The chaotic shambles of EPROM voltages, from 2708 through 2764
Computer memory used to require a lot of power supply voltages. Manufacturers gradually adopted simpler power-supply schemes, but legacy parts continued to require multiple voltages throughout the 1980s. The original Apple ][ included a negative power supply, -12 volts, solely to operate memory storage devices like the 4K and 16K dynamic RAMs and 2708 ROMs in peripherals. Apple switched to single-power-supply components whenever they could. Even though the Apple //c was mostly a single-voltage computer, it still needed a -12v power supply for two storage devices: the comparator inside the disk drive and the ROM in the AY-3600 keyboard controller.
And that poses a hurdle for programming EPROMs from that era. EPROM manufacturers tried to simplify their power supply requirements, but they also evolved toward requiring elevated VCC above +5 volts during programming because it enabled them to reduce the programming pulse width by a factor of 10, 50, or 500!
- 2708 8-Kbit (1024 byte) required three power supply connections for ordinary operation, plus VPP=26 volts for programming. It also imposed the peculiar requirement to elevate the Chip Select pin to 12 volts during programming. The 2708 also required that all bytes be programmed in a single pass, without the ability to program individual bytes. Uncommon, but not rare.
- 2716 16-Kbit (2048 byte) represented the first transition, reducing the quantity of power supplies. Texas Instruments retained three-supply design in their TM2716, and imposed a further complication to elevate VCC=12 volts during programming. Other manufacterers switched to a single-supply design. When TI adopted a single-supply, they designated it TM2516 to distinguish it from the incompatible TM2716. Motorola manufactured both styles, using TI's part number for the three-supply version and a conventional Motorola MM2716 part number for the single-supply version. Most manufacturers gradually migrated from VPP=25 volts to VPP=21 volts, and much later VPP=12.5 volts.
- 2732 32-Kbit (4096 byte) marked a second transition, reducing programming voltage VPP. Parts initially used VPP=25 volts, just like previous generations of EPROMs. But they soon migrated to VPP=21 volts, and later to VPP=12.5 volts.
- 2532 32-Kbit (4096 byte) seems to represent TI's effort to continue the numbering they adopted with their TM2516 variant, using different chip-select level than the 2732 and programming voltage VPP=25 volts. Uncommon, but not rare.
- 2764 64-Kbit (8192 byte) increased the pin count from 24 pins to 28 pins. Most manufacturers had switched to VPP=21 volts, and later they migrated to VPP=12.5 volts.
- 2564 64-Kbit (8192 byte) is yet another extension to TI's 25xx series, with programming voltage VPP=25 volts. It squeezed 8KB of ROM into a 24-pin package to make it easier to adapt circuits from their TM2516 and TM2532.
- 68764 64-Kbit (8192 bytes) is a strange beast from Motorola, with programming voltage VPP=25 volts. In order to squeeze 8KB of ROM into a 24-pin package it omitted the separate pins for chip-select versus output-enable, which prevented the chip from implementing standby or low-power mode.
This spreadsheet summarizes the voltage requirements for various EPROM types in the 8-kbit to 64-kbit range:
Note that this table is nowhere near complete. I don't even have specifications for all the EPROMs I own!
Some additional weirdness emerged from the chaos: manufacturers sometimes built EPROMs to their competitor's specifications and labeled them with competitor's part numbers to distinguish them. Here's a pair of Motorola TMS2716L EPROMs on a board found inside a Commodore PET. That's right, Motorola parts using a Texas Instruments prefix!