Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!ames!decwrl!ucbvax!husc6!encore!wcarroll
From: wcarroll@encore.com (Mr. New Dad)
Newsgroups: comp.periphs.scsi
Subject: Re: SCSI bus termination
Message-ID: <127627@jake.encore.com>
Date: 3 Aug 90 18:06:28 GMT
References: <210@rusux1.rus.uni-stuttgart.de>
Organization: Encore Computer Corp, Fort Lauderdale, FL
Lines: 47

From article <210@rusux1.rus.uni-stuttgart.de>, by wschmidt%decefix@decefix.iao.fhg.de (Wolfram Schmidt):
> Another Question:
> 
> Many SCSI host adapters for the AMIGA have external Mac compatible DB25 SCSI
> connectors.
> If you have internal drives the SCSI bus is terminated at both ends. Good.
> If you have external drives ... Good.
> If you have both the bus has 3 terminators.
> The host adapter sits in the middle of the cable. Shouldn't the host adapter's
> terminators be removed? As of my knowledge all HA have them soldered in.
> 
> Will SCSI work with this 3 terminators?
> 
> Wolfram Schmidt
> wschmidt@iao.fhg.de

SCSI termination is not some magical item that must be there for the bus to
work. It is simply a circuit that is used to eliminate reflections of signals
up and down the cable. It will only do this (to any real effect) when it is
at the physical end of the cable.

Yes, it is possible to operate a SCSI bus with three sets of termination.
Even more than three. But this is not recommended. Each termination network
lowers the impedance to ground, which increases the current each driver
must sink when it drives the line low. Most SCSI drivers are not specefied
to drive over-terminated lines. This means that if you over terminate the
bus, you will prematurely age your drivers best case, or blow them out
worst case.

Summary: Don't overterminate unless there is no way around it.

Note: I tend not to pay alot of attention to termination and often run
overterminated busses. But I don't do this on any permanent setups, 
usually leaving a bus overterminated for only a day or two. I have also 
never seen a SCSI device burn out it drivers from any cause and have 
never seen a bus fail because of too many terminations.

I still do not recommend it and waive all responsibility if you choose
to imitate my foolish actions and end up with ruined drives.



-- 
William R. Carroll  (Encore Computer Corp., Ft. Lauderdale FL)
wcarroll@encore.com         uunet!gould!wcarroll
   "Dan Quayle gives underachievers a bad name."
                                  -- Bart Simpson