The Epyx Fast Load Cartridge is a speed loader for the 1541 floppy disk drive, that is challenging the PLA a bit. And this is the reason, why I have made one.
Schematics (click to enlarge)
The capacitor C3 is doing "some analog thing" with the /EXROM signal. It is charcged via the pull up resistor for /EXROM - a 3k3 resistor connected to +5V, (actually the resistor network RP4) on the C64 mainboard. It holds the /EXROM signal low for a while after /ROML, /IO1 and /RESET are high. That means, the cartridge disables itself automatically, after it is not in use anymore. Depending on the logic family/levels used for a modern replacement PLA, this time does not match the requirements. And that is the reason, for the problems with some PLA reproductions.
The Epyx Fast Loader itself is an average fast loader, that reduces the loading time to about 19% of the original Commodore loading time.
The cartridge PCB is in the Versa64cart form factor with a reset button in the same position. Additionally, there is a jumper for selecting the cartridge image. I don't know, if it makes sense (are there other Epyx cartridges, that require the same hardware?), but it allows to use different EPROM sizes from 8k to 64k), it can also be configured with solder bridges.
Epyx Fast Load Rev. 0
Please find the project documentation and rescources on github: https://github.com/svenpetersen1965/Epyx-FastLoad