Newsgroups: comp.sys.mac.hardware.misc
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From: sje@mv.mv.com (Steven J. Edwards)
Subject: Re: Strange memory-related startup problems on a 7500
Message-ID: <Dps7Du.EnA@mv.mv.com>
Organization: MV Communications, Inc.
Date: Sat, 13 Apr 1996 03:37:06 GMT
References: <4klan5$d8k@wapping.ecs.soton.ac.uk>
X-Nntp-Posting-Host: mv.mv.com
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ajl@ecs.soton.ac.uk (Andy Lewis) writes:

>I'm getting confused. I've had my 7500 for four months now, and almost
>from the word go, I was been getting intermittent inconsistent startup
>problems.
>Its a 16 Meg 1 Gig 7500/100 with 256K L2 cache.

>It would crash 1/3 times in the early stages of booting, sometimes
>hanging before the happy Mac icon, most frequently hanging/crashing on
>the Welcome To Macintosh splash screen, but consistently booting OK
>when it got past that to the MacOS splash screen. It was much more
>likely to fail to start when booting from shutdown - if simply
>restarting there was very rarely a problem.

>I've tried a lot of things, including re-initialising the hard drive,
>and a clean install of the system. Diagnostic tools gave me an
>all-clear.

>I've now fixed the problem, it looks like, by moving the two 8Meg DIMMS
>into slots A1 & A2. I was expecting a slow-down from the loss of
>interleaving, but haven't noticed any change at all. But I recently
>read in a post that the first two DIMMS should always be in A4 & B4,
>without saying exactly why. Does anyone know if there's a problem with
>leaving the setup like it is? Or what the problem might have been in
>the first place?

Memory slots A4 and B4 are at the end of the memory bus and so DIMMs
in these slots serve as bus terminators.  A properly terminated bus is
needed to avoid signal reflection, a situation that often manifests
itself as random memory errors.

A PM7500 should always have at least two DIMMs and slots A4 and B4
should be filled for reliability reasons.

-- Steven (sje@mv.mv.com)

