Q: What is J1708?
J1708 is a automotive communications standard based on half-duplex RS-485. It uses the RS-485/422 differential transmitters with an improvement: Input to the transmit buffer is always active; the UART TX output drives the enable line of the transmitter. Thus, the output of the transmitter alternates between active and high-impedance states.
Benefits
The advantage of J1708 is that it reduces complexity of the transmitter. RS-485 requires the transmitter to be turned off once the transmitted data stream is complete. However, some UARTs only flag when the FIFO buffer is empty, not when the serializer has sent the last bit. RS-485 drivers must add timing routines to estimate when the last bit of data has been transmitted, which adds overhead to the serial drivers. J1708 eliminates the timing problem altogether.
J1708 also uses fewer GPIO lines. RS-485 requires four I/O lines from the processor (TX, RX, TX enable and RX enable). J1708 requires only two (TX and RX). No special software drivers are needed to convert serial data streams from logic levels to J1708.
Compatibility with 485/422
Since ADS systems include the resistors that pull the RS-485/422 +/- lines to the idle state, an ADS system configured as J1708 can communicate with another half-duplex RS-222 device or RS-485 network transparently without additional circuitry.
Availability
J1708 is a factory option on some ADS products (e.g. VGX, Graphics Client Plus, Graphics Master).
Further Reading
National Semiconductor App Note AN-915 (1993; good overview of SAE J1708)
For related information, please see the Serial Communications topic index.
Edited by akidder 2-Oct-2003: Add AN-915 link and section headings.
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What is J1708?
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Posted - 18 Feb 2003 : 11:25:14
