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THVD1406: Two THVD1406 to Implement a RS-485 Repeater

Neet
Intellectual 612 points
Part Number: THVD1406

Tool/software:

Hello E2E Team,

I am evaluating the THVD1406 auto-direction RS-485 transceiver for use in an RS-485 repeater application. Since this device includes automatic driver enable control, I would like to confirm whether two THVD1406 transceivers can be directly connected back-to-back to implement a bidirectional RS-485 repeater without requiring any additional control logic, pull-up/pull-down, delay circuits.

Proposed Configuration:

  • Two THVD1406 transceivers, each connected to separate RS-485 buses.
  • A1/B1 of the first transceiver connected to RS-485 Bus 1, and A2/B2 of the second transceiver connected to RS-485 Bus 2.
  • TXD and RXD of both transceivers cross-connected, i.e., TXD1 > RXD2 and TXD2 > RXD1, to enable bidirectional data flow.
  • No external DE/RE control, relying solely on the auto-direction functionality of the THVD1406.
  • Standard termination resistors (120ohm at both ends) and necessary pullup/pulldown resistors on A/B lines.

Key Questions:

1. Will this setup allow seamless bidirectional data forwarding between the two RS-485 buses, effectively functioning as a repeater?

2. Are there any timing, delay, or bus contention issues to consider when using two auto-direction transceivers in this manner?

3. Would any additional provisions (e.g., external logic, resistors, or filters) be required for reliable operation?

Kindly give the best guidance.

  • 0
    Guru 316690 points

    This will not work; a low signal will result in an infinite loop, i.e., neither transceiver will ever stop outputting low.

    You would need additional logic that prevents a transceiver from driving low when another node on the same bus segment is the only cause of the low signal.

  • 0
    TI__Genius 13512 points

    Hi Neet,

    What is your goal behind using an extender? What distances are you trying to reach? 

    While we haven't tested this use case with this device internally, we do have an existing reference design that shows something similar to this is possible. The logic in our repeater design is essentially the same as the internal logic of the THVD1406.

    You could prototype this by holding /SHDN and /RE high and adding pull-up resistors on R. Talking with my team, there could be an edge case that creates bus contention if both transceivers receive a high-to-low transition, but this is not necessarily a PHY issue and more of a data layer task to troubleshoot. Either way, I recommend looking at our reference design and reading through that for further background. 

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Intellectual 612 points

    Hello ,

    We want to isolate MODBUS sensor nodes on one side from a long section of cable that leads to the Master on other end. We cannot add isolated RS485 on each sensor node & hence are looking to add a repeater based isolator in between.

    Thank you for your insights. I would appreciate if you could help us with some more clarification on how do we wire up the test prototype - any specific issues we might have to be aware of at our intended speeds?

  • 0 in reply to Neet
    TI__Genius 13512 points

    Thanks for that info Neet.

    If your goal is to isolate the bus, then I would just use our reference design that I linked as it is fully isolated. That will save you time since that design is already proven rather than prototyping and testing a new concept (which has a chance of failing).

    However if you want to stick with the THVD1406, I can try to work out some recommendations for that concept. Just let me know. 

    Best,

    Ethan