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Original question:

ISOW1412: ISOW1412 vs ISOW1432

ISOW1412: common ground between devices

Hector Chen
Intellectual 330 points
Part Number: ISOW1412

Hi,

Because of the isolation mechanism, we will not see GPD on GND2 between one another, is this correct?

The main contribution of the common voltage is GPD. RS485 can operate within the common voltage between -7V and 12V.

Do we need to connect GND2 among devices? This means, for half-duplex applications, we need 3 wires for the differential pair and common ground.

Thanks.

Hector

  • 0
    TI__Mastermind 49415 points

    Hi Hector,

    Thanks for reaching out.

    The isolated RS-485 devices separate system ground from the RS-485 bus thereby limiting the amount of Ground Potential Difference (GPD) that is observed on the bus. With isolation in place, it is less likely for the bus to hit the limits of device common-mode voltage rating. For this reason, it is not necessary to run a ground signal between RS-485 devices. it should be fine to have a 2-wire half-duplex bus without ground and we expect the solution to work fine.

    Let me know if you have any other questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Intellectual 330 points

    Hi Koteshwar,

    Thanks for your comments.

    Without the ground, if all the devices on the bus work fine without violating the common-mode voltage rating, should all RS-485 devices be isolated? 

    Could you explain why it is less likely to hit the limits of common-mode voltage rating with isolation in place?

    Thanks 

    Hector

  • +1 in reply to Hector Chen
    TI__Mastermind 49415 points

    Hi Hector,

    Thanks for following up.

    Ground potential difference (GPD) between two systems occurs due to the potential difference between two Protective Earth (PE) reference points the systems are referencing to. This potential difference is roughly estimated by the RS-485 standard to be in the range of -7V to 12V for reasonable distances.

    When isolation is used, a floating ground, that is not referencing PE, gets created. This breaks the connection between the two PE reference points thereby eliminating the possible common-mode voltage that could have existed otherwise.

    If there are two RS-485 nodes, then only of them needs to be isolated to break the ground loop. If there are 10 nodes in the bus, then 9 of them need to be isolated to break all possible ground loops. Hence, it is a common practice to isolate all RS-485 nodes to keep the system uniform as against keeping only one node non-isolated.

    Please also note that isolation also serves another purpose of protecting the system from high voltage transients on the bus. Hence, isolation is generally used on all nodes. Let me know if you have any questions, thanks.


    Regards,
    Koteshwar Rao