options for replacing failed small ROMs in PDP-11

Christian Corti cc at informatik.uni-stuttgart.de
Sat Jun 25 04:16:00 CDT 2016

On Fri, 24 Jun 2016, Fritz Mueller wrote:
> 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.

Some years ago I had to replace one such PROM in my 11/45 that is 
responsible for the ALU conditional codes. Since the PROM contents is 
quite simple, I replaced that with a GAL with the corresponding logic 
equations. Not a pretty solution, but I can replace it with a new PROM 
when I get one.
A similar fault was in my RK05 "Cylinder Address and Difference" board, 
also changed the failed PROM with a GAL. The symptom was that the drive 
never "found" the desired track and instead, bumped the carriage to the 
inner stop.


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