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THVD1420: termination in multi-nodes use case

Jerry Chen
Expert 5110 points
Part Number: THVD1420

Hi team,

In my understanding, Rt might be needed for each node. Could you kindly help me understand the reason that Rt is not applied to the middle nodes?

Thanks.

  • 0
    Guru 272280 points

    Adding termination resistors to the middle nodes is likely to make the load too large for the driver.

    To avoid signal quality problems, you must keep the stub length short enough; see the discussion in section 4 of AN-979 The Practical Limits of RS-485.

  • 0 in reply to Clemens Ladisch
    TI__Guru 68742 points

    Jerry,

    As Clemens said, you don't want the overall bus impedance to be too low, as the RS485 drivers wont be able to drive the correct voltage levels while communicating. If the stub length cannot be shortened, then termination can be added to the middle nodes, but it must be much higher resistance value than the end nodes. This is because the parallel combination of all termination resistors should never be less than 54 ohms.

    Regards,

    Eric Hackett