RAM and PROM Board for
Early S-100 Systems

A 64K RAM/32K PROM Board With Very Flexible Memory Space Allocation

The FLEX64K board is designed for use in early S-100 systems in which memory space fragmentation can make it difficult to expand RAM and PROM as you might need. This board provides up to 64K of RAM and up to 32K of PROM. The PROM is implemented in EEPROM, however, the write enable line is normally disconnected, so you don't have to worry about accidental EEPROM write cycles.

In the top 16K of memory space (C000-FFFF), RAM and PROM can be enabled/disabled in completely independent 1K blocks. This allows working around fragmentation due to PROM, scratch pad RAM, memory mapped I/O, etc., that often breaks up this range of memory space.

From 8000-BFFF, RAM and PROM can be enabled/disabled in independent 4K blocks.

From 0000-7FFF, RAM can start on any 1K boundary and extend from that address through 7FFF. This allows keeping small RAM boards in your system (e.g, 1K, 4K) and having the FLEX64K board fill above those boards with RAM.

Phantom support includes blocking only reads during a Phantom cycle, blocking both reads and writes during Phantom, or ignoring Phantom.

Switches allow swapping the upper and lower half of the 32K EEPROM. This allows swapping between two 16K PROMs with the flip of a switch and running two completely different configurations without having to swap EPROMs or reprogram a part. Swapping halves can also solve programming issues when the address range to be programmed conflicts with PROM or hardware present in the system used for programming.

A switch connects the write line to the EEPROM to allow programming of the EEPROM. A utility for programming the EEPROM is accessible in the "Resources" section below.


Watch the FLEX64K in action

Ordering Information

The FLEX64K board is available as a bare board only. It is an easy board to assemble.

The cost of the bare PCB is $35 plus shipping. To order, contact me at the phone number or email address shown at the bottom of the page.


Links to Manuals, Software, and More

Documentation and Utilities


(972) 429-0947
Mike Douglas


deramp5113 followed by ASCII 0x40 followed by the Yahoo domain name.

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