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From: chai@hawk.cs.ukans.edu (Ian Chai)
Newsgroups: comp.sys.mac.hardware
Subject: Solution found to SCSI problem
Summary: Problems connecting multiple SCSI devices resolved in odd way.
Keywords: SCSI hard disk
Message-ID: <1991Mar22.014907.15725@hawk.cs.ukans.edu>
Date: 22 Mar 91 01:07:58 GMT
Organization: University of Kansas Computer Science Dept
Lines: 56

(By the way, does anyone know how to utilize the "Keywords" and "Summary"
fields?)

Anyway, I recently found a solution to my SCSI connection woes and so
I thought I'd share it on the net in case anyone else had this problem.

I have a IIx with the Apple's standard 40meg internal, and a Mac!Direct!
135 meg external, a Ruby Systems TX150 TEAC Tape backup drive, and an
old, defunct, Ehman Engineering 30meg.

When the Ehman drive died, I bought the Mac!Direct! to replace it, and
then found I had a problem: The Ehman had used Mac-like SCSI connectors,
while the MD uses standard shielded SCSI. "So, what?" you might ask,
"Didn't they ship the corresponding cable with it?" Well, yes, but
remember my Tape drive? Right: it has standard SCSI, so I now needed
a standard - standard, rather than another mac-mac.

Well, as a temporary fix, I hooked up the old, dead Ehman in the middle:
Mac :==| Tape |==: Ehman :==| MD ]

: Mac-type SCSI
| Std-type SCSI
] Terminator.

That worked fine. The MD is externally terminated, while the Ehman and
Tape are both "automatically terminating". (How *do* they do that,
anyway?)

But when I got the proper std-std SCSI connector last night, and took
out the Ehman, thus:
Mac :==| Tape |==| MD ]
It didn't work! I tried it the other way and:
Mac :==| MD |==| Tape
And it still didn't work. (Note: didn't attach termination, as the Tape
was "automatically terminating" and, in fact, had worked fine with the
Ehman in that configuration, before Ehman died.)
What worked was
Mac :==| MD |==| Tape ]
!!! So why did
Mac :==| Tape |==: Ehman :==| MD ]
work? And, in fact,
Mac :==| MD |==: Ehman :==| Tape
Mac :==: Ehman :==| Tape
and
Mac :==| Tape |==: Ehman
have all worked in the past... I think it has something to do with
Ehman's "internal termination" somehow doing the termination in the
Mac :==| MD |==: Ehman :==| Tape
case despite being in the middle.

Ian Chai
chai@cs.ukans.edu
2fntnougat@ukanvax.bitnet
-- 
Ian Chai        Internet: chai@cs.ukans.edu
                Bitnet:   2fntnougat@ukanvax