Path: utzoo!attcan!uunet!mcsun!hp4nl!fwi.uva.nl!dolf
From: dolf@fwi.uva.nl (Dolf Starreveld)
Newsgroups: comp.sys.mac.hardware
Subject: Re: A Miraculous Recovery
Message-ID: <1154@carol.fwi.uva.nl>
Date: 23 Jul 90 22:32:27 GMT
References: <40908@think.Think.COM>
Sender: news@fwi.uva.nl
Lines: 42

ephraim@think.com (Ephraim Vishniac) writes:
> A few weeks ago, I was complaining to anyone who would listen that the
> internal disk in my shiny new IIfx was useless.  If any external SCSI
> devices were connected, even in a 100% kosher configuration, the
> internal drive had intermittent read and write errors.  My Apple
> dealer was here twice to fix it, without any success.  Use of
> different terminators, filters, cables, and disks had no effect. 

> Last Friday, I fixed(?) the problem by accident by adding *another*
> external drive and making the external bus longer. The failing
> configuration included an Apple CD-SC, with or without a CDC Wren III
> disk.  Adding another Wren III, for a total of three external devices,
> seems to have cleared the problem completely.

> When failing, the internal drive would report write errors every
> couple of megabytes as I copied files onto it.  Attempts to run
> "successfully" copied applications from the internal disk usually
> ended in disaster.  After adding the new external disk, I copied about
> 60 megabytes of files onto the internal without any problems at all. I
> fired up SpInside Mac from the internal and it ran beautifully. 
This still sounds like a classic termination problem. Even though termination
may be theoretically right, transmission line theory tells you that the
so called stub length should not exceed a given length (some ilke 5"). Some
disk packaging companies do their internal cabling within the box wrong, making
it possible to exceed this limit (without you knowing so) depending on
whether you connect an incoming cable to connector 1 or 2. Also, there
is a specified minimum distance between two stubs, which again you may
have exceeded.
This may all sound a bit far fetched, but believe me I have a lot of experience
in the SCSI game (I seem to remember you did too. Didn't you write the
original Jasmine drivers?) and things like this do happen. The faster
hardware gets, the more sensitive it is. I still remember all the tricks I
could pull of with a MacPlus, a piece of flat cable with multiple connectors
on it to hookk up multiple devices. No longer so!

--dolf

--
Dolf Starreveld  Phone: +31 20 592 5056/5022 (FAX: 5155), TELEX: 10262 HEF NL
EMAIL:           dolf@fwi.uva.nl (dolf%fwi.uva.nl@hp4nl.nluug.nl)
SNAIL:           Dept. of Math. and Computing Science, University of Amsterdam,
                 Kruislaan 409, NL-1098 SJ  Amsterdam, The Netherlands