PDP-11/05 microcode dump?

Mattis Lind mattislind at gmail.com
Mon May 9 06:13:11 CDT 2016

2016-05-09 11:29 GMT+02:00 Christian Corti <cc at informatik.uni-stuttgart.de>:

> On Sat, 7 May 2016, Mattis Lind wrote:
>> What are the failure modes of PROMs?
> I had a PROM fail on the 11/45 CPU board. This PROM is responsible for the
> Conditional Codes handling. There was one output that had failed. I think
> that the output drivers may fail, because I was able to temporarily fix the
> problem by adding a pull-_down_ resistor to the (open-collector) output.
> That fix was unstable of course, and I ended up in replacing the PROM with
> a GAL (ugly fix because the pinout doesn't match well, and there is no
> space for a nice adapter solution)

In this case it was not a OC driver since it didn't affect all the
addresses in the memory. Nor was it a single bit due to NiCr fuse regrowth
since that should change the bit in one single direction as far as I
understand. In this case random bits, as it seems, were changed in both

Never seen this type of symptom before. But I have had all sorts of strange
problem with NS chips dated in the early seventies earlier.

Last time I had a PROM failure is what the internal address decoding that
failed which was easily seen as certain patterns repeated all the time


> Christian

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