Some time ago I needed to program an MCM68766 EPROM to upgrade the BIOS in an IBM PC 5150, and in the process discovered that hardware which can program these is rather difficult to come by.
So I set out to build my own. Knowing that the pin-out for 2708 EPROMs was quite similar to MCM68766 I thought I’d tick that off while I was at it, as this is another type that is very difficult to find hardware to program.
Why is MCM68766 difficult to program?
There is nothing algorithmically complicated about programming one of these – the main difficulty centres on the high Vpp voltage (26V). Most cheap EPROM programms have a max Vpp of 13V. That having been said, there are some cheap programmers which support the 2716 EPROM, also needing a Vpp of 26V, so quite why MCM68766 is such a difficulty, may also depend on other things, such as pin arrangements, or perhaps just limited demand for it.
Why is 2708/2704/TMS2716 difficult to program?
This type of EPROM is genuinely more difficult to deal with. It also needs the same high Vpp voltage, as well as an additional cocktail of voltages. +26V, +12V, +5V and -5V being the full list. All of these are required during programming – quite a headache for the designers of universal programmers. Models that do support it, require a special adapter which doesn’t come cheap. This adapter typically supplies the extra voltages which aren’t practical to provide through the universal socket. Like the MCM68766 – they are algorithmically simple to program, so no complicated software stuff to worry about.”