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Original question:

SN65HVD75: RS485 Power Over Bus

SN65HVD75: RS485 Power Over Bus

Jason Change
Intellectual 370 points
Part Number: SN65HVD75
Other Parts Discussed in Thread: TIDA-00527

Hi Hao,

We try to implement RS485 power over bus with SN65HVD75.

FW engineer said sometimes receiver wrong data.

Thus, I measured RS485 signal waveform as below.

The wrong data happened at high light red area (A-B < VIT-). 

Do you have any suggestion to solve this issue?

Ch1: RE, Ch2: R, Ch3: RS485 A, Ch4: RS485B, Math: A-B

Regards,

Jason

  • 0
    TI__Mastermind 25705 points

    Jason,

    How did you implement RS-485 power over bus, like TIDA-00527?

    www.ti.com/.../tidu993.pdf

    What's the data rate and cable length of the application? It looks like the bus has some reflection when it's entering the high impedance mode. You could try changing the 120Ohm termination to two split resistors with a big common mode capacitor to ground.

    Regards,

    Hao

  • 0 in reply to Hao L
    Intellectual 370 points

    Hi Hao,

    We follow TIDA-00527 to implement RS485 power over bus.

    The inductor and capacitor use 1mH and 10uF.

    The system data rate is 1Mbps with 2m cable length.

    I tried to build filter like you mention (60 Ohm + 60 Ohm + 10n), high frequency ringing has been cancel out.

    But R output still pull low around 30us in the end. 

    How to prevent pull low in the end?

    Ch1: RE, Ch2: R, Ch3: RS485 A, Ch4: RS485B, Math: A-B

    Regards,

    Jason

  • 0 in reply to Jason Change
    TI__Mastermind 25705 points

    Jason,

    Thanks for your update. A trick of this design is that it requires the DC balanced signal, since the DC voltage are established before and after the capacitor separately. You can refer to the last paragraph of the design overview section (page 2) for the detail. If it's hard to generate such signal, you may want to try with a clock as the input.

    Regards,

    Hao