Good luck repairing them old black screen long boards! However, if you like to tinker with the hardware, even the EPROM based versions seem to work for the average user and they are usually a little cheaper compared to the CPLD ones. But if you are into exploring the obscure and specific ways of your Commodore 64 where timing and other details really matters and you are using one of the older longs bords, the U17 PLAnkton is the safe choice. The easy drop-in-replacement would obviously be a CPLD based one. So, which PLA replacement do I recommend the most – a CPLD or an EPROM based? Well, it really comes down to how much effort you want to put into it. I tried the device in a few Assy 250407’s and I experienced no problems whatsoever.
The EPROM based PLA replacement should therefore be more than fine for the average user. However, I was told that it actually works with the Epyx FastLoad cartridge (I did not test this myself as I don’t have that particular cart…). KU14194HB) and the Super Zaxxon cartridge. This solution is not compatible with the Commodore 64 Version A long boards from 1982 ( e.g. Another solution is to use the 45ns Atmel AT27C512R EPROM with a PLAdapter PCB from Retro Innovations ( link). I got two of these EPROM based PLAs from a British buddy, mongooseman. The problem is that the chip is no longer produced. A suitable EPROM based solution could therefore be the M27C512-90B6 made by STMicroelectronics with a rating of 90 ns. The original Signetics 82S100 chips had a rating of 80ns. As written earlier, timing is crucial for a stable C64 system. The PLAdapter based PLA:Īnother approach for making a PLA replacement is using an EPROM. The replacement PLA costs about 16-23 Euros + shipping from Germany depending on the version/configuration. These are the Plethora of PLAs that are presented in this post… I got the U17 PLAnkton, Super PLA v3, The Ultimate PLA, Black Edition PLA and a ‘homemade’ EPROM based PLA from a friend. As there seem to be strong opinions on each of the approaches, this post will just exhibit some of the after market options to get a replacement PLA for a black screen Commodore 64. To do this, I gathered some of the available PLA versions (or parts to make my own PLA) that can be found online. In general terms, two approaches to create replacement PLAs have been presented. The first is CPLD ( complex programmable logic device) based while the other is EPROM ( erasable programmable read-only memory) based. In this respect, certain timing and electrical contraints must be meet, otherwise the machine becomes unstable and may not work at all! The propagation delay (or gate delay), which is the length of time that it takes for a logic gate to change, is very important for the C64 to work properly. Thus, replacing a broken PLA with another 30 year old MOS PLA chip, is most likely just a brief respite. In order to extent the lifespan of the PLAs functionality, a lot of effort has been put into creating replacement circuits. However, as seen on this 1986 Assy 250466 long board, Commodore was not very consistent with the parts that they put into their machines, so this motherboard has a PLA with the MOS label…Īll PLAs run hot (40-50☌), due to a a high power consumption (on a small area) which may be part of the reason for the high failure rate. Another reason is the corrosion of a protective layer inside the chip due to a manufacturing process. In 1986, another revision of the MOS 8700 was made (the 8700R2) and Commodore exchanged the MOS label to CBM but kept the revision number. At some point, Commodore started their own PLA production (after reverse engineering the Signetics PLA) and called them MOS 7700 and MOS 8700 with the label MOS 906114-01. A clone of the 82S100 is the Fairchild 93459 from 1977 and may also be found in the early versions of the Commodore 64. The first version was made by Signetics (the 82S100) back in 1975 and can be found in some of the early motherboard versions of the C64. The original PLA chip went through several revisions. Thus, the PLA is responsible for the implementation of the memory map of the Commodore 64. If the PLA is broken, the MPU and the VIC-II chips cannot access the correct memory and I/O devices anymore. Based on different input signals, the PLA creates chip select signals and controls which chip is to be connected to the data bus. The most common cause of a black screen on the old Commodore 64 long boards ( link) is a broken Programmed Logic Array (PLA) chip located at position U17.