Path: utzoo!utgpu!cunews!mitel!testeng1!stanfiel
From: stanfiel@testeng1.misemi (Chris Stanfield)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Terminator power: from SCSI cable or Power cable
Message-ID: <8064@testeng1.misemi>
Date: 28 May 91 18:56:00 GMT
References: <5440@dftsrv.gsfc.nasa.gov>
Reply-To: stanfiel@testeng1.UUCP (Chris Stanfield)
Organization: Mitel CAE Services
Lines: 27

In article <5440@dftsrv.gsfc.nasa.gov> jim@jagubox.gsfc.nasa.gov (Jim Jagielski) writes:
>
>I've noticed on some SCSI drives, like the 335MB Wren Runners (ST4385NM) that
>one can select whether you want the terminator power for that drive to come
>from the SCSI cable OR from the drive's own power cable. Now I know that Apple
>says NOT to have more than 2 sets of terminators or else you may overtax the
>line drivers on the SCSI chip, but what if you had these terminators powered
>via the power cord?
>

This isn't an Apple spec., it's part of the SCSI standard. You may not
have more than two terminators and they must be connected to opposite
ends of the bus, it doesn't matter where they are powered from. If you
have too many terminators, the bus impedance is wrong and the bus
drivers won't work properly. People will tell you they have had too
many terminators on and the bus worked - they were just lucky - it's
not guaranteed. The same thing applies to having only one terminator -
this will usually work if the bus is short. This mode may actually be
supported by the spec - I can't remember and we just moved and I can't
find my copy of said spec. Normally, term pwr is supplied via diodes
and is automatically supplied from the device rather than the host,
although the host will automatically supply power if the device is
turned off - that is what the diodes are for.

Chris Stanfield, Mitel Corporation: E-mail to:- uunet!mitel!testeng1!stanfiel
(613) 592 2122 Ext.4960
We do not inherit the world from our parents - we borrow it from our children.