6 pin DIN (DIN 45322) female at the computer.
|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 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 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.
|Note: in some models pin 6 is /RESET
Source: VIC-20 User Manual, VIC-20 Programmer's Reference Guide