Fixing a DEC Letterwriter (LA100)

Tony Duell ard at p850ug1.demon.co.uk
Fri Jun 13 21:54:34 CDT 2008


> 
> 2008/6/14 Tony Duell <ard at p850ug1.demon.co.uk>:
> > I think the 2114 is the most unreliable chip I've ever seen. And it
> > doesn't seem to be any particular manufactuer that's to blame either. I
> > would certainly check these very carefully.
> 
> I whipped all the 2114s out today and popped in a set of IC sockets.
> Thinking they couldn't possible /all/ be bad, I got two replacements,

I wouln't put much money on that :-)

> and have experimented with different combinations of old and new. Very
> scientific, I know ;-) I thought it was worth a try before hacking up
> a test rig.

 
> 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' :-)

Incidentally, 2114s fail in different ways. I've had data bits stuck high 
or low, I've also had address lines ignored (so that the same data is 
repeated twice). If you're going to test them, you want to check them 
pretty thoroughly, not jsut make sure you can store something.

> 
> > I seem to have dumps of 147E4 and 148E4 here...
> 
> Was just looking for those! I'd sure appreciate a copy if you could
> mail them or somehow get them online. I might try my luck at tracing

I'll get them on my way to you...

> through a bit of code to see which conditions are causing the light
> show.

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.

-tony



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