Did some reverse engineering. Added the schematic (Use at own risk!) but should be enough information to get it up and running.
The interface is most likely just an PWM input, for the moment of unknown freq. ... will test, .... and a reference freq. output for the mainboard to determine status of the motor-driver board. I guess it will output a freq. linear with the current rpm. Did not figure out how they control the elevation motor, but I have no plan for using that anyway.
Tachometer input is a sensor on the motor itself to be connected at JK50 (note, the notation is from the board)