Serial Port
From Mac Guides
Macintosh computers had printer and modem ports through the Power Mac G3 MiniTower (Beige). The two ports are identical and are named so just to avoid confusion. You can hook a Modem into a Printer port or visa versa. Other devices such as low-end scanners also used serial ports.
Serial ports were replaced with USB on the Power Mac G3 (Blue and White).
LowEndMac has a detailed description of the serial ports available on Macs

