HARDWARE CLINIC: SCSI Cabling and Termination   (5/94)

By Mark Minasi


In previous columns, I've discussed choosing SCSI host adapters, setting SCSI
ID values, and enabling or disabling SCSI parity.

This month, we'll tackle the last two steps of physically installing a SCSI
subsystem: cabling it and setting up its terminators.

Many of you will put only one or two SCSI devices on a PC, but SCSI can
easily support seven peripherals off a single SCSI host adapter. (A single
SCSI host adapter can actually support tens of thousands of devices--in
theory. But I wouldn't try it.)

Multiple devices are attached to a single SCSI host adapter via daisychaining
over several kinds of cables: a 50-conductor Centronics connector, a DB25
connector, a miniature DB50 connector, and a 50-pin ribbon cable. A
50-conductor Centronics connector looks like the connector on the printer end
of your parallel cable, only larger--your printer uses a 37-conductor
connector. A DB25 connector is the kind that you find on a serial port or on
a parallel cable--on the PC end. Miniature DB50s look somewhat like DB25s,
but there are 50 small pins or holes in the same space that the DB25 uses for
25 pins or holes. Also, DB50 connectors generally have buttons that you must
press to connect or disconnect them.

Some external SCSI devices have two Centronics 50 connectors on the backs of
their cases--two connectors so that they can be part of a daisychain. Older
SCSI devices--CD-ROM drives that follow the pre-SCSI-2 standard, mostly--are
the only devices that use the DB25. The miniature DB50 shows up on the backs
of some SCSI host adapters and on a few devices. For example, the
Hewlett-Packard ScanJet IIc has a miniature DB50 connector.

You can pretty much ignore the DB25, but you can't ignore the Centronics 50
and the miniature DB50, so there are three possible kinds of SCSI cable you
might have to lay your hands on: miniature-DB50-to-miniature-DB50,
miniature-DB50-to-Centronics-50, and Centronics-50-to-Centronics-50 cables.
And you have to be careful where you get them. To explain that, let me relate
a short war story.

I installed a CD-ROM drive on a server so that I could install Windows NT
Advanced Server on my LAN. (Windows NT Advanced Server is terrific, by the
way; if you're thinking about moving to it, don't hesitate.) The CD-ROM drive
used the Centronics 50 connector, so I pulled a standard SCSI cable with a
Centronics 50 connector on each end out of my cable pile. I plugged the
CD-ROM drive into the SCSI adapter's interface port on the back of my PC, and
the problems started. My tape drive and hard disk started acting up. Running
Chkdsk revealed lost clusters and invalid subdirectories on my hard disk--but
the messages referred to different parts of the disk every time I ran Chkdsk!
The tape drive wasn't recognized about half of the times I booted the system,
although it had been working fine for months--clearly, the new guy on the
block (the CD-ROM drive) was causing trouble.

I changed the host adapter to SCSI, giving up SCSI parity in the process and
engendering a sense of deep foreboding. Then I turned to installing Windows
NT Advanced Server. The README file that came with it said, "The SCSI and
CD-ROM support built into Windows NT 3.1 requires that CD-ROMs provide SCSI
parity to function properly." Oops.

I started fussing with the SCSI devices to get the CD-ROM drive to support
SCSI parity. There was a jumper included on the CD-ROM drive to control
whether or not SCSI parity would be used, so why wouldn't it support SCSI
parity?

On the off chance that I had a bad cable, I went back to the cable pile to
see what else I had. I found another dual Centronics 50 cable identical to
the cable that I was using and another dual Centronics 50 cable that was
about twice as thick as the first two. I tried swapping the original thin
cable for the other thin cable. No difference. But when I used the thicker
cable, everything started working! I got full SCSI-2 support, as well as SCSI
parity.

A few calls to cable places confirmed that there were two kinds of dual
Centronics 50 cables. The thin ones work fine for SCSI but not for SCSI-2.
The thick ones are good for both. Look for cables from Amphenol, Quintec, and
Icontec, and your cables should work fine under SCSI-2.

External devices, as I've said, tend to have two SCSI connections on them so
that they can support the SCSI daisychain. Internal devices, by contrast, use
only a single 50-pin header connector. Internal SCSI cables are just
50-conductor ribbon cables, looking somewhat like fatter-than-usual hard disk
cables.

Now that you know about cabling, before popping the top back on your PC,
there's one more thing that needs doing. You must terminate the SCSI chain.

To terminate is to provide a voltage and resistance on either end of a cable,
so that the entire bus has a particular set of electrical characteristics.
Without this resistance, the SCSI cables cannot transport data around without
significant error rates. (This process will work sometimes, despite what some
people claim, but it won't work reliably.) Complicating things a bit, as
you'll see, is the fact that there are two kinds of termination, and they
aren't really compatible. Passive termination is employed by earlier SCSI
devices. Active termination is employed by SCSI-2 devices.

I've said that some SCSI devices are installed internally in the PC and
connect to the host adapter with a ribbon cable. Other devices are installed
externally and connect to the host adapter with one of the three kinds of
common external SCSI cables. There are also internal and external
terminators, as well as SCSI devices that have terminators built in. The
terminators you'll see include the following.

o An internal SIPP (Single Inline Pin Package) terminator on the host adapter
and/or hard disk
o A separate external SCSI terminator (reliable only under SCSI, not SCSI-2)
o A device with built-in termination that's enabled or disabled with a DIP
switch or a jumper
o A device with built-in termination that cannot be disabled

An internal SIPP terminator looks like a colored plastic blob with a row of
little metal legs sticking out of it. You'll find SIPP-type adapters usually
on SCSI hard disks or host adapters. SIPPs often show up on the host adapter
itself because it needs termination, and SIPPs don't take up much space. If
you have a device that terminates with SIPPs, you'll probably see three of
these SIPPs on a host adapter. If you don't need termination on a device
equipped with SIPPs, just remove them (gently--you may need to reinstall them
one day) by working them out with needle-nosed pliers. Once you've done that,
put them in an envelope, seal it, label it "SCSI terminators," and put it
with your valuables.

Newer host adapters don't terminate with SIPPs, however. Adapters like the
Adaptec 1542C can terminate or not with a software command. If this doesn't
sound like manna from heaven to you, it's a sure bet that you've never spent
hours opening and closing a PC, flipping jumpers, and trying to get
everything working. This simplicity is a feature I'd look for in a host
adapter.

Older SCSI systems make use of external SCSI terminators, terminators that
can attach to one of the Centronics 50 connectors on the back of a SCSI
device. External terminators look like Centronics connectors without cables
attached to them. They clip onto one of the Centronics connectors on the back
of the last external device on your SCSI daisychain. The external Centronics
50 connector terminator may not work in a SCSI-2 environment. You're best off
these days looking for SCSI devices that offer active termination. The
explanation is coming up.

If you look at the back of an external SCSI device, you may notice a switch
labeled Termination. It can be flipped on or off, so if this SCSI device is
the last on the chain, all you need to do is flip the switch on. If this
device isn't the last device on the chain, flip it off.

You've already read that SCSI uses passive termination, and SCSI-2 uses
active termination. What's the difference? Are they compatible?

First, the differences. Passive termination employs two resistors on either
end of the SCSI bus. A 220-ohm resistor is tied to the termination voltage
(one of the SCSI lines), and a 330-ohm resistor is tied to a ground. Active
termination is a more reliable approach that uses a single 110-ohm resistor
to the termination line.

Second, what about compatibility? While you may experience different results,
most of the SCSI-2 setups I've worked with require active termination. If you
use one of the old plug-type terminators, SCSI-2 may not work. Active
termination requires electrical power provided by some device (the SCSI
drive, the host adapter, or whatever), so there's no simple plug that will
provide active termination on a SCSI-2 chain. You must have a device that
supports active termination on both ends of a SCSI-2 chain, or you're likely
to experience problems.

One more thought before I go: A number of devices offer the ability to
convert a simple parallel port into a SCSI device; perhaps the best known is
from a company named Trantor. In my experience, these devices will not work
unless the SCSI device that you're trying to attach can provide active
termination. This represents just one more reason to doublecheck that the
SCSI devices that you buy can provide active termination. 

COMPUTE, May 1994


Transmitted:  94-04-04 17:10:55 EDT

