[I am writing this when I am very tired. It will be terse.]
Now that the upper manual is basically controllable by MIDI, it’s time to look at the lower. The lower manual is more complicated than the upper. It has two sets of contacts for each key; one of which fires off the bass and chords, and one of which is ‘accompaniment’.
Essentially, then, the organ needs two sets of opto-isolators, one for each set of contacts. This means that each pin on the MIDI board needs to switch two LEDs; and for sanity’s sake, it’s probably a good idea to stick a tolerably heavy-duty diode (I have obtained industrial quanities of 1N4003s) along with the LED to make sure that current is only going where we think it is going.
First: looking at the ‘bass and chords’ contacts. These are in a scanmatrix arrangement; and so need a diode next to each key to avoid squirting current back up the scanmatrix pins on the mystery chip which seems to be running all this. (The diodes can be seen on the wiring for the lower keying; pps 8-9 in Part 2 of the TG-44 service manual, which I linked to previously.)
This means that, instead of our previous circuit, for each key, we have:
This is the basic outline for relayboard version 2b (2a is identical to 2 except that the two rows of chips can have different Vccs). This /should/ be interchangeable with v2 (so long as I’ve understood the organ correctly). In terms of board layout, it makes things a little denser; although the diodes on the MIDI side can sit right next to the resistors, the diodes on the other side will necessarily stretch things out a little. (I will post photos when the diodes arrive and I’ve had time to fiddle).
Each scanmatrix pin on the magical chip of doom deals with one octave, or twelve notes. It’s easiest then to match each octave to one row on a relayboard.
Plan for the ‘accompaniment’ keyswitches to come later.