Pasi Lassila has introduced HEXcart, a new cartridge designed for the Commodore 64 with a practical focus on diagnostic work. The cartridge can store up to sixteen 8KB or 16KB ROM images and supports both standard and Ultimax ROM configurations. ROM selection is handled through a hex switch, allowing it to function without software menus — a helpful feature when working with a malfunctioning C64.
HEXcart includes two flash ROM chips: one for software and the other for hardware control logic. The software ROM must be at least 256KB to hold all sixteen 16KB ROM slots. The second ROM controls the EXROM and GAME signals, which determine how the cartridge maps into the C64’s memory space. No jumpers or extra switches are needed — the logic ROM configures everything automatically.
A latch chip (U3) captures the position of the hex switch during power-up or reset. This lets users change the selected ROM while the machine is powered on, with the new ROM taking effect after the next reset. If this feature isn’t needed, U3 and a few supporting components can be left off the board, replaced with simple jumpers.
The hex switch directly controls the ROM slot selection by setting the highest address lines of the software ROM. It also selects one of the first sixteen bytes of the logic ROM, which in turn configures the GAME and EXROM lines appropriately for the type of ROM being loaded.
One useful feature is the ability to disable the cartridge by setting the hex switch to an empty slot. Empty slots default to a value of FF in flash memory, forcing both GAME and EXROM lines high — making the C64 behave as if no cartridge is present. However, this trick does not apply to the C128 since its cartridge handling works differently.
For full documentation, schematics, and source files, visit the HEXcart GitHub repository:
https://github.com/1c3d1v3r/HEXcart
