Power up the board through J1 with a good 9-10VDC well filtered.
Feed the signal from the cassette port into Pin 4 of JP2; pins
1 and 5 connect to the REMOTE relay inside the M100/2. The signal
from the computer (generated from the 80C85 SDO) is squared by
U2 and converted into a current proportional to its frequency
on Pin 3 (CP2). Thanks to an internal OP-AMP and output transistor,
the voltage on pin 4 is proportional to the input frequency. Q1
buffers the output and operates as a current amplifier. The final
output voltage applied to the motor is returned to pin 7 for negative
feedback control. RL1 allows polarity reversal on the motor.
Briefly, MOTOR ON-MOTOR OFF commands will make RL1 switch on and off (motor forward/reverse), the frequency generated by SDO will control the speed.