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ISO1450: [RS-485] Inquiries related to communication level

Part Number: ISO1450
Other Parts Discussed in Thread: SN65LBC184

Hi, TI Support Team

The following two parts are being used by the customer for RS-485 communication.

BCU side: ISO1450

PCS side: SN65LBC184

During the test, the following symptoms were confirmed and we will contact you.

1. The communication level of Tx/Rx is different during RS-485 communication.

What could be the cause?

2. There was no functional problem because the communication level was different.

Is there any chance of a problem?

Please refer to the attached circuit and waveform below for better understanding.

If you have any additional questions related to this inquiry, please make a request.

Thanks.

Regards,

MJ

  • Hi MJ,

    Thank you for reaching out and for sharing details related to your queries. Please see my inputs below.

    1. The communication level of Tx/Rx is different during RS-485 communication.

    What could be the cause?

    It is completely normal to see different output voltage swings from different RS-485 transceivers as the driver specifications of different devices can be different. Even though their output swings are different, they are all usually comply to the minimum RS-485 electrical specifications. Hence, this is not a problem for communication.

    2. There was no functional problem because the communication level was different.

    Is there any chance of a problem?

    That is right, no communication issue is expected as both RS-485 transceivers that customer is evaluating comply to RS-485 electrical specifications. The HIGH/LOW thresholds are closer to 0V for both devices while the output swing is >5Vp-p, there is sufficient margin from the actual levels compared to the thresholds and hence, no communication error is expected.

    Regarding the termination resistor to be used, it should be about 120Ω used only at two RS-485 nodes located on the opposite ends of the RS-485 bus. Please refer to the below two documents for more information on RS-485 terminations, thanks.

    https://www.ti.com/lit/pdf/slla070

    https://www.ti.com/lit/pdf/slla272


    Regards,
    Koteshwar Rao

  • Hi, Koteshwar

    Thank you for your kindly reply.

    I have further inquiries.

    ISO1450 output level is currently +-5V (p -p 10V).

    Is it possible to set this level to +-2.5V (p-p 5V)?

    Please let me know if there is a possible way.

    Thanks.

    Regards,

    MJ

  • Hi MJ,

    There is no setting in device to change the output voltage swing and this is usually not available for most RS-485 transceivers. The primary reason is that there is no need for such reduction in the voltage swing.

    The voltage drops as it goes through the communication cable and hence it is desirable to have a higher voltage swing at the source. This doesn't cause any issue to any other node having lower output swing as they are all still compatible to communication without any issues. Thanks.


    Regards,
    Koteshwar Rao