PLA replacement for Commodore 64 (C64 & SX-64) Breadbin - 906114-01
Designed by Daniël Mantione.
A design so good in fact that we now often use these as a matter of course when refurbishing and futureproofing C64s.
Replaces both 906114-01 and the older N82S100N
Works with boards: 326298, 250407, 250425, 250466, 251103 and 251137
Does not work with shortboard 250469 - as this has a different chip design
Works with Epyx Fastload
Works with Super Zaxxon cartridge
Even works with modern C64 games like Sizzler, Super Mario and Darkness
Runs cooler than original PLA (no need for heatsinks and helps lower the temperature in the case). Perfect for replacing the original PLA.
Build details and more info available here.
PLA Replacement for 906114-01 Commodore 64 [PLA20V8]
How good is it?
Pretty good. Allthough I was amining at low-cost easy to build PLA, rather than building the best PLA in the world, the PLA has until now passed everything thrown at it. Meanwhile a lot of C64 boards have been tested, with perfect results and a lot of C64 software and cartridges have been tested. In this regard it is relevant the GAL20V8B exists in three different speeds: 10ns, 15ns and 25ns. The 25ns version is the best choice from a theoretical point of view, but with a small adjustment (discussed below) I was able to get perfect results with 15ns chips as well. 10ns chips have not been tested, but are likely too fast.
Does Super-Zaxxon work?
Yes, it does work perfectly! Super-Zaxxon switches banks by means of accesses to the address lines of the C64 and as such, noise on the C64 address lines can cause undesired bank switches. This is a particuler concern for EPROMs, which are known to cause switching noises on address changes. The way the GAL20V8 works, with and AND array and an OR array just like the original PLA, this kind of noise is not expected and indeed, the cartridge works perfectly.
Can I use an alternate KERNAL?
Yes! An alternate KERNAL cartridge is a real PLA torture, because a KERNAL cartridge changes the C64 mode two times within a half cycle and thus likes a responsive PLA. This has been tested both with the 1541 Ultimate II and with the Easyflash 3 and works perfectly.
Does it work in all C64 mainboards?
Yes, except for the short board assy 250469 of course, because it uses a completely different type of chip. Assy 326298, 250407, 250425, 250466, 251103 (SX64) and C64 Reloaded MK1 have been tested. Assy 250441 and KU-14194HB have not been tested yet, but from what I know about these boards, I am confident about compatibility. Both PAL and NTSC tests have been done.
Does Epyx Fastload work?
Yes, it works fine. The Epyx Fastload cartridge is a rather questionable design that performs analog tricks on digital lines. In order for the Epyx Fastload to work, a PLA needs to have similar analog properties and be stable at the border voltage between 0 and 1. This is the case for the GAL20V8B.