U2 is a Complex Interface Adapter (CIA). Parallel port signals PA3-PA7 control the serial bus interface.
PA3 is the Attention (ATN) output. This signal is inverted by U8 before being transmitted to
a device on the bus. PA4 is the clock output. Data transmitted from the C64 to a device on the bus
is synchronized by this clock signal. U8 inverts the output PA4. PA5 is the data output. U8 inverts
this output also. Data transmitted from a device on the bus to the C64 is synchronized by a clock
generated by the transmitting device. The Clock signal is input on PA6. Data transmitted from a device
on the bus to the C64 is input on PA7. When a device on the bus wants to communicate with the
C64, SQR IN goes "low" indicating service is requested.
The User Port
Parallel port B of U2 (PB0 - PB7) is made available on the user port. Parallel data transfers with external
device are made very easily through this parallel port. SP2 and SP1 are bi-directional serial ports.
CNT1 and CNT2 are bi-directional synchronizing clock signals for each serial bus.