options for replacing failed small ROMs in PDP-11

Brent Hilpert hilpert at cs.ubc.ca
Fri Jun 24 22:36:33 CDT 2016

On 2016-Jun-24, at 7:41 PM, Fritz Mueller wrote:
> 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.

> 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?

Many/most of the common bipolar fusible proms are Schottky class, so are quite fast.
Take a look at  74S188 / 74S288 as a starting point. Down in the 20-30ns range.

> 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.

Are there pull-up resistors anywhere along the output/data lines? If not, it is more likely a tri-state device.

> 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?

Probably more than you want to bother with, but blowing fusible proms generally isn't all that difficult, I've hacked a burner on a breadboard with an RPi (substitute other microcontroller as desired), 2-3 common TTL ICs and a few transistors.

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