Fixing a DEC Letterwriter (LA100)
steve at cosam.org
Mon Jun 16 04:06:36 CDT 2008
2008/6/14 Tony Duell <ard at p850ug1.demon.co.uk>:
>> Anyway - the original problem is gone. With the (half) new RAM I'm now
>> getting what at least appears to be a proper fault indication on the
>> LEDs: both DSR and POWER/FAULT are flashing. This combination isn't in
>> the user manual so my guess is it's failing the POST. If any of the
> Maybe soemthing like 'RAM Error' :-)
That was my first guess too. Running the ROM through a simulator and
forcing the RAM test to fail confirms it. The problem? I'd managed to
gouge out a bit of track while removing the ICs - just so happens it
was one of the very tracks connecting /all/ the RAM to the address
bus, so checking for continuity between the sockets I'd soldered in
didn't reveal the problem.
So I fixed that and robbed a couple more 2114s from another machine.
So all RAM is replaced but, lo and behold, ONLINE and CSR are back
with a vengeance. So I'm basically back to square one, although not
entirely as it would at least appear the RAM is OK. It could of course
still be failing in one of the peculiar ways you mentioned and still
pass the POST... Otherwise it would appear the original LED
combination was directly asserted by the processor, so it should be
traceable in the ROM code.
>> > I seem to have dumps of 147E4 and 148E4 here...
> I'll get them on my way to you...
Many thanks! You may also want to send them on to Pete Turnbull for
inclusion in his DEC ROM archive.
> IIRC, there is a custom chip in this machine, which sits on the processor
> bus. I ahe no idea what the registers in that chip do, so any read/write
> access to that is essentially unknown.
I've not come across it yet; let's hope it stays that way ;-)
"The great thing about standards is that there are so many to choose from."
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