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TIDA-01401: Does the shield of the RS485 cable need grounding?

Hui Chen
Intellectual 770 points
Part Number: TIDA-01401

Hi TI Engineer：

       I have two questions.

       1.In many circuit systems, a parallel scheme of resistors and capacitors is used to connect the external earth ground with the internal GND.In this way, the DC is isolated, while the AC is a low impedance.What are the benefits of doing this for EFT testing?How to select resistance value and capacitance value?Panasonic servo driver, this capacitor value is 0.1uF.

       2.In the EFT test mentioned in the paper, does the RS485 cable shield connect the earth?In that case, the RS485 cable shield should be single ended or double ended grounding,and Why?

thank  you

  • 0
    TI__Expert 3570 points

    Hello,

    thank you for your interest in our TI design.

    In variable speed drives, the encoder interface connector metal case is connected to protective earth (PE). Vendors of drives typically request shielded cables and the shield is then connected to PE too. The encoder metal case and therefore the connector shield is also connected to protective earth (PE),  as the encoder is typically mounted to the motor and the motor case is connected to PE too. If we now connected a shielded cable the shield will be connected to PE on both sides of the cable. You can find more details on the video here:

    To your question 2: Yes both sides of the cable shield are connected to PE. Why: The standard requires we test the equipment such as TIDA-01401, as it will be used in the field. Have a look at a instruction manual of a variable speed drive customer. I cannot mentioned customer names here though.

    To your question 1: As you describe often a high-impedance parallel RC network is used to couple the protective earth (PE) to the signal GND. We chose 1000pF and 2.2MOhm for our design, as an empirical custom specific value. We did EFT tests with the RC and without the RC and didn't see a significant difference in the EFT test results. Therefore we marked it not populated in our schematics and consider it customer board specific to chose the optimum value, as customer board.

    Regards,

    Martin Staebler