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SN65HVD1781-Q1: bitrate vs 201 transceivers

Bas Reijnen
Prodigy 20 points
Part Number: SN65HVD1781-Q1
Other Parts Discussed in Thread: SN65HVD1781

We are investigating the use of the SN65HVD1781 in a RS485 bus topology with 201 transceivers connected through a bus structure built with 110 ohms transmission lines (total length 7-10M, stub length < 1cm)
we want to achieve a 500Kbit/s bitrate but from the datasheet it is not 100% clear whether the SN65HVD1781 is capable of driving the receiver capacitance of all the transceivers at this bitrate.

  • 0
    TI__Mastermind 25705 points

    Bas,

    The capacitance of A/B pins can be found on section 6.6 page 5:

    COD Differential output capacitance 23 pF

    If it's transformed to signal-ended structure, it's about 46pF to ground. Since the cable is long and modeled as transmission line, the cap load of 201 transceivers would not be lumped into one load. In one experiment, we drove 256 nodes that were divided into 16 segments over 50 meters. The waveforms on the signal chain all looked decent. Please let me know if you have any questions.

    Regards,


  • 0 in reply to Hao L
    Prodigy 20 points

    Hello Hao,

    I do believe it is possible to drive the 256 nodes, but I wonder what is the maximum bitrate achieved in this scenario with the mentioned driver?

    regards,

    Bas

  • 0 in reply to Hao L
    TI__Mastermind 25705 points
    Sorry I forgot to mention that the data rate we applied was 1Mbps in the test. The device should be able to handle it in your application. Please let me know if you have any questions.