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THVD1505: THVD1505 polarity correction feature

MIKHAIL SALOV
Expert 6940 points
Part Number: THVD1505

Good day Colleagues,

Could you explain if the THVD1505 (or another similar device with auto-polarity correction feature) requires all of the transcievers be supporting this feuture on the bus for proper opeation?

Or any technical limitations of using the THVD1505 with another trancisevers without auto-polarity correction feature?

Thank you, Mikhail

  • 0
    Guru 317260 points

    It is possible that the A/B lines are exchanged only at the connector of one device. In this case, only this device would need a polarity-correcting transceiver.

  • 0
    TI__Guru 52352 points

    Hi Mikhail,

    If the bus is connected correctly - the polarity is correct on all receivers w.r.t. driver then the THVD1505 work like a standard RS-485 receiver and can be implemented in the bus. That being said - if a non auto-polarity correction device is used on the bus and there is a polarity issue the receiver will not fix the data-stream and the data stream received on that node could be inverted. 

    However all the THVD1505 is really doing - is inverting the datastream when a polarity issue is detected - as it requires a fail-safe biasing network to set a "default" state of logic high - so that if there is a polarity issue the device will know because at idle the bus with a cross-wire fault would show as a logic low. Using the same type of architecture you can use a standard RS-485 device with the fail-safe biasing network - however instead of the receiver inverting the data stream the MCU would be able to tell at bus idle that there is a cross-wire fault because the default would be R = Low - which then the MCU or other controller can invert the datastream itself. So if you mix auto-polarity devices and standard only the nodes with auto-polarity correction will invert the datastream when a cross wire fault is detected and the standard nodes will send an inverted datastream to its controller - you can use firmware in the controller to help invert the datastream so that the application is still "auto-polarity" - but in general this is harder to do than just to ensure the bus is fully auto-polarity compliant by using auto-polarity devices - in some cases that may not be possible but the risks are listed above if you mix devices. 

    I do have a brief app note on the THVD1505 just as a good reference piece for you and customer on designing with this device (and similar) : https://www.ti.com/lit/an/slla572/slla572.pdf?ts=1696606049549&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fen-us%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253Drs-485%2526nr%253D100

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson