We previewed the device earlier, but now full schematics are available online.. and should be usuable for things other than Paul Slocum’s cynthcart.
The MIDI interface translates midi signals to key presses on the C64. It also simulates two potentio meters, which can be used for the cutoff-, LFO-, pitch bent- and pulswidth-modulation. After installing the PCB into your C64, it’s possible to use the C64 as a regular machine, because the keyboard is still connected, although trough the PCB. One must however make some sort of link to the joystick port to enable the use of the potentio meter functionality (see video for an example -i’ve used an USB connector-).
The controller uses two midi channels. Channel 1 is for all note information; channel 2 is used for synth-functions/params. MIDI-CC channels 12 and 13 are used for the potentio meters.