I developed a MIDI retrofit for the Pearl DRUM-X. It uses an AVR microcontroller to process MIDI messages, read timing signals, and generate CV/triggers. Here's a block diagram.
Generating CV
The CV comes from a single source and is multiplexed to the 40 destinations (5 voices * 8 parameters. The tricky part it that the DRUM-X controls the multiplexers while the retrofit board generates the CV. Two signals are used to synchronize them. I've labeled the signals "Count Reset" and "Clock". "Count Reset" sends a pulse when the multiplexers are pointed at the first destination. "Clock" sends a pulse each time the destination is changed.
Here are those two signals in the schematic:
Generating Triggers
The DRUM-X is velocity sensitive when sent variable voltage triggers. To take advantage of this I used a multi-channel DAC to generate similar triggers. The retrofit maps MIDI velocity messages to voltages for the triggers.
The DRUM-X has a jack built in that can receive triggers, likely from a proprietary device. I was able to reuse the connection for my retrofit. It's wired such that you can still connect and use pads/other external triggers. Here is a schematic of the trigger daughterboard.
