World Library  
Flag as Inappropriate
Email this Article


Article Id: WHEBN0000054180
Reproduction Date:

Title: LocalTalk  
Author: World Heritage Encyclopedia
Language: English
Subject: AppleTalk, LaserWriter, Snow White design language, Macintosh Office, Apple Industrial Design Group
Publisher: World Heritage Encyclopedia


Local cable and interior circuit board, port-side view
Rear view of auto-termination switch with dust cover removed
Interior of Apple LocalTalk interface box. In 1989, these boxes typically cost US $90 each. The connectors feature automatic electrical termination of the LocalTalk signal bus; insertion of a LocalTalk bus cable depresses a normally closed switch behind the connector, disabling termination for that connector.

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. CSMA/CA was implemented as a random multiple access method.

Farallon's competing PhoneNet transceiver.

Networking was envisioned in the Macintosh during planning, so the Mac was given expensive RS-422 capable serial ports. The ports were 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 230.4 kbit/s bit rate is the highest in the series of standard serial bit rates (110, 150, 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 38400, 57600, 115200, 230400) derived from the 3.6864 MHz clock after the customary divide-by-16. 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 side-by-side 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 useful life of PhoneNet was extended with the introduction of LocalTalk switching technology by Tribe Computer Works. Introduced in 1990, the Tribe LocalSwitch was a 16 port packet switch designed to speed up overloaded PhoneNet networks.

The widespread availability 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 and applications where Ethernet was not available, 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 available, and was particularly popular on PowerBooks. This enabled all but the earliest Macintosh models to access a high speed Ethernet network.

With the release of the iMac in 1998 the traditional Mac serial port — and thus, the ability to use both LocalTalk and PhoneNet — disappeared from new models of Macintosh. LocalTalk-to-Ethernet bridges were introduced to allow legacy devices (especially 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

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.