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ISO7721: bidirectional isolator in PROFIBUS interface

Part Number: ISO7721
Other Parts Discussed in Thread: PROFIBUS, ISO721, ISO1432, ISOW1432,

Hello,

I'm designing an isolated PROFIBUS interface, that shall be able to communicate at the maximum speed of 12 Mbps. I came across TI's Application Report SLLA177A (PROFIBUS Electrical-Layer Solutions https://www.ti.com/lit/an/slla177a/slla177a.pdf), where two single-channel ISO721 are used for TX/RX signal isolation. Also, design examples from other companies use two single-channel isolators. 

Can I use a bidirectional isolator like ISO7721D for TX/RX signals coming from the transceiver? What are the pros/cons if using this solution?

Thanks in advance,

Alessio

  • Hi Alessio,

    Thank you for reaching out to us and for your interest in TI isolators.

    The application report SLLA177A that you are referring to is one of our older reports. Although all the content described in it is still accurate, the devices referred are very old where single-channel digital isolator (ISO721) is used to isolate the channels as multi-channel devices weren't available during that time (2005).

    We now have many devices that have much more integration and much better performance than ISO721. Please see below some of the device suggestions for your requirement.

    1. ISO1432 - Isolated RS-485 transceiver that integrates digital channels and RS-485 transceiver into a single package supporting datarate upto 12Mbps.
    2. ISOW1432 - Isolated RS-485 transceiver with integrated DC-DC that integrates digital channels, RS-485 transceiver and also an isolated DC-DC converter including the power transformer into a single package supporting datarate upto 12Mbps.

    Please note that the above two devices are PROFIBUS compatible as stated in the datasheet.

    If you still would like to keep the isolator and transceiver separate, then yes, ISO7721 along an RS-485 transceiver can be used. Please refer to below application report that describes various methods to implement RS-485 isolation.

    How to Isolate Signal and Power for an RS-485 System (Rev. C)


    Regards,
    Koteshwar Rao

  • Hi Rao,

    many thanks for your help.

    Is there any timing requirement for PROFIBUS isolators, apart from >12Mbps baud rate capability?

    A Youtube video on TI channel ( https://www.youtube.com/watch?v=lxFeFx2A6dM ) states <2ns skew in propagation delay, but it is very hard to find an isolator with such a low value. I cannot find this specification in the standard 61158-2, that only refers to a 2ns cable skew between the differential pair signals. Can you please help me with that? Where does that specification come from?

    Regards,
    Alessio

  • Hi Alessio,

    I believe the requirements specified in IEC 61158-2 should be sufficient to meet your application requirements. The specifications referred in the YouTube video are general guidelines to help achieve good performance on PROFIBUS. Thanks.


    Regards,
    Koteshwar Rao

  • OK, thanks for your help.

    Regards,
    Alessio