Mac to C64 Nullmodem Cable
The RS-232 standard on the C64 is a little bit strange. It uses inverted TTL level for the signals. The RS-422 ports on the Macintosh has both an inverted and non-inverted input. By using the inverted instead of non-inverted the inverted C64 level is back to normal.
8 PIN MINI-DIN MALE to the Macintosh.
Source: [The Hardware Book]