options for replacing failed small ROMs in PDP-11

Don North north at alum.mit.edu
Fri Jun 24 20:28:30 CDT 2016

On 6/24/2016 4:36 PM, Fritz Mueller wrote:
> 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?
>         cheers,
>              --FritzM.
Almost 100% certainty the part already there is a small bipolar TTL PROM. What 
would you think it otherwise might be?

For a lot of these logic replacement applications DEC used the open collector 
version, but it might be tristate variation. Check schematic.

Also, the microcycle on the 11/45 (and 11/70 for that matter, basically the same 
design) is 150ns, not 30ns.

There are various clock timing pulses (tp1, tp2, etc) but the datapath / control 
unit microcycle is 150ns.


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