At 16:45 1/19/99 -0500, Changhsu Liu wrote:
>Are there speed limits on different SCSI cables (e.g., DB68, DB50,
>centronic 50)? I have a ultra-wide scsi card but I don't have any DB68
>cable or ultra-wide scsi HD. I bought a converter to convert db68 to
>db50 scsi cable. Just wonder if using converter would make the scsi
>slower than using its own scsi cable. Maybe it does not matter since
>my drive is not ultra-wide anyway.

There is at least one potentially serious problem with this regarding
termination. I'll explain below.

Cables with 68 wires (called wide cables) can potentially carry twice
as many MegaBytes per second as cables that only have 50 lines (called
narrow cables). A wide SCSI bus carries two bytes (16 bits) of data in
parallel along 16 data lines every clock cycle. But a narrow bus is only
carrying one byte (8 bits along 8 data lines) in parallel. Sort of like
how a two lane highway can carry along twice as much traffic as a one
lane highway during an equal amount of time. (In a 50 line SCSI cable,
half the lines are at ground potential to provide shielding, 8 lines
carry the data, and the rest of the lines carry the control signals.)

Whether the connectors on a narrow cable are Centronics50 or DB50 or
Micro-D should not affect the amount of data per second (the bandwidth)
that the cable can handle. (Using an Apple25 cable is bad however,
because the 25 ground lines are not present.)

A problem can occur when you convert from wide SCSI down to narrow SCSI.
Each end of a SCSI bus must be electrically "terminated" to prevent
signal reflections. (In addition, when *active* electronic components
are used in the terminating module, signal voltages are kept at the
correct levels.) When you go from wide to narrow somewhere along the
SCSI bus, the upper byte of data lines ends. You must terminate these
"upper" data lines here or suffer the consequences. Effects of improper
termination can be anything from slower than normal operation, data
corruption, system hangs during heavy use, or a refusal of the computer
to even boot. The effects may be obvious or, possibly worse, may be
transtory.

Your 68-to-50 converter may or may not contain termination circuitry.
If you bought an inexpensive converter or it was not specifically stated
that it terminates the "high byte" (the upper byte of data lines), then
likely it is non terminating. So does this mean you have a problem?

Attaching a wide converter to some controllers tricks the controller
into thinking that there is a wide cable attached and the controller
doesn't itself automatically terminate the high byte. For this reason,
some controllers will let you turn on termination of the high byte
from within the controller's BIOS Setup. Unfortunately, some of these
controllers have a bug that keeps this from working. And some
controllers won't let you set the termination manually at all.

Supposing that there are no termination problems, and your cables and
converter are of good quality, and the whole length of the cables is
not "overly long", there should be no slowdown upon converting down to
narrow SCSI as long as the drive is a narrow drive.

>Let me ask in another way if my question is unclear. Compare...
>SCSI Card <=> 68pin SCSI cable <=> external ultra-wide SCSI HD

This would be the correct way to attach a *wide* drive to a wide capable
controller. (Didn't you mentioned above that your drive was not a wide
SCSI drive?) But suppose you have a wide drive now.

>[Or] Ultra-wide SCSI Card <=> SCSI converter (from 68pin to 50pin) <=>
>50pin SCSI cable <=> SCSI converter (from 68pin to 50pin) <=>
>external ultra-wide SCSI HD

This defeats the whole purpose of having a wide SCSI bus. There *will*
be a slowdown...at least potentially...in the speed that the drive and
controller can communicate. (The end result may or may not be below the
maximum rate that your hard drive can pull data off its platters.) But
more seriously, suppose the wide drive and the wide controller get
confused and both assume that there is a wide cable connecting them?
They each might try to send data along all 16 data lines. But the narrow
cable would have only 8 data lines... You could lose half your data.
(You might have to make a change in your SCSI controller's BIOS Setup
to cancel *wide* data transfer between the drive and the controller.)

Imagine a traffic jam caused by construction along a two lane highway
where traffic must trickle through along one lane for a stretch. This is
bad enough, but now imagine if one of the two lanes just ended without
warning so that all the cars in that lane drove off the road and over a
cliff. This is like what might happen if the drive and controller tried
to transfer data using wide SCSI without realizing that a narrow cable
was being used.

Two final points. The more connections and converters you use, the more
the SCSI signals might get degraded. The effects might be enough to cause
problems, particularly at Ultra SCSI frequencies. Also, Ultra SCSI may
require that the cabling used be no longer than 1.5 meters. This would be
the total of the lengths of all the cables used with your single channel
SCSI controller.

Regards,
Bill

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