How to Lower Your PLA: The PLA264

In my previous post I described in detail how I created a ROM replacement for the Commodore Plus/4 computer and how I made sure that it would fit into the very tight space of the closed case.

Five in a row: The four ROM chips of the Plus/4 to the left and the PLA to the right.

The four ROM chips of the Plus/4 are sitting all in one row, and then there is a fifth IC right next to them: the 251641-02 PLA. This PLA chip is the same for all TED based Commodore computers, the C16, the C116, and the Plus/4. In the past, I used the excellent PLA16V8 from D. Mantione as a replacement. The PLA16V8 is quick to build from cheap parts that are easily available. And for a C16 with it’s roomy case, it’s a perfectly adequate solution.

The original PLA16V18 is much too tall to fit under the keyboard of the Plus/4.

With the Plus/4, however, it’s all about space again: There are only a few millimeters between the original PLA and the keyboard above it. So while the PLA16V8 works fine on a Plus/4 motherboard, it is far too tall to fit inside the Plus/4’s case.

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The XCPLA – Yet Another PLA Replacement

I’ve covered the PLA and the question how to replace a broken one before. First, when I repaired one of my C64 from back in the day, needed a quick replacement, and used an EEPROM on a simple adapter board. In a follow-up post I used a simple circuit suggested in a talk by Eslapion to test this PLA replacement for glitches and found plenty. Last but not least, I created a replica of the Super Zaxxon cartridge also to be used for testing the PLA. There is a lot more information online on this topic like the excellent Post on Breadbox64.

The broken PLA from a C64 mainboard.

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