Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!ames!coherent!dplatt
From: dplatt@coherent.com (Dave Platt)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Double termination on a //ci ??
Message-ID: <49073@coherent.coherent.com>
Date: 9 Mar 90 18:28:22 GMT
References: <-531597662@cpl-mfh.UUCP>
Reply-To: dplatt@coherent.com (Dave Platt)
Organization: Coherent Thought Inc., Palo Alto CA
Lines: 55

In article <-531597662@cpl-mfh.UUCP> martya@cpl-mfh.UUCP (Marty Adelman) writes:

>                           ... He called up Rodime tech support again and
> they suggested to doubly terminate the SCSI interface, seems that the new
> higher speed SCSI was causing RFI? noise to bounce back and might be
> causing his problem.  He tried this and sure enough his problems went away.
> I have personally never heard of this problem, has anyone else heard about
> this and is the explanation plausible???

I've heard similar reports.  Yes, the explanation is entirely plausible.

The SCSI specification calls for the SCSI bus to be terminated precisely
twice... once at each end of the bus.  This ensures that the "not-busy"
voltage levels on the bus lines are correct, and that signals travelling
down the bus are absorbed by the terminators, rather than reflecting
back along the bus and interfering with later signals.

The majority of Macintosh hard-disk setups are actually _not_ in
conformance with the SCSI specification... they are terminated only
once, at the internal or external disk drive.  The Mac doesn't include a
terminator, even though the Mac is often the device at one end of the
bus.

Up until now (Mac IIci) we've usually been able to get away with a
singly-terminated bus... the reflections haven't been enough of a
problem to cause bus-jamming, I guess.  Apparently, the IIci's SCSI
interface is fast enough to be troubled by the bouncing signals in some
cases.

If you have a IIci and an external hard disk, you'd be best off to have
two terminators:  one attached to the external hard drive (either on the
drive controller or plugged into the drive case's SCSI port), and one at
the IIci end of the bus.  Unfortunately, the Mac does not use the
standard 50-pin SCSI jack... it uses a DB-25... and it's not easy to
find SCSI terminators that use DB-25 connectors.

I understand that one of the big Mac disk-sellers (MicroNet Technology,
I think) is making a terminator module which can be connected _inside_
the IIci case... it plugs into the internal 50-pin SCSI header, I think.
This is probably the best way to go, if you can get such a beast... it
places the termination at the near end of the bus, minimizing
reflections.

Putting a second terminator at the external drive case is a possible
alternative... it'll help regulate the voltage levels, but I don't know
that it would really do a good job of damping down reflections on the
bus.

It might help to use a _very_ short SCSI cable to the external drive
case... a 1' cable rather than a 2' or 3' cable.
-- 
Dave Platt                                             VOICE: (415) 493-8805
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