Service manual

The Serial Interface
UC3 is a 6522 Versatile Interface Adapter (VIA). Two parallel ports, handshake control,
programmable timers, and interrupt control are standard features of the VIA. Port B signals (PB0-
PB7) control the serial interface driver ICs (UB1 and UA1). CLK and DATA signals are bidirectional
signals connected to pins 4 and 5 of P2 and P3. ANT (Attention) is an input on pin 3 of P2 and P3
that is sensed at PB7 and CA1 of UC3 after being inverted by UA1. ATNA (Attention Acknowledge) is
an output from PB4 of UC3 which is sensed on the data line pin 5 of P2 and P4 after being
exclusively "ored" by UD3 and inverted by UB1. UC3 is selected by UC7 pin 7 going "low" when the
proper address is output from the processor. UC3 resides at memory locations $1C00-$1C0F.