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RS485 interface chip behavior when it has no power applied, but the bus is signalling

Erich
Prodigy 210 points

I have a customer who wants to use RS 485 in a multi drop configuration. In the application some sensor nodes will be powered down, so the RS485 transceiver will also thus be powered down.  Will this affect the operation of the bus? I suspect not as I would guess the RS485 transceiver will be in a high impedance state, so for all purposes not part of the bus.

Any comments / suggestions welcome

  • 0
    TI__Mastermind 21555 points

    Hi Erich,

    All nodes, whether powered or not will present some loading to the bus. With that being said, you are correct. You will easily be able to have some powered nodes and some unpowered nodes while communicating on the bus. The spec that I would play close attention to is the input bias current in the datasheet. We spec this with VCC powered up and at 0 volts.

    The lower the max leakage current the less loading the unpowered device will present to the rest of the bus. The leakage current is normally in the low 100's of µAs, and therefore will not have much effect.

    Please let me know if you have any other questions.

    John