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.
