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SN65HVD1781-Q1: communication waveform fluctuates.

Tommy
Intellectual 565 points

Part Number: SN65HVD1781-Q1

Hi E2E,

When using SN65HVD1781QDRQ1 for design testing, the communication waveform fluctuates.

Is there any suggestion for improvement?

  • 0
    Guru 321960 points

    This is only common-mode noise; the differential voltage looks OK.

    Ensure that the decoupling capacitor C1501 is as near the transceiver's VCC pin as possible. But that ringing has a very low frequency (I'd estimate about 500 kHz); check if it also occurs in the VCC and GND lines (measure pin 5 against GND at the power supply).

  • 0 in reply to Clemens Ladisch
    Intellectual 565 points

    Hi Clemens,

    Thanks for your advice.

    This situation was improved by removing the common-mode inductors.

    However, There is always a short period of low level in the beginning, I do not know what factors will cause it?

  • 0 in reply to Tommy
    Guru 321960 points

    This happens when RS485_EN goes high before RS485_DI has a valid signal. In theory, UARTs are high when idle; is this not the case in your system?

  • 0 in reply to Tommy
    TI__Guru 54042 points

    Hi Tommy,

    Do you have a scope shot of the Enable signal and the DI signal during the change - I suspect that Clemens could be correct on the DI pin voltage not being correct - but I am not 100% sure as it seem per schematic that the DI pin should be held high in case of Idle.

    Best,

    Parker Dodson