GeoPort

GeoPort is a serial data system used on some models of the Apple Macintosh. GeoPort slightly modified the existing Mac serial port pins to allow the computer's internal DSP hardware or software to send data that, when passed to a digital-to-analog converter, emulated various devices such as modems and fax machines. GeoPort could be found on late-model 68K-based machines (the AV series) as well as many pre-USB Power Macintosh models. Some later Macintosh models also included an internal GeoPort via an internal connector on the Communications Slot. Apple GeoPort technology is now obsolete, and modem support is typically offered through USB.

Early during the development of the Apple Macintosh, Apple engineers decided to use the Zilog 8530 "Serial Communications Controller" (SCC) for most input/output tasks. The SCC was relatively advanced compared to the more common UARTs of the era, offering a number of high-speed modes and built-in software for error checking and similar duties. The speed of the system was based on an external clock signal sent to it by the host platform, normally up to about 1 Mbit/s, which could be "divided down" to run at slower speeds as low as 300 bit/s. The SCC had two channels, which could be run at different speeds, and even different voltages, to allow communications with a wide variety of devices and interfaces.

Initially the engineers had envisioned using the SCC to support a packet-based protocol known as "AppleBus". AppleBus would allow peripheral devices to be plugged into a daisy-chain configuration in a manner surprisingly similar to the modern Universal Serial Bus. However, as development continued, Apple's networking project, AppleNet, was being canceled due to high costs and a rapidly changing marketplace. Team members working on AppleBus quickly shifted gears, producing the AppleTalk system running on the SCC ports rather than AppleNet's plug-in expansion card.

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