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SN75179B: The R pin shows abnormal signal when AB pins are floating.

Wayne Wang20
Expert 4380 points
Part Number: SN75179B
Other Parts Discussed in Thread: THVD1500

Hi team,

My customer is using SN75179B in a inverter project as the 422 interface. To make a default state when the interface is floating, customer adds pull up resistors on both R pin and D pin. For most cases, when the AB pins are floating, the R pin is a stable high level. However, for some specific devices, the R pin shows abnormal signal when the AB pins are floating. Please see the abnormal waveform on R pin as below picture. The purple curve is the waveform on R pin when the AB pins are floating.  It shows some periodic low level. The picture is the screenshot of one of the periodic low level. Do you have any comment on this issue?

Best regard,

Wayne

  • 0
    TI__Guru 62920 points
    Hi Wayne,

    When the RS-422 interface is floating, the input differential voltage to the transceiver will be close to 0 V. This is also where in the input switching threshold is set, so small fluctuations in the pin voltages (due to noise, etc.) could result in the R output toggling between low and high states.

    If you want to avoid this, you could place a pull up resistance on the A line and a pull down resistance on the B line. This is often referred to a failsafe biasing, and is useful in establishing a positive differential voltage at the transceiver input when the bus is undriven (resulting in a stable high-level receiver output). Another solution would be to populate a different transceiver with this failsafe biasing built in. One example of this would be THVD1500.

    I hope this helps; please let me know if it is unclear or if you have further questions.

    Regards,
    Max