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ISO7741: RS-232/422/485 isolated with transceiver on secondary side

Expert 5455 points

Replies: 2

Views: 26

Part Number: ISO7741

Hello expert,

My customer would like to use below block diagram for their RS232/422/485 isolated application. We have some questions below, may we have your help on it? Thanks a lot!

  • Could we use ISO7741+ISO7742 to implement 3T5R application?
  • We need to use different direction for ISO7741 and ISO7742. But it should be okay as long as we set the right direction, right?
  • For multi-protocol transceiver, what should we need to check? (ESD level)

Best regards,

Ann Lien

  • HI Ann,

    Thanks for reaching out.

    ISO77xx family of devices can certainly be used to isolate data lines connected to RS-232/422/485 applications. The channel configuration to be used depends on the number of channels needed in each direction. Please see my inputs below.

    • Looking at the devices you have already chosen, it seems like you have assumed VCC2 of ISO774x connection to Host and VCC1 to Transceiver. With this connection, ISO7741 + ISO7742 should meet your 3T5R requirement.
      • Though the requirement is already met using above 4x devices, an alternative option, which would also equally work, would be one from the list below.
        • ISO7761 + ISO7720
        • ISO7762 + ISO7721
        • ISO7763 + ISO7720
      • If the isolation requirements demand a package with 8mm creepage/clearance like DW then both solutions should take up same space. But if isolation requirements allow smaller packages with ~4mm creepage/clearance then ISO776x based solution should yield a smaller size.
    • That is correct. VCC1 and VCC2 circuits are exactly identical for ISO77xx, you can use them in either direction.
    • The requirements should ideally come from customer end product, I do not believe the transceiver has any say here. Transceivers are chosen based on application EMC requirement. Some of the EMC requirements would also be application to digital isolator and I believe ISO77xx should meet most of customer requirements without any issue.

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

    Koteshwar Rao

  • In reply to Koteshwar Rao:

    Hello Koteshwar,

    Thanks for your detail reply!

    Best regards,

    Ann Lien