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Standard for automotive network is ratified
Harris Semiconductor (Melbourne, FL) and Motorola (Phoenix, AZ) plan chip implementations of the Society of Automotive Engineers (SAE) J1850 interface, ratified as a standard last month by SAE's Multiplex Committee. Based on work by engineers at Chrysler, General Motors, and Ford, the standard may achieve rapid (in relatively geological automotive time scales) success. If so, it will eventually end the use of proprietary network standards for vehicles, and possibly for smaller markets in need of similar standards as well.
One incentive to quick adoption is a California legal requirement. Starting with the 1994 model year, cars must dump emissions data to an external scanner. The current way of doing this uses an I/O block made to the ISO9141 standard. Such blocks are different for each proprietary bus, so a standard bus would be cheaper for all car makers.
The J1850 standard provides both a single-wire physical link passing 10.4 kbits/s, adequate for present-day bandwidth requirements, and a two-wire link passing 41.6 kbits/s, necessary within a few years. Each device that sends messages has a priority code built into its address. Contention resolution always lets the higher-priority sender through.
The standard also prescribes message structure, timing, bit sequence, and error-checking. Encoding is synchronous, variable-pulse-width on the single wire, and pulse-width-modulated on the double wire.
Neither Motorola or Harris Semiconductor has announced actual J1850 ICs yet. Motorola's IC is expected to be a microcontroller, probably 68HC11 or '05, with the bus transceiver integrated. Harris plans a two-chip solution, with a 68HC05 core in the controller chip and the transceiver on another chip. EEPROM and bondout versions are planned, as are development tools.
–Rodney Myrvaagnes
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