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.
Do's and don't's on interconnections
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.
SCSI bus types
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:
FWD: Fast Wide Differential
FND: Fast Narrow Differential
SE: Single Ended
FN: Fast Narrow
etc.
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 > 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.
Single ended buses
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 ) 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.
Differential buses
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.
Terminators
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 not
mix 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: every SCSI bus has 2 (two)
terminators, one at each end of the bus. 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.
Terminator power
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!
Device addressing
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.
Bus layout
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.
Stick to the linear bus rule!Using SCSI with FreeBSD
About translations, BIOSes and magic..
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
translated 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 all operating systems on the disk use
the same translation 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:
Feb 9 19:33:46 yedi /386bsd: aha0 targ 0 lun 0:
Feb 9 19:33:46 yedi /386bsd: sd0: 636MB (1303250 total sec), 1632 cyl, 15 head,
53 sec, bytes/sec 512
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.
SCSI subsystem design
FreeBSD uses a sort of layered SCSI subsystem. For each different
controller card a so called 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 /sys/scsi. 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.
Kernel configuration
Dependent on your hardware, the kernel configuration file must
contain a line which describes your hostadapter. This includes
I/O addresses, interrupts etc. Consult the man page for your
adapter driver to get more info.
Although it is probably an obvious remark: the kernel config
file should reflect your actual hardware setup. So, interrupts,
I/O addresses etc must match the kernel config file.
An example from the kernel config file (they live in
/sys/i386/conf BTW), with some added comments (between
[]):
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]
So, the ahb driver is used for the Adaptec 1740, the aha driver
for the Adaptec 154x etc. If you have more than one card of the
same type in your system you get an ahb1, ahb2 line etc.
The example above supports 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 man 4 scsi to check for the latest info on the SCSI
subsystem. For more detailed info on hostadapter drivers use eg
man 4 aha for info on the Adaptec 154x driver.
Tuning your SCSI kernel setup
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:
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 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.
Rogue SCSI devices
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:
Feb 25 21:03:34 yedi /386bsd: ahb0 targ 5 lun 0:
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:
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 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.
Busmaster host adapters
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:
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).
Tracking down problems
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.
Check for loose connectors and cables.
Check and doublecheck 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 at least one device provides terminator power to the bus.
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 hostadapter to use slow bus speeds.
Further reading