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SN65HVD75: SN65HVD75: RS485 Power Over Bus Glitch

Blindu Marcel
Intellectual 325 points
Part Number: SN65HVD75
Other Parts Discussed in Thread: TIDA-00527, , THVD8010, THVD8000

We try to implement RS485 power over bus with SN65HVD75 using TIDA-00527 design reference.



The system data rate is 921600bps with 3m cable length. But R output still pull low around 60us in the end.

How to prevent pull low in the end? Do you have any suggestion to solve this issue?

I attach pcb schematic and a oscilloscope graph with D of SN65HVD75 of DRIVER and R of SN65HVD75 of RECEIVER.

  • 0
    TI__Mastermind 25705 points

    Hi,

    First of all I would like to introduce you the newly released THVD8000 and THVD8010 (power over RS485 transceiver). The devices integrate the function of TIDA-00527. I believe it would be easier to make the design with them.

    Back to the issue, can you capture the waveform of the bus when R was low? It might be due to the bus noise or poor signal integrity. The schematic looks good to me. However I'm not sure how well the 120Ohm termination matches the cable impedance.

    Regards,

    Hao

  • 0 in reply to Hao L
    Intellectual 325 points

    Hi.

    This are the signals: R -> Yellow, 485_B -> Magenta and RS485_A -> Violet

    Best Regards

  • 0 in reply to Blindu Marcel
    TI__Mastermind 25705 points

    It looks like the glitch is due to the different setting time of the wires. Can you try a different termination value to check the difference? If not already used, is it possible to apply twisted wire? Another thing to tweak is increasing the common mode cap (C8) value. Again, I think it's worth trying THVD8000, which has taken such issues into account in design.

    Regards,

    Hao 

  • 0 in reply to Hao L
    Intellectual 325 points

    Hi,

    I'm on holiday now and I'll be back the week of January 11 and I'll get back to you that week.

    Thanks a lot and best regards