The Macintosh SE modem port connects external devices to the Macintosh via a Mini-8 connector. The port is driven by an RS423 line driver
(an enhanced version of the RS232 format).
The modem port's General Purpose Input (GPI) pin is connected to the SCC's* Receive/Transmit Clock. This feature supports such devices as a synchronous modem, which provides separate transmit and receive clocks.