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SN75176B: Erroneous pulse on A/B outputs

Derwys Munoz
Prodigy 40 points
Part Number: SN75176B
Other Parts Discussed in Thread: THVD1500

Hello,

I'm trying to perform half-duplex serial communication using RS-485.

I'm using IC SN75176 to do it and some micro controllers.

My issue is this one:

- I'm connecting together Pin 2 & 3 (DE,RE). If Pin 2 & 3 are Low (RX mode) pin 4 (TX line) is idle (high level) and I want to transmit, when Pin 2 & 3 go high on output lines A B a HIGH - LOW - HIGH pulse appears while Pin 4 remains High.

This transition is interpreted as an erroneous start condition by receivers.

Do you know why this pulse is produced and how to solve it?

  • 0
    TI__Guru 62920 points
    Hi Derwys,

    Would it be possible to capture the waveforms of the A and B signals (pins 6 and 7) when this pulse appears? Seeing the waveforms may give us a little better understanding what could be going on. (This isn't the behavior I would normally expect from this IC.)

    Also, can you please provide a schematic of the interface circuit or otherwise let us know how the chip is connected and what components may be on the RS-485 lines (termination resistances, etc.)?

    Regards,
    Max
  • 0
    Prodigy 40 points

    Hello Max,

    Thanks for your fast response,

    Yesterday I changed some PSU capacitors and now I can't see the mentioned pulse any more, but I guess the problem is with SN75176 master,

    please look attached waveform.

    Master device's pulses shape is not good but these are received well by slave devices.

    Best Regards.

  • 0 in reply to Derwys Munoz
    TI__Guru 62920 points
    Derwys,

    I'm glad to hear the system is functional, although the "A" signal coming from the master side seems degraded (typically it would be the mirror image of the "B" waveform). Are there any components (series resistances, pull-up/pull-down resistances, filter capacitors, clamping diodes, etc.) on the A line that are not present on B? Or, does a new unit installed on the master side show the same behavior?

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 40 points
    Max,
    On master side there is only a failsafe/load resistor network, VCC-600 ohm-130 ohm-600 ohm-GROUND.
    It would be the same load for both A / B lines, and the same load for slave devices.
    I'm looking for a new 75176 for replace it on master side.
    There would be convenient to replace all them for SN65#####?
  • 0 in reply to Derwys Munoz
    TI__Guru 62920 points
    Derwys,

    Yes, the SN65175B devices will function the same as the SN75176B devices, so if you have those more readily available they could be used. (If you are looking for a new part number in general, you might consider our newer THVD1500 device in this application. It's pin-to-pin equivalent to SN75176B but has improved ESD/noise immunity, wider operating voltage range, and supports a higher node count.)

    Max
  • 0 in reply to Miguel Robertson
    Prodigy 40 points
    Thank you very much Max...