options for replacing failed small ROMs in PDP-11

Fritz Mueller fritzm at fritzm.org
Fri Jun 24 18:36:29 CDT 2016

Hi All,

In bringing up and debugging my PDP 11/45, I found that one of my GRA 
(M8101) spares has a failed ALU subsidiary ROM.  It's a pretty standard 
little 32x8 ROM in a 16-pin DIP, and the truth table is in the 11/45 
print set.

I wonder what the replacement options are for parts like these?  In 
particular, given the 30ns micro-cycle on the KB11-A, and the fact that 
the propagation time for the ALU downstream of this is roughly 20ns on 
its own, I'd be worried that an off-the-shelf bipolar PROM might be too 
slow here.

I'm still a little slow on reading the microcode flows, so its not clear 
to me exactly how many micro-cycles there are on the critical path for 
the E-class instructions where this ROM is used.  Maybe its not an issue.

Anybody every try replacing one of these with a bipolar PROM?  Any other 
suggestions for how to repair parts like these?


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