PDP-11/05 microcode dump?

Mattis Lind mattislind at gmail.com
Sat May 7 01:45:53 CDT 2016

> For the data paths board (M7260), both major versions appear to contain the
> same PROMs (going by the DEC part numbers), but the chip location (Exx)
> numbers are all different.
> For the control board (M7261), the C, E ('early' version) and F ('late'
> version) etch revisions each contain mostly the same PROMs, but apparently
> with slight differences between the sets of PROMs in each (as reflected in
> different DEC part numbers). For details see:
>   http://gunkies.org/wiki/PDP-11/05#Control_PROMs

Thanks. The change of PROM on etch F also explain the note  "Aux control is
asserted one micro step earlier..." on page 91 in the schematic on

Now I am convinced that if I program a new A04A2 PROM the machine should
behave much better. The interesting is when comparing the bad chip with the
list that approximately half of the nibbles are OK. Then the rest has
mostly one or two changed bits.

What are the failure modes of PROMs?

BTW. The failing chip was yet another NS chip in plastic


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