U7 is a 6510 microprocessor. One of the features of the 6510 is a built in parallel I/O port (P0-P5).
P3 - P5 control most of the cassette interface circuitry. P3 pin p6 of U7 outputs the write data signal
to connector CN3 on pins E and 5. P4 is an input that senses the play switch depressed on the cassette
deck. P5 is on output that controls the cassette motor. When P5 goes "low", Q2 cuts off, CR2 regulates
Vb of Q1 at 7.5 volts, this forward biases Q1 and Q3, passing current through the cassette motor
coil. U1 is a Complex Interface Adapter (CIA). Parallel ports, serial outputs, and Timers are standard
features of the CIA. Read data enters on pins D, 4 of CN3. U1 accepts the read data signal on the
FLAG input pin 24.