Back in the days of old, computers used EPROMs to store their most vital data – usually character maps and a BASIC interpreter. The nature of these EPROMs meant you could write to them easily enough, but erasing them meant putting them under an ultraviolet light. Times have changed and now we have EEPROMs, which can be erased electronically, and Flash, the latest and greatest technology that would by any other name be called an EEPROM. [Nicholas] wanted an alternative to these 27xx-series EPROMs, and found his answer in supercapacitors.
[Nick]’s creation is a mostly non-volatile memory built around an old 62256 32k SRAM. SRAM is completely unlike EPROMs or Flash, in that it requires power to keep all its bits in memory. Capacitor technology has improved dramatically since the 1980s, and by using a supercap and one of these RAM chips, [Nick] has created a substitute for a 27-series EPROM that keeps all its memory alive for days at a time.
The circuit requires a small bit of electronics tucked between the EPROM socket and the SRAM chip; just enough to turn the 12 Volts coming from the EPROM programming pin to the 5 Volts expected from the SRAM’s Write Enable pin. This is accomplished by a few LEDs in series, and a 0.1F 5.5V supercap which keeps the SRAM alive when the power is off.
As for why anyone would want to do this when modern technologies like Flash can be found, we can think of two reasons. For strange EPROM sizes, old SRAMs abound, but a suitable Flash chip in the right package (and the right voltage) might be very hard to find. Also, EEPROMs have a write lifetime; SRAMs can be written to an infinite number of times. It’s not the best solution in every case, but it is certainly interesting, and could be useful for more than a few vintage computing enthusiasts.
This project makes us think of another where an LED may have been supplying keep-alive power to some volatile memory.