Fixing a DEC Letterwriter (LA100)

Steve Maddison steve at
Mon Jun 16 15:23:39 CDT 2008

2008/6/16 M H Stein <dm561 at>:
> 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.

Yep, I figured that one out the long way. I only have some user docs
so this wasn't listed, just things like "out of paper" and "cover
open". I'm now back to the original CTS and ONLINE combination (no

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

If only! There are however 8085 simulators, so fed the RAM test a bad
value and the thing went into a loop. These LEDs are driven off a
couple of I/O ports so I could match the values written to the ports
to the LED combination from the schematic. CTS and ONLINE are however
mapped into memory at 0x6000 and I've not found any specific code
which would cause them to both illuminate, although there are several
candidates. So it could still be a hardware problem...

Steve Maddison

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