GeoPort was 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 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 many pre-USB Power Macintosh models. Some later Macintosh models also included an internal GeoPort known as the Communications Slot. Apple GeoPort technology is now obsolete, and modem support is typically offered through USB.
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Prior to the introduction of the Apple Macintosh, Apple engineers on the Apple Lisa team were working on the design of a local area networking system known as "AppleNet", based on the Xerox XNS protocol stack running at 1 Mbit/s.[1] There were also plans to offer AppleNet on the Apple II. Mac development had been ongoing for some time at this point, but the $500 price of AppleNet led Steve Jobs to refuse its inclusion on the low-cost machine. The Macintosh engineers did manage to make some room for a "peripheral bus" instead, using the $5 Zilog 8530 "Serial Communications Controller" (SCC) instead of the more common, and almost free, UARTs that were used on most designs of the era.
The SCC drove two ports and could be operated in either of two modes, RS-232 mode at 12 V, or RS-422 mode at 5 V using pairs of wires supporting differential signaling. In the former mode the ports operated like the serial ports on any contemporary computer, and, with an appropriate cable, could be used with any common serial device, like modems. In the latter 5 volt mode, the SCC could operate at speed up to 250 kbit/s, which was put to use driving the ImageWriter series of printers and some other devices. On the Macintosh, one of the SCC's ports was labeled "modem" and the other "printer", although there was no logical difference between the two.
Given the poor sales of the Lisa system, development efforts on that project were dramatically scaled back in September 1983 in favor of the Macintosh. AppleNet for both the Lisa and Apple II became a casualty in October. Cancellation of AppleNet gave the vague plans for a peripheral bus new importance, briefly becoming "AppleBus".[2] Gursharan Sidhu led development of what eventually developed into AppleTalk and its associated LocalTalk hardware, developing a truly "Mac-like" plug-n-play networking system running at 230 kbit/s.[3]
Although the SCC's internal clocks supported any transmission speed up to 250 kbit/s, in practice these sorts of speeds were rarely obtainable. Data was provided to the SCC by the host CPU, which was also loaded down with other tasks. In RS-232 mode more constant polling was needed, and the best expected performance varied from 9600 bit/s on the Mac Plus, to 115,200 bit/s on later machines.
AppleTalk allowed for relatively high speeds because its packetized nature allowed the CPU to send blocks of data to the port at high speed when it wasn't busy doing something else. In this case the speeds were primarily a function of the internal clocks in the SCC. A number of vendors provided network connectors with their own clocks that ran at much higher speeds, up to 1 Mbit/s. A lack of standardization precluded these from becoming common.
Since both serial ports were driven by the single SCC, and those in turn by software drivers running on the CPU, the CPU's load could dramatically affect throughput. Using LocalTalk and the serial port at the same time generally halved the performance of the latter. 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 IOPs offloaded the low-level driver code from the host CPU, running the floppy disks and serial ports, only interrupting the CPU when there was data to transfer. The IIfx also introduced a DMA mode for the SCSI-bus, but this was not available for use by the IOPs. The IOPs also appeared on the Quadra 900 and Quadra 950.[4]
After the early Quadra machines, Apple started a short-lived experiment including AT&T 3210 digital signal processors (DSP) in the "AV" series machines. The DSP was primarily intended to offer audio and video digitization support, accessed though the custom 60-pin "Digital Audio/Video" (DAV) port on the rear of the machines. However, there was also the possibility of using the DSP to emulate a modem, another common task for DSPs. Using the relatively advanced 3210 would offer higher performance than existing modems, which used lower-end DSPs, and much better performance than softmodems using the host CPU.
In order to support this mode of operation, the serial ports would have to operate at high enough speeds to allow the DSP to send the signal, in digital form, to an external analog to digital converter, or "line adapter" as it was referred to in Apple terminology. To operate at these speeds, as well as handle the higher voltages and powers of the phone system, the adapter needed to have a separate power source. Apple solved this problem by shoehorning in a 9th pin to the existing 8-pin Mini-DIN connector. The result was GeoPort.
In GeoPort mode the SCC was placed in a high-speed mode using external clocking, similar to the earlier high-performance LocalTalk replacements. 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. In GeoPort mode, speeds up to 2 Mbit/s were supported.[5]
The only widely known device to make use of the GeoPort was Apple's own "GeoPort Telecom Adapter". Initially released as the "pod" with the AV Macs in 1993, it later became its own separate product in early 1995, when it was released for PowerPC-based Macintosh machines that did not include AV-related hardware out of the box.[6] The Adaptor eventually came in three versions, an internal model that plugged into the Communication Slot II (CS II), and two external versions (the GeoPort Telecom Adapter and the GeoPort Telecom Adapter II) all providing a standard RJ-11 phone line connector. The CS II was essentially an internal GeoPort with various other signals as well. 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.[7] They were also notorious for poor quality connections, dropped calls and poor support from existing software; many pundits recommended not using them.[8]
SAGEM also introduced an ISDN adaptor for GeoPort, the "Planet-ISDN Geoport Adapter", or SPIGA, which they now sell in a USB version. Global Village also produced a variant of its compact Teleport Modems for Apple Performas and third-party Mac computers that used the 9th pin to supply power, but these were true hardware modems.[9]
During the evolution of GeoPort, the telecommunications market was undergoing major changes with the introduction of newer all-digital PBX systems. A number of these companies offered some sort of computer telephony integration, although they were all based on their own standards, typically connected to the computers via a serial port at relatively low speeds. Although these signals could have been routed using existing networking standards like Ethernet, most companies used custom systems, like Switch56.
Apple started an effort to promote GeoPort as a standardized computer interface to PBX systems.[10] They envisioned different GeoPort adapters for different back-end systems; a Switch56 interface could be used with Northern Telecom systems, while another would be used to connect to the InteCom switches that Apple used. Apple claimed that the "same GeoPort phone pod will be able to support TAPI applications on a Windows PC and Telephone Manager applications on a Mac."[5]
To connect those systems to the Macintosh, in particular, Apple created the Macintosh Telephony Architecture (MTA) and promoted it as a standarized telephony application program interface. The MTA consisted primarily of three parts, the "Telephone Manager" which handled call-control, the "Telephone Tool" that mapped Telephone Manager commands onto specific hardware, and finally a set of telephony-related Apple Events that would allow simpler operation from any Macintosh system.[5] MTA was essentially Apple's analog of Microsoft's TAPI or Novell's TSAPI.
To promote the system, in late 1994 Apple organized the Versit alliance along with AT&T, IBM and Siemens to attempt to standardize the commands that the PBX systems responded to.[11][12] Novell announced that they would adapt TSAPI to work on top of the Versit standards. All of these efforts were hampered by a lack of standardization among PBX vendors, and their lack of real support for GeoPort adapters.
After two years of effort with little to show for it, Apple eventually gave up on Versit, and telephony in general.[13] The main problem was that the various PBX companies relied on vendor lock-in to keep their existing customers coming back to them for newer products, so the very concept of a standardized system was seen as a problem rather than a solution. Additionally there was no support for high-speed serial on the basic PC, so users would have to buy an add-on card if they were going to use GeoPort, at which point it made just as much sense to buy a custom adaptor on a different card.
The GeoPort was dropped, along with SCSI and ADB, when Apple introduced the iMac in 1998; early demonstration models had a software modem based on the GeoPort Telecom Adapter’s technology, but this was replaced with a hardware 56K modem by the time the iMac shipped.
The table below shows the name and purpose of the various pins in the GeoPort-enabled serial connector when used in GeoPort, RS-422 (LocalTalk) and RS-232 modes.
Pin # GeoPort RS-422 RS-232 Name 1 SCLK HSKo DTR Serial Clock (out), Handshake Out, Data Terminal Ready 2 SCLK HSKi DSR Serial Clock (in), Handshake In, Data Set Ready 3 TxD- TxD- TD Transmit data (-ve signal) 4 GND GND GND Cable ground 5 RxD- RxD- RD Receive data (-ve signal) 6 TxD+ TxD+ Transmit data (+ve signal) 7 TxHS GPi CD Wakeup/DMA Request, General Purpose input, Carrier Detect 8 RxD+ RxD+ (ground) Receive data (+ve signal) 9 +5 V Power, 350 mA maximum