The expansion box allows up to 4 interface cards to be connected to the COMX. The expansion box firmware ROM maps to memory location @E000-@EFFF. Location @E000-@E7FF also overwrites location @1000-@17FF of the BASIC ROM, this trick extended basic with commands and logic to switch between different interface cards.
Next to the standard firmware ROM there is an adaptation made by F&M (Frank and Marcel van Tongeren) this adapted ROM added a screen editor feature to COMX BASIC. Both ROM versions are included in the Emma 02 emulator.