Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!coherent!dplatt
From: dplatt@coherent.com (Dave Platt)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Can the SCSI bus be terminated w/o powering up noisy device?
Message-ID: <47013@improper.coherent.com>
Date: 17 Feb 90 01:18:12 GMT
References: <17008@boulder.Colorado.EDU> <1990Feb15.204213.29062@chaos.cs.brandeis.edu>
Reply-To: dplatt@coherent.com (Dave Platt)
Organization: Coherent Thought Inc., Palo Alto CA
Lines: 32

In article <1990Feb15.204213.29062@chaos.cs.brandeis.edu> dan@chaos.cs.brandeis.edu (Dan Schwarz) writes:

> That's really odd - most of the SCSI devices I have worked with that are
> internally terminated, did not require power when the computer was operating.

Some drive-designers do a good job in this respect.  For example, the
Teac N50 streaming-tape drives have a very nice termination-drive circuit.
Their internal termination resistors normally accept power from the tape
drive, and also feed power out to the SCSI bus... but can also accept
termination power from the bus if the tape drive is powered down.

Other vendors are less generous.  I've seen a number of external drives
(e.g. the LaCie Cirrus) which would _neither_ provide power to the bus
when operating (so that external terminators could be used), _nor_
accept power from the bus for their internal terminators (so that the
terminators would work correctly if the device was powered off).
If you have a peripheral of this sort in your system, and use its internal
terminators, you MUST have it powered on at all times... otherwise it
clamps the bus, and nothing will work.  It's not always possible to
pull the device's internal terminators and use external ones, either...
these devices won't provide power to external terminators, and neither
(alas) does the Mac Plus.

The SCSI-1 specification forbids having powered-off devices attached to
the bus... so these "unfriendly" devices are not really in violation of
the spec, I think.  I'd much rather buy devices that have a robust
terminator-power circuit, however!
-- 
Dave Platt                                             VOICE: (415) 493-8805
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