CONFIRMED! It is actually spitting out proper MIDI on the serial line! It gives you the following:
MIDI sync, incl. start and stop messages.
Automation data for all of the LFO settings (both the knobs and the switches) and the EG shape. This suggests that the LFO and envelope may be created in software.
I have not yet hooked up an optocoupler and fed the thing MIDI from the other end, but I will. Shorting the serial out and serial in ports glitches the thing, so it most probably is capable of receiving MIDI.
Here's the pinout for the pin connector marked serial on the board.
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1 Pin 20 PH0, TB0IN0, /BOOT
2 Pin 12 RXD0
3 Pin 11 TXD0
4 Pin 9 Vdd (3 V)
6 Pin 29 PF6/SCK1
I believe this header was intended for factory programming of the firmware. When pin 1 is held low, the MCU enter a special programming mode, according to the datasheet. Pin 6 goes is routed up to somwhere near the power switch and is probably used to detect when a unit is turned on to begin the programming.
CN13 (which comes with no connector attached) would be used for debugging, but it is likely that debugging is disabled so you can't dump the firmware etc. Still haven't looked into that. (I don't have JTAG tools readily available, so...)
But what we're interested in is pins 2-5. Pin 2 is for receiving MIDI. Pin 3 is where MIDI comes out. If you're going to try to add an optocoupler, you'll also need Gnd and Vdd.
MIDI is a current loop protocol, so 3.3 V is no problem for standard MIDI gear given that you adjust the output resistors accordingly. Following the standard MIDI convention, I connected 3.3V and the output as usual, but replaced the usual 220 ohm resistors with 150 ohm ones. Works well enough.
I also got an idea: If the monotribe can receive MIDI, that may open up the possibility of throwing the internal oscillator out the window and using the it as a self-tuning MIDI CV interface for modulars. However, I've found that at least my monotribe tracks badly and sometimes drifts a little. This is especially obvious when comparing it to a well-tuned computer playing the same MIDI notes. Also, it's using just 5V internally for the analogue part (3.3V for the digital) and the CV range may be even less, so perhaps it wouldnn't be of much use for 1 V/oct systems.
More to come...