Well, I thought that tonight would be the night when I got the MPG-200 up and running with the JX-3P for the first time. Alas, I failed miserably ;-)
I've checked the output from both the MPG-200 and the PG-200, and they both output the same signals. BUT - I've forgotten that I implemented running status support on the MPG-200 (which means that the midi device generating the CC messages does not have to send the controller number again as long as it is the same controller as the previous one). The JX-3P on the other hand, does not support running messages, so it has to explicitly get a controller (or address as it is called in the PG-200 service notes) message for every data message. I have forgotten to implement this and instead just send the new controller value. Not a hard thing to fix, but missing none the less.
Here is a picture of my setup btw:
To the left is a macbook connected to a saleae.com logic probe, which I used to decode the PG-200 signals. The white square to the right of it is a prototype card/solderless breadboard, with all the necessary electronics (except for the microcontroller and associated circuitry) for interfacing the JX-3P. Behind that is my good old Mikroelektronika EasyPIC3 development kit, connected to a different PC (behind the synth). This kit holds the microcontroller. To the right of it, just beneath the big screen is the JX-3P, and below it is the guts of the PG-200 (the box is actually under it). To the far right is my Australian Redback edition Little Phatty which I only use as a midi controller in this setup.