Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!rutgers!ub!csn!ltepper
From: ltepper@csn.org (Larry Tepper)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Solution found to SCSI problem
Keywords: SCSI hard disk
Message-ID: <1991Mar23.190342.523@csn.org>
Date: 23 Mar 91 19:03:42 GMT
References: <1991Mar22.014907.15725@hawk.cs.ukans.edu>
Organization: Colorado SuperNet Inc.
Lines: 66

In article <1991Mar22.014907.15725@hawk.cs.ukans.edu> chai@hawk.cs.ukans.edu (Ian Chai) writes:
>... 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.

I work for Compatible Systems.  We make Ether+, a SCSI<->Ethernet for the
Macintosh.  We also made QuickShare, a SCSI interface for the PC bundled
with software to let it talk to a Mac.  I have had lots of fun over the
last several years wasting time rearranging chains of SCSI peripherals.

I would be suspicious of the "automatically termination" in your tape
drive, especially considering that when you put a terminator on the
tape drive, things started working.  Perhaps only one of the two SCSI
connectors on the tape drive provides automatic termination, using a
mechanical widget to detect whether or not a cable is plugged in.  Either
the widget is broken or you plugged your cable into the wrong one.

Basically, here's what I've learned about SCSCI chains over the years
(or at least I think I've learned this :-):

	Sometimes, SCSI chains with > 2 terminators will work.

	Often, short SCSI chains with 1 terminator will work.

	Occasionally, long SCSI chains with 1 terminator will work.

	The faster the Macintosh, the lower the odds that the above three
	will work.

	Almost always, SCSI chains with terminators at each end of the chain
	will work.  Note that any Macintosh without an internal drive is not
	terminated.  They are almost ALWAYS at one end of the bus.

	Sometimes, apparently valid SCSI chains won't work in a particular
	arrangement, but rearranging devices fixes the problem.  Right now,
	for instance, my SCSI chain looks like this:

		[Internal disk==Mac IIsi==External disk==CD ROM==Ether+]
	
	If I switch the order of the external disk and the CD ROM, I can't
	access the external disk anymore.  SCSI Probe doesn't see it.

We've cooked up a three-way 25-pin connector for use on Mac's w/o internal
drives.  It plugs into the back of the Macintosh, and provides two 25-pin
female connectors.  You then plug two terminated halves of a SCSI chain
into it, and voila, the SCSI chain "almost always works".
-- 
Larry Tepper, Compatible Systems Corp.  303-444-9532, FAX 303-444-9595