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SN65HVD1781-Q1: bus lines, A & B not outputting differential

Ryan McKenna
Prodigy 20 points
Part Number: SN65HVD1781-Q1

I'm trying a design with the RS485 transceiver mentioned in the title on 2 different cards but using the exact same transceiver design. The schematic design is almost identical to the reference design in the datasheet but with a termination resistor (120 ohm) and a pull-up/pull-down fail safe resistor (10k) on A and B respectively. One card works perfectly and on the other card the A & B lines are outputting at the same polarity and not in differential mode. Both circuits were tested using a USB to 485 converter to the SN65HVD1781-Q1 circuit.

  • 0
    TI__Guru 62910 points
    Hi Ryan,

    The most likely explanation would be if either the A or B pin does not make good contact with the board or cable. (If just one line is connected, then the differential signal will appear in-phase since the low-impedance termination keeps the voltage similar between both lines.)

    Can you please double check that you have good electrical continuity on both lines? One useful debug step would be to probe the IC's A/B pins directly when it is transmitting. Another thing you could try is removing the termination resistance and seeing if the signal on one half of the differential pair goes away.

    If that fails, can we please take a look at some scope shots of the A and B lines? I'd like to see the amplitude and general signal quality to see if that inspires any other ideas.

    Thanks,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 20 points
    Max,

    Thanks for the quick response. It did turn out that one of the lines was shifted a pin over on a connector so it was floating. Thanks for the help.