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 (At the Computer)
??? USERPORT (To the C64).

8 PIN MINI-DIN MALE to the Macintosh.
DZM 12 DREH to the C64 UserPort.

  Mac C64  
GND+RXD- 4+5 1+12+A+N GND
RXD+ 8 M TXD (PA2)
    D+E RTS+DTR (PB1+PB2)

Source: [The Hardware Book]