J-PLA, C64 Replacement PLA

Commodore 64s have a PLA (Programmable Logic Array) chip, which creates for example chip select signals based on various input signals. Chip select signals control which component is connected to data bus.
Originally Commodore used Signetics 82S100 PLA-chip, but later they (MOS) started manufacturing compatible chips in their own factory.
PLA chips found in C64's can have numbers like 82S100, 93459PC, 7700, 8700-001, 906114-01, 251064-01.
All of these chips produce the same logic, but signal levels and timings vary.

PLA is one of the most commonly failing parts in Commodore 64.
Symptomps of the failure vary from black screen to minor instability.
Failed PLA can cause for example garbage on screen, screen color problems and problems with cartridges.
"The Pictorial C64 Fault Guide" can help you with the fault diagnosis: http://derbian.webs.com/c64diag/

Original PLA's run hot and partly because of that, they fail quite often.
One can replace a failed PLA with an original one (or recently programmed NOS 82S100 or PLS100), but a longterm option is to replace PLA with a modern CPLD-based alternative.

J-PLA reproduces the original logic completely and it's timing and voltage levels makes it very compatible with original PLA (no incompatible hardware or software found yet). J-PLA runs cool, draws less current than original and it is based on
a very reliable Xilinx CPLD chip.

This is my hobby project and my contribution for keeping Commodore 64's alive!