[1] 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 and PiPPiN.
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 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.
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 LocalTalk system running on the SCC ports rather than AppleNet's plug-in expansion card.
As a result, performance on a Mac Plus was generally limited to about 9600 bit/s in RS-232 mode, and could only reliably maintain half of that if LocalTalk was active at the same time.
Apple was aware of these problems, and starting with the Macintosh IIfx they attempted to address them by including two dedicated "Input/Output Processors" (IOP), customized MOS 6502s running at 10 MHz.
The DSP then generated signals and sent them over the serial bus to the adapter, which converted them to analog and amplified them to telephone levels.
Initially the adapter driver software could support speeds up to 9600 bit/s, but later upgrades introduced full V.34 compliance, running at up to 33.6 kbit/s.
In practice the GeoPort Adaptors tied up huge amounts of CPU time, and led to performance problems throughout the system.
During the evolution of GeoPort, the telecommunications market was undergoing major changes with the introduction of newer all-digital PBX systems.