PROM 4 (Discontinued)

IMSAI PROM 4

The PROM 4 board provides up to 4K of addressable Read-Only-Memory, utilizing the Intel 3702-1702 PROM devices. The board contains 256 bytes of memory for each 8702-1702 chip installed.

Address lines A0 through A7 are run directly to all PROM positions to select one of the 256 internal byte positions, while address lines A8 through A11 are used to select and enable one particular PROM position through 8205 decoders. Address lines A12 through A15 are jumper-selected to determine the board's enabling address.

The PROM 4 comes in two configurations.

• The PROM 4-4 provides the full 4K or EPROM storage in sixteen 1702A chips.
• The PROM 4-512 contains two EPROM chips that provide 512 bytes of PROM storage. The board may be fully populated, in 512-byte increments, up to 4K bytes by adding 512 Byte Memory Module (MM702-5) sets.
• The MM702-5 is a set of two EPROM chips to add 512 bytes of storage to a PROM 4-512 board.

Current Status

As of 2024, the PROM 4 board is no longer in production. The primary reason for the retirement is that the 8702A and 1702A EPROMs require a dedicated programmer (unlike modern ones like the 2716 and later). These chips are nearly impossible to find and command exorbitant prices.

Successors

There are no replacements currently in production. If you need an EPROM board for your system, you may find a bare board at S100 Computers.

Resources

Manual

• PROM 4 Rev-3 User Manual

Schematics

• PROM 4 Rev-3 Schematic

Spare Parts

While most of the parts on the PROM 4 are generic and commonly available (except the 1702A EPROMs), these parts may be somewhat difficult to source.