Path: utzoo!censor!geac!jtsv16!uunet!cs.utexas.edu!usc!gem.mps.ohio-state.edu!lavaca.uh.edu!uhnix1!moray!siswat!buck
From: buck@siswat.UUCP (A. Lester Buck)
Newsgroups: comp.sys.ibm.pc.rt
Subject: Re: What is "differential" SCSI?
Message-ID: <471@siswat.UUCP>
Date: 10 Nov 89 08:16:37 GMT
References: <9037@spool.cs.wisc.edu> <1989Nov6.192632.9113@arnor.uucp> <509@ursa-major.SPDCC.COM>
Organization: Photon Graphics,  Houston
Lines: 44

In article <509@ursa-major.SPDCC.COM>, dyer@spdcc.COM (Steve Dyer) writes:
> Can someone here tell me what "differential SCSI" is, and how it's
> different from the SCSI I'm used to on Macs, {DEC,VAX}station 3100s,
> Adaptec 1542s, and Suns?

The physical SCSI bus comes in two flavors - single-ended and differential.
Single-ended is much more common.  Single ended has each signal on a single
wire, with all the extra lines as grounds.  Differential uses two lines per
signal, with a balanced plus and minus signal.  Differential interfaces have
much better noise immunity due to elimination of common mode noise.
According to the SCSI standard, differential cables can be up to 25 meters,
while single-ended cables are limited to 6 meters.  This allows significant
distances between the drives and the adapter, as in a computer room
environment.  The disadvantage of differential SCSI is that the driver chips
are quite large, relatively expensive, and take a lot of power, so they
have not been integrated onto the SCSI protocol chips common today.
Differential and single-ended are not compatible on the same bus, though
most differential implementations have circuitry to detect single-ended
devices and shut down the differential drivers before they are damaged.
Rancho Technology sells a bi-directional converter box for about $150 that
can connect the two flavors, if you must mix interfaces.

IBM was somewhat odd in choosing a differential SCSI implementation for
their adapter.  The 9332 SCSI disks (are these called Athens?) are also
differential drives.

> We've got lots of IBM Athens 400mb drives, some of which would be nice to
> attach to our {DEC,VAX}station 3100s, and some people are asking about
> attaching DEC RZ53s to the IBM SCSI controller (yes, I know that the
> AOS 4.3 SCSI driver seems wedded to the weirdness of the Athens drive--
> I'm concerned with things I have no control over like hardware
> incompatibility, and not with presumably tractable driver problems.)

I would suggest just the opposite.  The whole SCSI architecture has removed
virtually all of your hardware incompatibility problems, but software can be
a nightmare.  As I have said before in this newsgroup, I have experience
with a number of SCSI implementations, and the IBM RT version is some of the
worst hardware and the worst software (AIX/VRM) that I have ever run across.
Maybe things are a little better when you have full source for the drivers.
I hope IBM gets it together with the MicroChannel SCSI adapter that they
will soon announce.

-- 
A. Lester Buck		...!texbell!moray!siswat!buck