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THVD1406: THVD1406 @10kbit/s not working

Jorge Sampaio
Prodigy 30 points
Part Number: THVD1406
Other Parts Discussed in Thread: THVD1400

Hi

THVD1406 auto direction does not work at low data rates.

The output pulse "H" is only 8us at high level then output goes "idle" until next output pulse "L".

DE* is Low, SHDN* is Open.

Other device in the bus does not recognize the received data.

Is it possible to have a pulse "H" larger than the 8us "t device-auto-dir" to allow communication at lower data rates?

Thank you very much.

  • +1
    Guru 317320 points

    The THVD1406 does not have a /DE pin.

    The is no transceiver with a longer tdevice_auto-dir. This 8 µs interval is not supposed to cover the entire transmission; its purpose is just to force the bus out of the "L" state. The receivers are supposed to read a "H" state when the bus is in the idle state; most modern receivers have fail-safe inputs, but for older receivers, you can add fail-safe resistors to the bus. What receivers are you using? Can you show an oscilloscope trace of what the receivers see (compute A-B to see the differential bus voltage)?

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Other nodes are closed products from other manufacturers, may have any kind of transceiver. I can't change or see it.

    Thank you.

  • 0 in reply to Jorge Sampaio
    TI__Mastermind 48228 points

    Hi Jorge,

    So are you saying you won't be able to retrieve scope shots of the current issue? 

    Have you checked to see if other transceivers on the bus recognize data at a faster data rate such as >250kHz? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 30 points

    No, I can't see signals inside the product. This is a product installed at a customer. I can't open it.

    In our case the bus is working at 10kbps fixed.

    THVD1406 has been working for other customers without problems. Just in one case it did not work.

    THVD1400 works (with DE*).

    We have bought THVD1406 because THVD1400 was not available in stock.

    It seemed to be compatible but it is not a real alternative in our case.

    So, for me it is solved with THVD1400, as long as it is available.

  • 0 in reply to Jorge Sampaio
    Guru 317320 points

    You can measure the A/B bus voltages anywhere on the bus.

  • 0 in reply to Jorge Sampaio
    TI__Mastermind 48228 points

    Hi Jorge,

    The THVD1400 should be available for a long period of time since it is a fairly new device. 

    Without scope shots it is hard to debug the issue on a signal level. The only major difference between the THVD1400 and THVD1406 is the auto-direction control implemented in the THVD1406. Maybe this is causing the issues you are seeing when the device is on the bus.

    After a period of time tdevice_auto-dir, VOD sits at 0V, receiver is enabled and the drivers are disabled for the THVD1406. 

    When you say the device is failing, do you know how it is failing, or is it just that the device doesn't work on the system but other transceivers do? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 30 points

    Dear Tyler,

    Your answer tells exactly what I have written in my first report.

    The auto-direction implemented in THVD1406 does not work in some cases.

    I found it the hard way at a customer installation.

    I saw the waves you refer at the customer installation.

    So THVD1406 is working as intended, but this is not acceptable because it does not comply with standard RS485.

    Best regards

    Jorge

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    No, I cant go back to the customer.