PLA Replacement

  • Many Commodore 8-bit computers implement the Signetics 82S100 FPLA, commonly known as a “PLA” chip by owners. This is a one-time programmable logic device that was programmed by Commodore. This part is one of the most common failures for all computers that use it.

    Unfortunately, the 82S100 has not been produced in quite some time and finding a device programmer that supports it is also difficult. Without a drop-in replacement available, users began seeking an alternative. Some have reconstructed the logic equations and building replacements using more modern programmable logic parts. Others have attempted to replace the PLA with an EPROM. Both methods have had varied success.

    EPROMs are available with much faster access times than were available when the Commodore computers were produced. It is now possible to obtain 45ns EPROMs with timing characteristics similar to 82S100. In June 2008, I purchased some AM27C512-45 parts and began experimenting with them as a replacement for the 82S100. I was able to successfully repair my CBM 8296D using this method and have since used the technique on several other models.


    The 82S100 and 27C512 have similar pinouts but are not compatible. Several pin swaps must be made before a 27C512 can be installed in a socket intended for an 82S100. This could be done by producing a small daughterboard or by hand-wiring.

    To save the cost, I came up with a simple technique of modifying a standard 28-pin DIP socket. The 27C512 is installed on this socket, which is then installed into the computer’s socket intended for the 82S100.

    The first step is to solder four wires onto the socket. I used 30 AWG Kynar wire, the same type of wire that is typically used for wirewrapping. Four pins on the EPROM are then bent out slightly and the EPROM is installed on the socket. Finally, the wires are tacked to the EPROM.