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From: dyer@spdcc.UUCP (Steve Dyer)
Newsgroups: net.micro.pc
Subject: Re: PC/AT memory size/error; help needed
Message-ID: <235@spdcc.UUCP>
Date: Tue, 25-Mar-86 01:24:35 EST
Article-I.D.: spdcc.235
Posted: Tue Mar 25 01:24:35 1986
Date-Received: Wed, 26-Mar-86 04:45:28 EST
References: <227@spdcc.UUCP>
Organization: S.P. Dyer Computer Consulting, Cambridge MA
Lines: 35
Summary: problem solved; complicated explanation...

In article <227@spdcc.UUCP>, dyer@spdcc.UUCP (Steve Dyer) writes:
> In pirating my AT for a memory board to put in my AT clone, I find now
> that I cannot reboot the AT: I neglected to run SETUP from the diagnostic
> disk first to resize the CMOS RAM's idea of how much memory I have.
> Instead, the boot ROM sizes memory up to its new hi-water point, then
> reports a 201-- memory error, then reports a memory size error, then
> says "Run SETUP".  However, although the diskette then runs as if to
> boot from disk, the system then simply hangs.  The behavior is identical
> regardless of what copy of AT Diagnostics or DOS I load.

I relate the solution that others might avoid similar problems in the
future.  It is sobering and surprising to realize how subtle the problems
were, which had nothing to do with the amount of memory in the machine.
I was given a set of hard disk cables by the clone dealer; as it turned out,
I used the AT's original disk cables on the clone, because it had two hard
drives, and the AT's cables have two connectors.  This explains why the clone
came up and worked.  I used the spare cable which had only one connector on
the AT (which has only one drive.)

It turns out that the 'hanging' was the boot program waiting for the hard
disk.  The AT hard disk cable follows the IBM convention of TWISTING several
conductor pairs on the ribbon cable for drive C.  My single-drive cable was
NOT twisted, hence the hanging.  Things were further complicated by the fact
that my hard disk was lacking a termination resistor (it had previously been
used as a second drive) which also contributed to its misbehavior even after
replacement of the correct cable.

By unplugging the battery (and leaving it unplugged for > 5 minutes) I
could get the diagnostic program to boot.  Everything worked fine if the
# of hard drives was set to 0, and it hung as described above if the number
of hard drives was 1 or 2.  This led me to the problems with the hard disk.
-- 
Steve Dyer
dyer@harvard.HARVARD.EDU
{bbncca,bbnccv,harvard,ima,ihnp4}!spdcc!dyer