TRS80-100/102 DC Motor Interface through Cassette Port
I just thought of this little gadget. It is not yet tested, so think of it as a prototype.

Circuit Description

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.