Around three years ago, in april 2012, I decoded the PG-200 protocol. Using a Saleae logic probe I took a closer look at what happened while using the PG-200, and built a device that converts MIDI CC messages into PG-200 commands. This device, the MPG-200, also receives and transmits MIDI note messages, making it possible to completely control the JX-3P using MIDI.
To my knowledge, at the time I started the project only two other devices existed that could do this - the KiwiTechnics JX-3P upgrade and Patch Editor and the Organix Midi upgrade kit. These both require extensive modification of the JX-3P. Later, Mode machines made a PG-200 clone called the DT-200. I have yet to see the PG-200 protocol fully documented anywhere, but I may have missed something as I have not searched the web lately.
As a tiny gift to the synth community in honor of my daughter's birth, I now release all the information I have gathered. This document is based on the data found in the JX-3P/PG-200 service manual as well as a lot of work done by myself. Feel free to use it in any way you see fit, but I would be really happy if you acknowledged my contribution.
The complete protocol description can be found at http://www.xonik.no/mpg-200/pg-200/pg-200.html