a RS232 to Macintosh RS-422 cable. Same things they used in the modems back in the day. You could also make one by either cutting an old mac cable and rewiring it to a solderable DB-9, Or finding a solder type DIN port that fits the serial port of the mac. and using cat-5 cable between it, and a solder DB-9.

a cheatsheet of the cable wiring is on the net.