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RS-485 AC termination method

Hiroshi Sato
Expert 2590 points

Hello,

I have a question from our customer.

Please see attached file for datail and may I have your comment ?

 

Regards,

Hiroshi Sato6131.RS485-question.ppt

  • 0
    TI__Mastermind 28265 points

    10nF is a heavy capacitive load on the driver which can cause high peak currents during signal transitions that could driver output into current limit. To measure transient currents place 1 to 10 ohms in series with capacitor and measure voltage across this resistor.

    you said power cable is routed together with signal lines in one twisted cable. Is this the 5V supply power for transceiver or higher power values?

    thanks, Thomas

  • 0 in reply to Thomas Kugelstadt
    Expert 2590 points

    Hello Thomas-san,

    Thank you for your checking and sending comments.

    I'll tell the customer them and ask to take measure and optimization of the capacitor.

    About power cable, your understandings is correct. The voltage is 24V, industrial standard. The application is a kind of data logger.

    Is there any cause for concerns with 24V supply?

    Thanks and best regards,

    Hiro

  • 0 in reply to Hiroshi Sato
    TI__Mastermind 28265 points

    Hello Hiro-san,

    I am verry sorry for my late response due to many customer engagements. I have also found another possible solution for common-mode filtering which some customers are using. Please see picture below. Maybe your customer could try to place this "split termination" at both ends of the bus and check its performance.

    Also with regards to the 24V power cable, your customer might use some TVS (for example SM712) to protect the A-B data lines if the power cable might break and come in contact with the data cable. If both cable are in a conduit behind a wall (so they are fixed) than this might be hypothetically, but if the cable is flexible then breaking of the cable insulation could be possible.

    I hope this helps. Best regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    Expert 2590 points
    Hello Thomas-san,
     
    These are very useful information, thank you.
    I'll ask the customer to try "split termination" too.
     
     
    Thanks and best regards,
    Hiro
  • 0 in reply to Thomas Kugelstadt
    Intellectual 280 points

    Hello Thomas,

    I'm wondering what if let 1nF be shorted.  Anyway, this still could make A/B terminal more balanced with respect to GND.

    Am I correct?

  • 0 in reply to Fan Yi
    TI__Mastermind 28265 points

    Actually we found that split termination in RS-485 is not very helpful. It is more applied in CAN networks where during the recessive state the bus potential is balanced due to the split termination.

    regards, Thomas