The Highly Liquid boards are no longer available. I acquired a VPO console several months ago that uses 2 HL boards for pistons, 32 note pedalboard, and expression. The keyboards uses a different encoder. I started having trouble with one that had the 2 expression shoes connected. The pots were fine so it seemed I needed to tweak the code for the boards. I found some documentation and it seems to use MIDI Sysex messages for coding? I didn't want to experiment and wipe out something as everything else was working perfectly, so I went the Arduino route. I actually used an Arduino Leonardo (just $12) that is capable of being MIDI compliant thanks to the ATMEGA32u4 chip on board . So I just needed a USB cable, Windows and Hautpwerk recognized it accordingly, and I was done!.
Here is a project for wiring a complete 2-manual organ using an Arduino Mega (lots of inputs).
https://projecthub.arduino.cc/Larason2/midi-for-digital-organs-398d3e
** Note **
All Allen pedal boards, pistons, keyboards, toe studs, etc. are in a Scan Matrix design vs a discrete (common bus) design found on an other organ electronic brands (like older Rodgers) or pipe organ consoles. MidiBotique does sell encoders that work with standard matrices. I don't think the Allen pedalboard follows a standard matrix pattern. Not an issue for an Arduino since you can easily customize the code.
Danny B.