LocalTalk

LocalTalk

LocalTalk is a particular implementation of the physical layer of the AppleTalk networking system from Apple Computer. LocalTalk specifies a system of shielded twisted pair cabling, plugged into self-terminating transceivers, running at a rate of 230.4 kbit/s.

Networking was envisioned in the Macintosh during planning, so the Mac was given expensive multi-mode (RS-232/RS-422) capable serial ports. The port was driven by the Zilog SCC which could serve as either a standard UART or handle the much more complicated HDLC protocol which was a packet oriented protocol which incorporated addressing, bit-stuffing, and packet checksumming in hardware. Coupled together with the RS422 electrical connections, this provided a reasonably high-speed data connection.

The odd 230.4 kbit/s data transfer rate fell out of the use of the SCC and the clock frequency it was fed. This clock frequency, 3.6864 MHz, was chosen (in part) to support the common asynchronous baud rates up to 38.4 kbit/s using the SCC's internal baud-rate generator. When the SCC's internal PLL was used to lock to the clock embedded in the LocalTalk serial data stream (using its FM0 encoding method) a divide-by-16 setting on the PLL yielded the fastest rate available, namely 230.4 kbit/s.

There is a rumor that Steve Jobs was initially opposed to including any sort of networking on the Mac, and that the RS-422 port and its associated software support was developed largely in secret.

Originally released as "AppleTalk Personal Network", LocalTalk used shielded twisted-pair cable with 3-pin Mini-DIN connectors. Cables were daisy-chained from transceiver to transceiver. Each transceiver had two 3-pin Mini-DIN ports, and a cable to connect to the Mac's DE-9 serial connector. Later, when the Mac Plus introduced the 8-pin Mini-DIN serial connector, transceivers were updated as well.

A variation of LocalTalk, called PhoneNet, was introduced by Farallon Computing. It used standard unshielded twisted pair telephone wire with 6 position modular connectors (same as used in the popular RJ11 telephone connectors) connected to a PhoneNet transceiver, instead of the expensive shielded twisted-pair cable. In addition to being lower cost, PhoneNet-wired networks were more reliable due to the connections being more difficult to accidentally disconnect. In addition, because it used the "outer" pair of the modular connector, it could travel on many pre-existing phone cables and jacks where just the inner pair was in use for RJ11 telephone service. PhoneNet was also able to use an office's existing phone wire, allowing for entire floors of computers to be easily networked. Farallon introduced a 12 port hub which made constructing star topology networks of up to 48 devices as easy as adding jacks at the workstations and some jumpers in the phone closet. These factors led to PhoneNet largely supplanting LocalTalk wiring in low cost networking.

The widespread introduction of Ethernet-based networking in the early 1990s led to the swift disappearance of both LocalTalk and PhoneNet. They remained in use for some time in low-cost applications, but as Ethernet became universal on the PC most offices were installing it anyway. Early models of Power Macintosh and the Macintosh Quadra supported 10BASE-T via the Apple Attachment Unit Interface while still supporting LocalTalk-based networking. For older Macintosh computers that did not have built-in Ethernet expansion options, a high speed SCSI-to-Ethernet adapter was popular, particularly on PowerBooks. This enabled all but the earliest models to access a high speed Ethernet network.

With the release of the iMac in 1998 the traditional Mac serial port disappeared — and thus, the ability to use both LocalTalk and PhoneNet — from new models of Macintosh. LocalTalk-to-Ethernet bridges were introduced to allow legacy devices (especially laser printers) to function on newer networks. For very old Macintosh computers, LocalTalk remains the only option.

Design Legacy

The LocalTalk connector had the distinction of being the first to use Apple's unified AppleTalk Connector Family design, created by Brad Bissell of Frogdesign using Rick Meadows' Apple Icon Family designs. LocalTalk connectors were first released in January 1985 to connect the Laserwriter printer initially with the Macintosh family of computers as an integral part of the newly announced Macintosh Office. However, well past the move to Ethernet, the connector's design continued to be used on all of Apple's peripherals and cable connectors as well as influencing the connectors used throughout the industry as a whole.

See also

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