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THVD1400V: THVD1400V wire resistance on Bus input/output & Digital input

Louis Lin
Prodigy 200 points
Part Number: THVD1400V

Tool/software:

Dear team,

Should we pay attention to the wire resistance highlighted in yellow?

Louis Lin

  • 0
    Guru 317370 points

    The image did not attach; you have to use the "Insert"/"Image" menu entry below the text box.

    In general, RS-485 is designed to tolerate the voltage drop over long wires; drivers generates more than ±1.5 V over the load, while receivers detect inputs as low as ±0.2 V.

  • 0 in reply to Clemens Ladisch
    TI__Prodigy 200 points

    Dear Clemens,

    Here is the updated image file. Do you mean that if the voltage does not drop below 0.2V, the communication will work fine? I believe this may be difficult to let voltage drop from 1.5V to 0.2V with a standard wire on the PCB. Therefore, I can conclude that using a standard wire is acceptable without needing to be overly concerned about the wire resistance.

    Louis Lin

  • 0 in reply to Louis Lin
    Guru 317370 points

    Yes; the total resistance of the marked traces is negligible.

  • 0 in reply to Louis Lin
    TI__Guru 52462 points

    Hi Louis,

    Clemens is correct - the specific highlighted traces shown above are going to have negligible series resistance.

    That being said there are a couple things that I wanted to point out:

    1. Series resistance of wiring / cabling is not negligible at distance - if the distance between nodes is large it will matter. For like 150mm to 300mm long traces it generally isn't an issue. 

    2. Ideally the traces are still impedance matched - RS-485 should use a 120 ohm characteristic cable / traces - so the trace should be 120 ohm characteristic impedance; so that will impact overall impedance of the trace. Impedance matching is strongly suggested with this device for ~4.7m and above (the device is slow enough where shorter bus systems really don't need to be impedance matched) - and that 4.7m is the point where the device could cause reflections at signal transitions regardless of data rate. 

    Best,

    Parker Dodson