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SN65HVD1781: RS485 communication direction switch problem

user505277
Prodigy 30 points
Part Number: SN65HVD1781

I use RS485 device SN65HVD1781. short pin RE and pin DE, use mcu IO to control direction.

When switch receiver to tranceive or switch tranceive to receiver, "0x00" will be received.

if no direction change, no "0x00" received, very stable.

I am sure, No signal interferenced.

change SN65HVD1781 direction caused pin A and pin B line voltage nosie.

how to avoid this problem, please help me, thank you very much.

  • 0
    TI__Intellectual 840 points
    Hi,

    There are a few things we would like to know:

    1. Is the noise causing bit errors?

    2. Would you mind sharing a schematic of your system?

    3. Could you send some scope shots of the data, DE/RE toggle, and lines A and B?


    Regards,

    Daniel Noel
  • 0 in reply to Daniel R Noel
    Prodigy 30 points

    1. no noise causing bit errors, switch direction causing bit errors.

    2. slave side use device SN65HVD1781(1Mbps), master side use device SP485EEN-L(10Mbps)

    if master side also use SN65HVD1781, it is ok.

    3. slave and master schematic

  • 0 in reply to user505277
    TI__Intellectual 840 points
    Hi

    When you switch DE/RE is it the slave or the master unit causing the error? I would like to see the A line and B line waveform when DE/RE is toggles or during the error event, to have a little more insight as to whats happening. I also see you have failsafe biasing on both end of the bus, where as you only need 1 failsafe network, you might want to try and remove one and see what that does.

    Regards,
    Daniel Noel
  • 0 in reply to Daniel R Noel
    Prodigy 30 points

    Hi

     toggle DE/RE every 2s, no data send on bus,just  toggle DE/RE. yellow->A, blue->B. thank you very much.

  • 0 in reply to user505277
    TI__Intellectual 840 points

    Hi,

    Your A line seems to be fine, but B line looks to be a little off. I ran your setup according to the schematic you provided  and here is what I got.

    Yellow->A, Blue->B, Purple ->Receive, and Green-> DE/RE

    Your B line looks to be very close to my Receive line, you might want to double check to see if something is miss labeled.

    Also here are some close up scope grabs of the rising and falling edge of the DE/RE pins to maybe help you with some debugging.

    Rising edge

    Falling edge

    I hope this helps please let me know if you are have any further issues. 

    Regards,

    Daniel Noel