The RS-232 on the C64 is a little bit strange. It uses inverted TTL level of 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
| Mac | C64 | ||
|---|---|---|---|
| GND+RXD- | 4+5 | 1+12+A+N | GND |
| RXD+ | 8 | M | RS232 TXD (PA2) |
| TXD+ | 6 | B+C | RXD (FLAG2+PB0) |
| D+E | RTS+DTR (PB1+PB2) |

