Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!apple!mattd
From: mattd@Apple.COM (Matt Deatherage)
Newsgroups: comp.sys.apple2
Subject: Re: What's wrong with my hard drive
Message-ID: <52529@apple.Apple.COM>
Date: 7 May 91 00:27:06 GMT
References: <1991May6.010943.5215@utstat.uucp> <52494@apple.Apple.COM> <1991May6.081109.14843@nntp-server.caltech.edu>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 68

In article <1991May6.081109.14843@nntp-server.caltech.edu> toddpw@nntp-server.caltech.edu (Todd P. Whitesel) writes:
>Matt, he said that his drive doesn't have internal termination -- if he
>removes it, one end of the cable won't be terminated at all. Sure,
>it will probably still work, but the SCSI spec explicitly states that
>both ends should be terminated.
>
You see why I didn't want to answer this question?  It turns into a holy
way _every single time_.  It's like potato chips - you can't answer it just
once.

Or, like happens here, someone (probably me) gets confused about the original
question somewhere along the line, or people interpret a response for a specific
situation as appropriate always, or...never mind.  Let's just deal with it.

There are four unique situations hinging on two boolean questions:
1)  Do you have a Rev. C or a High Speed SCSI Card?  (Non-Apple cards are NOT
    being discussed here.)
2)  Do you have more than one SCSI device on your chain?

(We will assume that all drives have no internal termination.  If yours does,
it counts a drive with a terminator.)

The key points here are that the SCSI Specification says the bus should be
terminated on both ends, but a) the High-Speed Card has a terminator on-board
and the Rev. C card does not, and b) short chains or slow throughputs can often
let you bend this slightly with no ill effects.  So...

IF YOU HAVE A REV. C CARD AND ONE SCSI DEVICE:

You need one terminator.  (Since the Spec says you need two and the card has
none, you'd think you'd need to monkey with this somehow, but the SCSI chain
is so short and the throughput slow enough that one terminator works just
fine.)

IF YOU HAVE A REV. C CARD AND MORE THAN ONE SCSI DEVICE:

You need a terminator on the first device and a terminator on the last
device (just like the Spec says).

IF YOU HAVE A HIGH-SPEED SCSI CARD AND ONE SCSI DEVICE:

You can use zero or one terminators.  (Since the card is terminated, you
should need a terminator on the device, according to the Spec, but the
SCSI bus is usually so short that the lack of another terminator causes no
problems.  If you have a longer cable or if no terminator is not working,
try adding one.)

IF YOU HAVE A HIGH-SPEED SCSI CARD AND MORE THAN ONE SCSI DEVICE:

You need one terminator on the last device.  Adding more terminators could
cause problems (the card is terminated).

------------------

Note that in all cases one of the SCSI devices must supply termination _power_,
since for either SCSI card to do so would seriously overuse the specified
power per slot in the system.

Questions?  Keep 'em to yourself.  If anyone around you asks these questions,
print this out and nail it to their forehead.  Ten-penny nails work best.

(yes, that last paragraph was a joke.)
-- 
============================================================================
Matt Deatherage, Developer Technical  | The opinions expressed herein are
Support, Apple Computer, Inc.         | not those of Apple Computer, and
Personal mail only, please.  Thanks.  | shame on you for thinking otherwise.
============================================================================