8/1/2026 at 6:57:37 AM
> 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.
For some years, after the older DTL logic circuits were mostly replaced by faster TTL logic circuits, which had a +5 V power supply, there remained available some DTL ICs that were intended for industrial applications in noisy environments, where speed was less important than immunity to noise, so those DTL ICs used higher supply voltages, e.g. +15 V, for which TTL circuits could not be made.
Eventually, those high-voltage DTL circuits were replaced by the RCA CD4000 CMOS logic circuit series, which could use the same high supply voltages while consuming much less power.
Decades later, the cost of regulated power supplies decreased by orders of magnitude, so it became possible to ensure much less conducted noise for the electronic devices used in industrial environments, which mostly eliminated the demand for high-voltage logic circuits, allowing the use of modern microcontrollers.
by adrian_b