C64 RS232 User Port
Available on the Commodore C64/C128. Software emulated. The signals does not have true RS232 levels. It's TTL level, and RXD/TXD is inverted. It's just the normal User Port, used as a RS232 port.
24 PIN MALE EDGE (DZM 12 DREH) at the computer.
Source: [The Hardware Book]