GeoPort was a serial data system used on some models of the Apple Macintosh. GeoPort modified the meaning of the existing Mac serial port pins to add a new high-speed DMA channel that allowed the Macs internal sound hardware to emulate various devices such as modems and fax machines. GeoPort could be found on late-model m68k-based machines (the AV series) as well as all pre-USB Power Macintosh models.
The basic idea behind GeoPort was to allow one to build a low-cost analog communications device by using the hardware already built into the Mac for most of the data handling. Early machines were not fast enough to emulate these devices, but the AV-series added a AT&T 3210 DSP which could handle this with ease, and the later PowerMacs PowerPC had enough raw processing power to do this without the DSP helping. Since this hardware was already built into the Mac, all that was needed externally was a line adaptor which included a digital-to-analog converter to convert the digital sound values generated by the computer to analog signals, and vice versa. Better yet, the same system could be used to handle voice, which required too much bandwidth to easily send over existing serial ports.
The problem was that the normal Mac serial port did not have the speed needed to send the resulting sound data to the line adaptor. Fully supporting a phone system requires between 56 and 64 kbit/s, and while the Macs RS-422 mode could run at up to 230 kbit/s, this required a considerable amount of the CPUs time to mediate the transfer, leaving little time for it to actually generate the signal. However, the RS-422 ports on the Mac also supported higher speeds through the use of an externally-generated clock signal. Instead of the serial drivers internal clock, which topped out at 230 kbit/s, devices could supply their own clock signal for rates up to 920 kbit/s in each direction. This had been used by a small number of networking adapators in the past, and the main reason this was not more widely used is that the clock required power, which the port did not supply in RS-422 mode.
Apple solved the power problem by modifying their existing 8-pin DIN connector with the addition of a 9th pin shoehorned into it, serving double-duty in providing power to an attached device, as well as serving as an indication of a classic device if there was nothing connected to it. With the additional power, the external line adaptors could power their own clock devices and send data at higher rates. In the case of GeoPort, this allowed the Macs CPU to quickly squirt data to the device, leaving it more time to handle the actual data.