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THVD1439: Communication waveform

Shuji Ishiwata
Mastermind 7815 points
Part Number: THVD1439

Tool/software:

Hi All,


I have a question about THVD1439.
Is it necessary to pull up the A(6pin) and pull down the B(7pin)?

Below are the waveforms when each terminal is pulled up and pulled down.
The waveform will be normal.

Below are the waveforms when the pull-up and pull-down resistors are removed from each terminal.

The waveform becomes dull when the A terminal pull-up is removed. Do you know why?

Best Regards,
Ishiwata

  • 0
    Guru 316690 points

    RS-485 is a differential bus. Only the difference A−B matters.

    You need fail-safe biasing resistors only when there is some receiver without fail-safe inputs on the bus. (The THVD1439 and all other modern transceivers have fail-safe inputs.)

  • 0
    TI__Guru 72771 points

    Hi Ishiwata,

    Shuji Ishiwata said:
    Is it necessary to pull up the A(6pin) and pull down the B(7pin)?

    You can refer to Clemen's comment. Adding those resistors are for helping with noise when the bus is idle or if receivers on the bus do not have internal fail safe protection.

    Shuji Ishiwata said:
    The waveform becomes dull when the A terminal pull-up is removed. Do you know why?

    If you remove the external resistors and disable the driver (DE low) the differential bus no longer has any sort of biasing on it. So when the driver is disabled, the A/B nodes will leak charge into the air or leakage currents through parasitic paths. This will look like a slow RC discharge like what you show on the waveform. Basically from the A/B nodes, the 'R' looks very large so the waveform is slower.

    With external resistors present, the RC discharge will be faster depending on the resistor value you choose.

    -Bobby

  • 0 in reply to BOBBY
    Mastermind 7815 points

    Hi All,


    Thank you for your answer.

    I understand that when DE is low, the differential bus has no bias, so the RC discharge takes longer.

    Is pull-up and pull-down the way to shorten the discharge? Are there any other ways?


    Best Regards,
    Ishiwata

  • 0 in reply to Shuji Ishiwata
    Guru 316690 points

    The slow discharge is not a problem when it is the same for A and B. Please show an oscilloscope trace with computed A−B.

  • 0 in reply to Shuji Ishiwata
    TI__Guru 72771 points

    Hi Ishiwata,

    Clemens is correct. If your concern is the R pin which looks like it may be seeing a glitch then you should see if you can get a scopeshot of the A-B math on the scopeshot to see if this signal is seeing a glitch. 

    Shuji Ishiwata said:
    Is pull-up and pull-down the way to shorten the discharge? Are there any other ways?

    I don't really know of any other methods to do this other than making the bus load lighter. It's an RC so either change the R (can't be changed without external pull ups and downs) or change the C which is the cable length......

    -Bobby