors etc are listed there in
painstaking detail.
.P
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.
.P
.H2 "Differential buses"
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.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..).
.P
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.
.P
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.
.P
.H2 "Terminators"
\*h
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\*(SN \*h
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.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.
.P
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 built-in terminator.
There are special terminators you can stick onto a flat cable
bus.  Others look like external connectors, so a connector hood
without a cable.  So, lots of choice as you can see.
.P
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 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.
.P
Please keep in mind that terminators for differential and
single-ended buses are not identical. You should \fBnot
mix\fR the two variants.
.P
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: \fBevery SCSI bus has 2 (two)
terminators, one at each end of the bus.\fR 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.
.P
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.
.P
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.
.P
.H2 "Terminator power"
\*h
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\*(SN \*h
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.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?
.P
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.
.P
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.
.P
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.
.P
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.
.P
In newer designs auto-restoring fuses are used who 'reset'
themselves after some time.
.P
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!
.P
.H2 "Device addressing"
\*h
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.P
Because the SCSI bus is, ehh, a bus there must be a way to
distinguish or address the different devices connected to it.
.P
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
more information.
.P
Beware of multiple devices configured to use the same ID. Chaos
normally reigns in this case.
.P
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
data lines.
.P
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).
.P
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.
.P
.H2 "Bus layout"
\*h
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.P
SCSI buses are linear. So, not shaped like Y-junctions, star
topologies, cobwebs or whatever else people might want to
invent.
.P
You might notice that the terminator issue discussed earlier
becomes rather hairy if your bus is not linear..
.P
The electrical characteristics, it's noise margins and
ultimately the reliability of it all are tightly related to
linear bus rule.
.P
\fBStick to the linear bus rule!\fR
.P
.H2 "Using SCSI with FreeBSD"
\*h
.XS \n%
\*(SN \*h
.XE
.P
.H2 "About translations, BIOSes and magic..."
\*h
.XS \n%
\*(SN \*h
.XE
.P
As stated before, you should first make sure that you have a
electrically sound bus.
.P
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
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.
.P
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.
.P
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 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?
.P
The SCSI BIOS itself presents to the system a so called
\fBtranslated\fR 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.
.P
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 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.
.P
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.
.P
It is very important that \fBall\fR operating systems on the disk use
the \fBsame translation\fR 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.
.P
Failing to observe the translation issue might be un-bootable systems or
operating systems overwriting each others partitions. Using fdisk
you should be able to see all partitions.
.P
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:
.P
.DS L
.ft R
.eo
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
          
.ec
.DE
.ft P
.LP
.P
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.
.P
.H2 "SCSI subsystem design"
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.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.
.P
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 \fC/sys/scsi\fR. See the man pages in section 4
for more details.
.P
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.
.P
.H2 "Kernel configuration"
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.P
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 \fInot\fP imply LINT will actually get you to a
working kernel at all.
.P
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.
.P
An example based on the FreeBSD 2.0.5-Release kernel config 
file LINT with some added comments (between []):
.P
.DS L
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# 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.
#

[For a Bustek controller]
controller	bt0	at isa? port "IO_BT0" bio irq ? vector btintr

[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 Adaptec AHA154x controller]
controller	aha0	at isa? port "IO_AHA0" bio irq ? drq 5 vector ahaintr

[For an Ultrastor adapter]
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]

	  
.ec
.DE
.ft P
.LP
.P
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 
\fIonly\fP attach them when they match the target ID and
LUN specified. 
.P
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.
.P
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.
.P
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.
.P
Although the old style still works, you 
are \fIstrongly\fP 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.
.P
.DS L
.ft R
.eo
controller      ahb0    at isa? bio irq 11 vector ahbintr	[driver for Adaptec 174x]
controller      aha0    at isa? port "IO_AHA0" bio irq 11 drq 5 vector ahaintr [for Adaptec 154x]
controller      sea0    at isa? bio irq 5 iomem 0xc8000 iosiz 0x2000 vector seaintr [for Seagate
ST01/02]
controller      scbus0

device          sd0	[support for 4 SCSI harddisks, sd0 up sd3]
device          sd1
device          sd2
device          sd3

device          st0	[support for 2 SCSI tapes]
device          st1

device          cd0     #Only need one of these, the code dynamically grows [for the cdrom]
          
.ec
.DE
.ft P
.LP
.P
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.
.P
Use \fCman 4 scsi\fR to check for the latest info on the SCSI
subsystem. For more detailed info on host adapter drivers use eg
\fCman 4 aha\fR for info on the Adaptec 154x driver.
.P
.H2 "Tuning your SCSI kernel setup"
\*h
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\*(SN \*h
.XE
.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.
.P
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:
.P
.DS L
.ft R
.eo
options         "SCSI_DELAY=15"         #Be pessimistic about Joe SCSI device
	  
.ec
.DE
.ft P
.LP
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
working.
.P
.H2 "Rogue SCSI devices"
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.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. 
.P
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:
.P
.DS L
.ft R
.eo
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
	 
.ec
.DE
.ft P
.LP
.P
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.
.P
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.
.P
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.
.P
.H2 "Busmaster host adapters"
\*h
.XS \n%
\*(SN \*h
.XE
.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.
.P
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.
.P
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.
.P
The solution is of course obvious: switch to a lower data transfer rate
and try if that works better. 
.P
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:
.P
.DS L
.ft R
.eo
options        "TUNE_1542"             #dynamic tune of bus DMA speed
	  
.ec
.DE
.ft P
.LP
.P
Check the man pages for the host adapter that you use. Or better
still, use the ultimate documentation (read: driver source).
.P
.H2 "Tracking down problems"
\*h
.XS \n%
\*(SN \*h
.XE
.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.
.P
.LI
Check for loose connectors and cables.
.LI
Check and doublecheck the location and number of your terminators.
.LI
Check if your bus has at least one supplier of terminator power
(especially with external terminators.
.LI
Check if no double target IDs are used.
.LI
Check if at least one device provides terminator power to the bus.
.LI
Check if all devices to be used are powered up. 
.LI
Make a minimal bus config with as little devices as possible.
.LI
If possible, configure your host adapter to use slow bus speeds.
.P
.H2 "Further reading"
\*h
.XS \n%
\*(SN \*h
.XE
.P
If you intend to do some serious SCSI hacking, you might want to
have the official standard at hand:
.P
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.
.P
Many X3T10 draft documents are available electronically on the SCSI
BBS (719-574-0424) and on the ncrinfo.ncr.com anonymous ftp site.
.P
Latest X3T10 committee documents are:
.LI
AT Attachment (ATA or IDE) [X3.221-1994] (\fIApproved\fP)
.LI
ATA Extensions (ATA-2) [X3T10/948D Rev 2i]
.LI
Enhanced Small Device Interface (ESDI) [X3.170-1990/X3.170a-1991]   (\fIApproved\fP)
.LI
Small Computer System Interface - 2 (SCSI-2) [X3.131-1994] (\fIApproved\fP)
.LI
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:
.LI
"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
.LI
"Basics of SCSI", a SCSI tutorial written by Ancot Corporation
Contact Ancot for availability information at:
Phone: (415) 322-5322  Fax: (415) 322-0455
.LI
"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
.LI
"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
.LI
"The SCSI Bench Reference", "The SCSI Encyclopedia", and the "SCSI Tutor",
ENDL Publications, 14426 Black Walnut Court, Saratoga CA, 95070
Phone: (408) 867-6642

.LI
"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
.P
On Usenet the newsgroups comp.periphs.scsi
and comp.periphs
are noteworthy places to look for more info. You can also
find the SCSI-Faq there, which is posted periodically.
.P
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.

.P
.H2 "ESDI hard disks and FreeBSD"
\*h
.XS \n%
\*(SN \*h
.XE
.P
\fICopyright (c) 1995, Wilko Bulte 
\fC<wilko@yedi.iaf.nl>\fR.
.br
24 September 1995.\fP
.P
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.
.P
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 standardised by ANSI.
.P
Capacities of the drives are boosted by putting more sectors
on each track. Typical is 35 sectors per track, high capacity
drives I've seen were up to 54 sectors/track.
.P
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.
.P
.H2 "Concepts of ESDI"
\*h
.XS \n%
\*(SN \*h
.XE
.P
.H2 "Physical connections"
\*h
.XS \n%
\*(SN \*h
.XE
.P
The ESDI interface uses two cables connected to each drive. 
One cable is a 34 pin flatcable edge connector that carries
the command and status signals from the controller to the
drive and viceversa. The command cable is daisy chained
between all the drives. So, it forms a bus onto which all
drives are connected.
.P
The second cable is a a 20 pin flatcable edge connector that
carries the data to and from the drive. This cable is radially
connected, so each drive has it's own direct connection to the
controller.
.P
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.
.P
.H2 "Device addressing"
\*h
.XS \n%
\*(SN \*h
.XE
.P
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.
.P
On PC type controllers the first drive is set to address 0, 
the second disk to address 1. \fIAlways make sure\fR you
set each disk to an unique address! So, on a PC with it's
two drives/controller maximum the first drive is drive 0, the
second is drive 1.
.P
.H2 "Termination"
\*h
.XS \n%
\*(SN \*h
.XE
.P
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.
.P
So, one and \fIonly\fR one drive, the one at 
the fartest end of the command
cable has it's 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 \fInot\fR in the middle.
.P
.H2 "Using ESDI disks with FreeBSD"
\*h
.XS \n%
\*(SN \*h
.XE
.P
Why is ESDI such a pain to get working in the first place?
.P
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.
.P
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.
.P
.H2 "ESDI speed variants"
\*h
.XS \n%
\*(SN \*h
.XE
.P
As briefly mentioned before, ESDI comes in two speed flavours.
The older drives and controllers use a 10 Mbits/second
data transfer rate. Newer stuff uses 15 Mbits/second.
.P
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 \fIand\fR drive 
documentation to see if things match.
.P
.H2 "Stay on track"
\*h
.XS \n%
\*(SN \*h
.XE
.P
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.
.P
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 diskspace.
.P
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.
.P
.H2 "Hard or soft sectoring"
\*h
.XS \n%
\*(SN \*h
.XE
.P
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.
.P
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 \fIunformatted\fP 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.
.P
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.
.P
In general, experiment with sector settings before you install
FreeBSD because you need to re-run the low-level format
after each change.
.P
.H2 "Low level formatting"
\*h
.XS \n%
\*(SN \*h
.XE
.P
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. 
.P
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.
.P
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).
.P
.H2 "Translations"
\*h
.XS \n%
\*(SN \*h
.XE
.P
Translations, although not exclusively a ESDI-only problem, 
might give you real trouble.
Translations come in multiple flavours. 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!).
.P
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. 
.P
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 \fIall\fR 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 after it's kernel has
started no longer uses the BIOS. More on this later.
.P
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).
.P
\fIWhatever 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\fP
.P
While on the subject of translations, I've 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 powerup it
read the info and presented itself to the system based on
the info from the disk.
.P
.H2 "Spare sectoring"
\*h
.XS \n%
\*(SN \*h
.XE
.P
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. 
.P
In most cases the remapping is done by using N-1 sectors on
each track for actual datastorage, 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.
.P
The problem is that the translation from \fIbad\fR to \fIgood\fR
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.
.P
\fISo: 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.\fP
.P
.H2 "Bad block handling"
\*h
.XS \n%
\*(SN \*h
.XE
.P
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 \fIbad144\fR 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. 
.P
When the disk is in operation, the diskaccesses are checked 
against the table read from the disk. Whenever a blocknumber
is requested that is in the bad144 list, a replacement block
(also from the end of the FreeBSD slice) is used.
In this way, the bad144 replacement scheme presents 'perfect'
media to the FreeBSD filesystems.
.P
There are a number of potential pitfalls associated with 
the use of bad144.
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 \fIevery\fP 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.
.P
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.
\fINote\fP that the restriction is not that only the root
\fIfilesystem\fP must be within the 1024 cylinder limit, but
rather the entire \fIslice\fP that contains the root filesystem.
.P
.H2 "Kernel configuration"
\*h
.XS \n%
\*(SN \*h
.XE
.P
ESDI disks are handled by the same \fIwd\fRdriver as
IDE and ST412 MFM disks. The \fIwd\fR driver should work
for all WD1003 compatible interfaces. 
.P
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. 
.P
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
\fC/sys/i386/conf\fR BTW).
.P
.Ds
# 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
.ec
.DE
.LP
.P
.H2 "Particulars on ESDI hardware"
\*h
.XS \n%
\*(SN \*h
.XE
.P
.H2 "Adaptec 2320 controllers"
\*h
.XS \n%
\*(SN \*h
.XE
.P
I succesfully installed FreeBSD onto a ESDI disk controlled by a
ACB-2320. No other operating system was present on the disk.
.P
To do so I low level formatted the disk using NEFMT.EXE 
(\fIftp\fRable from \fIwww.adaptec.com\fR) 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 'free configurable' option in the system
BIOS to allow the BIOS to boot it.
.P
Before using NEFMT.EXE I tried to format the disk using the
ACB-2320 BIOS builtin formatter. This proved to be a showstopper,
because it didn't give me an option to disable spare sectoring.
With spare sectoring enabled the FreeBSD installation
process broke down on the bad144 run.
.P
Please check carefully which ACB-232xy variant you have. The
x is either 0 or 2, indicating a controller without or with
a floppy controller on board. 
.P
The y is more interesting.  It can either be a blank, 
a "A-8" or a "D".  A blank indicates a plain 10 Mbits/second
controller. An "A-8" indicates a 15 Mbits/second controller
capable of handling 52 sectors/track.
A "D" means a 15 Mbits/second controller that can also
handle drives with > 36 sectors/track (also 52 ?).
.P
All variations should be capable of using 1:1 interleaving. Use 1:1,
FreeBSD is fast enough to handle it.
.P
.H2 "Western Digital WD1007 controllers"
\*h
.XS \n%
\*(SN \*h
.XE
.P
I succesfully 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.
.P
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
WDFMT.EXE from www.wdc.com Running this formatted my drive
just fine. 
.P
.H2 "Ultrastor U14F controllers"
\*h
.XS \n%
\*(SN \*h
.XE
.P
According to multiple reports from the net, Ultrastor ESDI 
boards work OK with FreeBSD. I lack any further info on
particular settings.
.P
.H2 "Further reading"
\*h
.XS \n%
\*(SN \*h
.XE
.P
If you intend to do some serious ESDI hacking, you might want to
have the official standard at hand:
.P
The latest ANSI X3T10 committee document is:
.LI
Enhanced Small Device Interface (ESDI) [X3.170-1990/X3.170a-1991]   
[X3T10/792D Rev 11]

On Usenet the newsgroup comp.periphs is a noteworthy place to look 
for more info.
.P
The World Wide Web (WWW) also proves to be a very handy info source:
For info on Adaptec ESDI controllers see .
For info on Western Digital controllers see .
.P
.H2 "Thanks to..."

\*h
.XS \n%
\*(SN \*h
.XE
.P
Andrew Gordon for sending me an Adaptec 2320 controller and ESDI disk 
for testing.
.P
.H2 "* Tapes and backups"
\*h
.XS \n%
\*(SN \*h
.XE
.H2 "* Serial ports"
\*h
.XS \n%
\*(SN \*h
.XE
.H2 "* Sound cards"
\*h
.XS \n%
\*(SN \*h
.XE
