Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!mailrus!wasatch!uplherc!esunix!jsnow
From: jsnow@esunix.UUCP (John Snow)
Newsgroups: comp.sys.apple
Subject: Homebrew SCSI pitfalls
Keywords: SCSI Termination
Message-ID: <1431@esunix.UUCP>
Date: 9 Aug 89 17:08:26 GMT
Organization: Evans & Sutherland, Salt Lake City, Utah
Lines: 27

Here is a word of warning to those of you building homebrew SCSI drives for the Apple II family.  APPLE VIOLATES THE SCSI SPECIFICATIONS for terminating the    SCSI bus!  The SCSI spec requires that the SCSI bus be terminated by 220/330
resistors at BOTH ends of the bus.  The Apple II SCSI interface card does
NOT terminate the cable.  Apple normally gets away with this violation by
supplying a very short SCSI cable to connect the SCSI interface card to the
first disk drive where the SCSI cable IS terminated.  This is poor design
practice but normally works.

I purchased a SCSI cable from Tulin along with a Tulin A-hive case and power
supply.  The SCSI cable Tulin sent me was 6 feet long -- much too long to
work when there is no termination at one end.  Sure enough, I spent a week
trying to figure out whether my drive or my interface card was bad because
things just weren't working.  After a really frustrating week of trying
different computers, drives, and interface cards and hooking a SCSI bus
analyzer up to my system, I decided that the long cable and lack of
termination was the problem.  I hacked the 6 foot cable down to 18 inches
and wired on a new connector and what do you know, it works!

What would it have cost Apple to terminate things properly, less than a buck
to put in the resistor packs and sockets to allow them to be removed if the
CPU was not at the end of the cable!


-- 
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John F. Snow                         UUCP: {ihnp4,decvax}!decwrl!esunix!jsnow 
Evans & Sutherland Computer Corp.      or:         uunet!utah-cs!esunix!jsnow
Salt Lake City, Utah                 AppleLink PE: JohnSnow    GEnie: J.SNOW2