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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. 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 that 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 initially opposed including networking on the Macintosh, and that the RS-422 port and its associated software 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 six-position modular connectors (same as 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「LocalTalk」の詳細全文を読む スポンサード リンク
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