options for replacing failed small ROMs in PDP-11

Fritz Mueller fritzm at fritzm.org
Fri Jun 24 21:41:17 CDT 2016

Thanks for the info, Don -- learning a lot about this stuff as I go...

I had wondered if the part might have been a mask ROM rather than a 
PROM.  And wrt. timing, I was certainly mistaken to call the nominal 
interval between the clock pulses a microcycle.

So after staring at the flows and prints a little more closely, it looks 
to me now like the IR will be latched at FET.10 t6 (which is really 
IRD.00 t1?) then there is the rest of intervening IRD.00 during which 
time control signals can propagate to and through decode logic and the 
subsidiary ROM and ALU, then the ALU results are latched into the 
shifter at EXC.80 t2 or EXC.90 t2.  So that's a solid 150ns there minimally?

 From the prints, it looks like this is an open-collector part -- I 
don't see it called out, but the chip select is wired active and I can't 
see that the outputs have any other drivers.

So that's good news for repairing my board!  Which brings on the next 
question: do folks here have a recommendation for a good programmer to 
try and track down on eBay for programming these sorts of parts?


On 06/24/2016 06:28 PM, Don North wrote:
> 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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