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SN65176 A,B Outputs are Asymetric

DerekG
Intellectual 985 points

Can you help clarify why the A,B outputs from the SN65176 waveforms below are asymmetric? The circuit  below shows external termination network (13k pullups and 120 parallel)

 

  • 0
    TI__Expert 5175 points

    Greetings -

    The RS485 output should be symetrical, the waveform posted appears odd.  Usually the driver VOL is around 1V, the green looks OK, the yellow is not switching low.  The VOH levels appear a bit low also, I would have expected to see north of 3V. 

    I assume the 65176 is connected to DIFF BUS nodes, suggest to test that with just a 120 Ohm load (remove SBI chip and pull up and pull down).  If you now see symetrical swing, then add the pull up and pull down, check again, then add the SBI device.  This will help isolate where the additional current / loading might be coming from.  Also insure the pull up in indeed 13k Ohm, it might be wrong value, and is whacking the VOL on that side with too much current.

    John Goldie

    DPS APPS / SVA / TI

     

  • 0
    TI__Mastermind 28265 points

    Hello Derek,

    here are my 2 cents. the common-mode filter circuit does not look symmetrical. Could it be that you are missing a connecting point from the lower Diff.Bus line to the middle point of the two CL capacitors at the output ?

    regards, Thomas

  • 0
    TI__Mastermind 28265 points

    Derek,

    I simulated your filter circuit and agree with John that there is probably some resistive mistmatch involved. I got similar results to yours when I replaced the lower 13k resistor with a 13 ohms resistor. This could be a simple tolerance ring issue (not that you would be able to see those on these tiny thin film resistors anyway) or you might have some parasitic soldering connection lowering the impedance towards ground.

    Cheers, Thomas