All signals are active low.
In This signal is not used on the C64. On C128 it is replaced with Fast Serial Clock for the 1571 disk drive. When the C64 brings this signal LOW, all other devices start listening for it to transmit an address. Usually, the address byte will be followed by one to two commands for the device addressed, meaning the secondary address and channel on the peripheral. Out This signal is for timing the data sent on the serial bus.