1541 CIRCUIT THEORY
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.