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SN75176B: R pin received unexpected signal while the differentiated signal is 0(A-B) in noise environment

Benjamin Zhou
Expert 4000 points
Part Number: SN75176B
Other Parts Discussed in Thread: THVD1550, THVD1500

Hi Team,

  My customer reported an issue that the R PIN of 75176 received an unexpected signal:

   The R PIN of SN75176 received an unexpected signal with differentiated signal is 0 while the load (motor driver) is active, but the R PIN stayed as normal without load(motor driver is de-active). The 2 RS485 nodes are connected by a 25m cable connected to the bus; 

   It’s obvious affected by the noise, but the differentiated signal kept as normal, could you kindly give comments on 2 items?

  • Reason analysis

  • Solution recommended

   More details like schematic and test waveform could be found as attached file: SN75176 received unexpected signal when AB differentiated signal is 0.docx

   Expect for your reply, thanks.

 

Best Regards

Benjamin

  • 0
    TI__Guru 62910 points

    Hi Benjamin,

    The input high/low switching threshold for SN75176B is near 0 V, so if the signal is held near this threshold even a small amount of coupled noise could result in toggling on the R output.  That seems to be what is happening here.  The easiest solution is to provide a differential bias to the receiver that is greater than its worst-case (maximum) VIT+ threshold of 0.2 V.  If the 0-V differential input is the result of an open or high-Z/idle bus, then this could be accomplished via the use of a pull-up resistance on A and a pull-down resistance on B.  (I see some included on the schematic and the "normal result" does show this offset on A-B; were these pull-up/down resistances populated when then failing result was obtained?)

    If the differential voltage is 0 V due to a short between A and B then external pull-up/pull-down resistances wouldn't solve the problem.  In that case, the bets solution should be to change to a transceiver with integrated fail-safe function (which ensures the R output is high when the differential input is 0 V).  I'd recommend THVD1550 as a p2p drop-in update to SN75176B.

    Max

  • 0 in reply to Max Robertson
    TI__Expert 4000 points

    Hi Team,

      This issue has been solved by adjust the value of resister. also THVD1500 has been verified could fix this issue.

    Best Regards

    Benjamin