Commodore VIC-20 Programmer's Reference Manual page 251

3 SERIAL ATN IN/OUT
4 SERIAL CLK IN/OUT
5 SERIAL DATA IN/OUT
6 NO CONNECTION
SERIAL SRQ IN: (SERIAL SERVICE REQUEST IN)
Any device on the serial bus can bring this signal LOW when it
requires attention from the VIC. The VIC will then take care of the
device.
SERIAL ATN IN/OUT: (SERIAL ATTENTION IN/OUT)
The VIC uses this signal to start a command sequence for a
device on the serial bus. When the VIC brings this signal LOW, all
other devices on the bus start listening for the VIC to transmit an
address. The device addressed must respond in a preset period of
time; otherwise the VIC will assume that the device addressed is
not on the bus, and will return an error in the STATUS WORD.
SERIAL CLK IN/OUT: (SERIAL CLOCK IN/OUT)
This signal is used for timing on the serial bus.
SERIAL DATA IN/OUT:
Data on the serial bus is transmitted one bit at a time on this line.
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