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From: toddpw@tybalt.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: APPLE HIGH SPEED SCSI Vs Ramfast
Message-ID: <1990Oct25.200430.28671@nntp-server.caltech.edu>
Date: 25 Oct 90 20:04:30 GMT
References: <0093EB2AE6574820.00000111@dcs.simpact.com>
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scottr@gnh-applesauce.cts.com (Scott Rothstein) writes:

>The need for terminators is questionable. I have a 105 meg Unimac hooked up to
>my GS, and no SCSI terminator is needed. (Course, when it first arrived in the
>mail, and there was no terminator included, I was a bit annoyed. I called the
>place I bought it from, and they said the new SCSI card didn't need
>terminators). The point is, the HD works....

SCSI termination is a black art. Terminators are essentially resistors that
provide a 'cushion' for the signals moving down the wire. They can take the
form of either external terminator packs or of 3 little resistor packs (these
look like little chips with one row of pins) inside the drive or on the SCSI
card.

The DMA SCSI has a terminator built into it. The RAMFast has a dip switch
selectable terminator built into it. Your HD most probably has three little
resistor packs inside it (my Quantum 105 did), i.e. it probably has termination
built into it too. You can remove the HD's terminators but you have to open the
drive case and get to the drive mechansim itself to do it.

The point here is, the need for EXTERNAL terminators is questionable. The need
for terminators in general is very important because you can have creeping data
errors (with no warning!) if your termination is questionable. 99% of the time
it will be painfully obvious that the termination is bad (i.e. GS/OS will crash
midway through booting, and not because of DMA problems).

The official word on SCSI busses and termination is -- two terminators, one at
each _end_. If there are only two devices (i.e. 1 computer and 1 drive) then
you can usually get away with only one terminator, although two are preferable.
You can also usually get away with three terminators on a three device chain,
however I wouldn't recommend it (I've had some questionable errors occur under
those conditions).

Hope this makes a reasonable amount of sense.

Todd Whitesel
toddpw @ tybalt.caltech.edu