Commodore 64 PLA (906114-01) Replacement

Commodore 64 PLA
If you are a little familiar with the hardware of the Commodore 64 you know that the PLA chip in the Commodore 64 is perhaps the most failing chip of the entire Commodore 64 today.
You will find references to these chips under the following references 906114-01, 251064-01 and the 82S100.

This PLA (Programmable Logic Array) is the custom chip that manages the memory map and memory bank switching.
Especially the chips that were produced in the years 1983 and 1984 suffer a lot from failure.

I once read somewhere that the chemical process that was used at the time to produce the chips, the main reason is that they now fall out (failed) in masses

The PLA and its modern Replacements

Since the MOS factories are long gone and there is no manufacturer that can still produce this technology, the community has been searching diligently for a way to replicate / emulate the workings of the logic that makes up this chip.


There are replacements based on a simple Eprom, there are replacements that use a high-end CLPD, each solution has its advantages and disadvantages.
One solution is a poorly (partly) functioning replacement that is said to destroy the entire Commodore 64 in the long term, the other solution is multifunctional, but then again has miniature islands and must be provided per machine / revision of (miniature) solder bridges.

Commodore 64 PLA Vervangers

Commodore 64 PLA Vervanger op basis van GAL's

 and then there's the brainchild of Daniël Mantione, who came up with a different approach for the PLA Replacement based on GAL chips.
And that approach, according to all kinds of tests, resulted in the 'most compatible' PLA replacement, without any known drawbacks.
In fact, it's so compatible that there's no need to do anything for any revision of the C64.
it has rightly proved to be a Drop-in replacement for all systems equipped with this PLA.
And with that it is THE replacement that has made the heart of many C64 (Lovers) beat faster🙂

Frequently Asked Questions :

Does Epyx Fastload work with PLA20V8
Yes, this one works fine.

Although the Epyx Fastload cartridge has a questionable design to say the least (Epyx playing analog tricks on lines that are digital).
In order for the Epyx to function smoothly, the PLA replacement needs the same analog properties as the original, the GAL20V8B has the same properties.

Does the PLA20V8 work in all C64 motherboards?
Yes, except for the shortboard, it uses a different kind of chip.

The shortboard (assy 250469) uses a completely different type of chip.
The following boards have been successfully tested, Assy : 326298, 250407, 250425, 250466, 251103 (SX64), and C64 Reloaded MK1.
Assy : 250441 and KU-14194HB have not yet been tested, but in all likelihood no compatibility issues are to be expected there either. For all tested boards, both the PAL and NTSC versions have been tested.

Can I also use alternative kernals with the PLA20V8 ?
Yes.


Alternative KERNALs can really put the PLA to the test, especially KERNAL cartridges, which change the C64 mode twice per half cycle and thus require a very responsive PLA.
This has been tested with the 1541 Ultimate II, and the Easyflash 3 cartridge and works flawlessly.

Does Super-Zaxxon work with the PLA20V8
Yes.

Yes it works without any problems!
Super-Zaxxon switches between address banks by calling the address lines of the Commodore 64 therefore disturbances on the address lines of the Commodore 64 can cause inadvertent switching of address banks.

 

This problem mainly occurs with PLA replacements based on EPROMs, which are known to cause disturbances during switching during address changes.
The GAL20V8 works with an AND array and an OR array, similar to how Commodore's original PLA was designed, so this disruption is unlikely, which is why the Super-Zaxxon cartridge works perfectly.

Is it already available?
Yes.

Price : 15 euros

Commodore 64 GAL PLA Close-up

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