Fixing a DEC Letterwriter (LA100) - test-drb@ccmp.vtda.org

16 Jun 2008

-----------ORIGINAL MESSAGE:
Date: Mon, 16 Jun 2008 11:06:36 +0200
From: "Steve Maddison" <steve at cosam.org>
Subject: Re: Fixing a DEC Letterwriter (LA100)

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.
-- 
Steve Maddison
http://www.cosam.org/
"The great thing about standards is that there are so many to choose from."

---------REPLY:

Various patterns of DSR and POWER/FAULT flashing are the diagnostic
failure codes; what pattern do you have that is not documented? 

Defective RAM is 8 flashes BTW.

You ran the ROM through a simulator??? There's an LA100 simulator?

m