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adrian_b 4 minutes ago [-]
> I suspect that diode-transistor logic was used instead of TTL for the output multiplexer because the simpler DTL circuit worked better with the high programming voltages. The DTL circuitry uses very large resistors biased to the programming voltage.
Yes, it was necessary to use DTL instead of TTL, because a TTL gate is equivalent with a DTL gate where the diodes are base-emitter diodes of bipolar junction transistors, but the base-emitter junctions of BJTs have a very low breakdown voltage, higher than the +5 V power supply of TTL ICs, but lower than the programming voltage required by PROMs (typically +12 V).
Therefore TTL gates would have been destroyed by the programming voltage.
laughing_man 1 hours ago [-]
Yeah... I remember designing these into circuit boards. They were much cheaper than EPROMs, so we did development with EPROMs and production with PROMs.
And then we desoldered and replaced all the PROMs when we found a problem.
ggm 5 hours ago [-]
I remember knowing about these at uni and then learning about reprogrammable UV erased PROM and thinking the same magic faries who do the link fusing probably had tiny soldering irons to fix them in the reprogrammable ones (we said if you let the magic fairy smoke escape from your circuit board it wasn't surprising it stopped working...)
sage981 5 hours ago [-]
$70 for 1024 bits is the part that lands. The 33x33 array with an extra row and column is a neat wrinkle.
Yes, it was necessary to use DTL instead of TTL, because a TTL gate is equivalent with a DTL gate where the diodes are base-emitter diodes of bipolar junction transistors, but the base-emitter junctions of BJTs have a very low breakdown voltage, higher than the +5 V power supply of TTL ICs, but lower than the programming voltage required by PROMs (typically +12 V).
Therefore TTL gates would have been destroyed by the programming voltage.
And then we desoldered and replaced all the PROMs when we found a problem.