> 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.
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.
I don't miss those days. Constantly having chips in the eraser while you debugged the running version on the board and then finally programming an OTP (One-Time Programmable) device when you were ready to ship. I remember Parallax had some development devices that were a battery-backed static RAM with a cable that plugged into an EPROM socket. Faster to program and no eraser needed.
Still, when cheap Flash came along in the mid-90's, it was a godsend.
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...)
The first computer I used (Sinclair Mk14) featured two fuse programmable ROMs. Fwiw the article says PROMs were replaced by EPROMs in the 1980s which is true but only for memory applications. PALs (programmable array logic, also made by MMI), which were the bread and butter of random logic design through the 1990s afaik also used fuse programmability. Eventually those were replaced by more dense CMOS chips: FPGAs and so on that use either EEPROM or RAM programming. There is also a factory programming technique used on large chips where fuses are "laser zapped" to lock/unlock functionality, set limits on clock speed and seal encryption keys, which I think is still in use today.
> 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.
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.
I don't miss those days. Constantly having chips in the eraser while you debugged the running version on the board and then finally programming an OTP (One-Time Programmable) device when you were ready to ship. I remember Parallax had some development devices that were a battery-backed static RAM with a cable that plugged into an EPROM socket. Faster to program and no eraser needed.
Still, when cheap Flash came along in the mid-90's, it was a godsend.
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...)
If you let the magic smoke out, you can't put it back in! You can only put the magic smoke in at a fab, in a cleanroom!
The first computer I used (Sinclair Mk14) featured two fuse programmable ROMs. Fwiw the article says PROMs were replaced by EPROMs in the 1980s which is true but only for memory applications. PALs (programmable array logic, also made by MMI), which were the bread and butter of random logic design through the 1990s afaik also used fuse programmability. Eventually those were replaced by more dense CMOS chips: FPGAs and so on that use either EEPROM or RAM programming. There is also a factory programming technique used on large chips where fuses are "laser zapped" to lock/unlock functionality, set limits on clock speed and seal encryption keys, which I think is still in use today.