John,

Differential drives like this use a high voltage signal between the host
adapter and the connected drive to increase signal to noise and make it
possible to use drives in a noisy environmnet or to use long cable lengths
to an external drive case. The case can be 25 -50 feet or greater from the
CPU. Normally SCSI is about 6 feet. SO you can have the computer in your
office and the external drive case in the garage!

Two things you need to be aware of of. You will need a differential host
adadpater (Adaptec 2944, 2744, NCR 825, etc). These cards are generally
available at e-bay.com for autction typically between $100-200.  Second, the
drives do not have on board termination. You will need a special active
differential terminator. If yoy are mounting this drive internally, it's
bets to try and find an internal differential terminator ($20). They are
rare, but can be found on e-bay. You can also get an external version but
then you need a gender changer to connect it to the internal ribbon cable.
If you put your dirve in an external case, then plug in hte terminator as
normal.

Note that only differential devices can be attached to a differential host
adapter. If you have other SCSI devices you'll need to continue using this
adapter for your single ended devices.

As you may have noticed, you'll get a deal on the drive, but it will cost
you to actually be able to use it! The more differential drives you buy to
use on your system, the better the deal. I have picked up a bunch of Quantum
AtlasII 9.1 gb drives for around $150 apiece at onsale.com.

Hope this helps. Good luck.

Regards, David Brenner

==============================================
David Brenner, Ph.D.                           [log in to unmask]
==============================================


-----Original Message-----
From: John D. Penberthy [mailto:[log in to unmask]]
Sent: Thursday, February 25, 1999 10:37 PM
Subject: Differential SCSI Drives?


I just bid on an IBM 9.1GB SCSI drive that specifies that it is a
differential drive.... What does differential mean?

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