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RS485 biasing for Modbus

Discussion in 'Misc Electronics' started by David, Aug 15, 2006.

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  1. David

    David Guest

    I am a little confused by the biasing requirements of RS485 bus for Modbus.

    I normally bias the RS485 driver with A > B to ensure a HIGH at any
    receiver on the bus when now transmission is in progress.

    The standard though states that B must be higher than A for idle state
    of the RS485 bus. If I bias like this, the micro in the device will
    continually think a start bit is being received due to a low on the
    RS485 IC (MAX485).

    All practical applications I have seen so far bias A > B.

    Could someone shed some light on this for me please.


  2. David

    David Guest

    Thanks for the link.

    It seems I was not dreaming afterall and that there is confusion over
    the biasing.

    So far my systems Bias A higher than B to be compatible with the
    microcontroller UART and not require inversion between the MX485 and
  3. This application note does not address Modbus, but is a very good
    document on how to bias RS-485:

  4. David

    David Guest


    Thanks for the link. I have the RS485 bus working fine for our own
    products. (Using 680R pull up and pull down resistors and 120R
    termination). The problem is that we are now producing Modbus slave and
    Master devices and need to clarify the Modbus requirement for A to be
    biased lower than B.

    If another manufacturers device is a Master and they bias A to be biased
    lower than B but our system is designed for bias with A more positive
    than B then we have an issue. I suppose if we state the requirements in
    the documentation the users can connect A of their master to B of our
    slave and visa versa.

    I imagine many applications (like ours) connects the RS485 interface IC
    directly to a micro controller. If this is the case, they need RX to
    idle "1" to prevent the micro UART interrupt continually being triggered.

    I suppose another way to do it is to avoid using A and B designations
    and simply mark the RS485 as (+) (-) and (Com).
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