2016-05-09 11:29 GMT+02:00 Christian Corti <[email protected]>:
> 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 directions. 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 /Mattis > > > Christian >
