options for replacing failed small ROMs in PDP-11

Jon Elson elson at pico-systems.com
Fri Jun 24 22:10:24 CDT 2016

On 06/24/2016 06:36 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?
Well, you could use a scope to find out what the good ROMS 
do for access time.


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