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RS232/RS422/RS485 Isolation

Discussion in 'Electronic Design' started by Matt, Oct 11, 2007.

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  1. Matt

    Matt Guest

    I have been tasked to design a serial card that can operate use RS232,
    RS422, and RS485. I found a transceiver (MAX3160) that can convert all
    three to signals my UART (TL16C554A) can handle. I started to worry
    about isolation to protect the system the card is in and also to
    eliminate any ground loops that can exist between my system and other
    systems so I began to look for an opto coupler for each serial
    channel. Because RS232, and RS422 use different voltage levels, I am
    having an issue locating an opto coupler that can handle either 232,
    or 422 depending on a jumper setting. Do I need to worry about opto
    coupling before the MAX3160 or can I do it after where all the levels
    are identical regardless of what serial standard I use. I would still
    want to optically isolate the signal ground at the connect however. Is
    this correct?

    Thanks for any help.
     
  2. James Beck

    James Beck Guest

    Why not isolate the data lines going TO the transceiver?
    Then all you need to worry about are logic level signals.
    I haven't looked up the MAX3160, but if it is a single supply IC then
    all you need to do is make an isolated DC to DC converter to power the
    IC.

    Jim
     
  3. Matt

    Matt Guest

    Jim,

    I hadn't thought of using an isolated DC-DC, good idea. Are you
    suggesting this sort of set up?

    UART==>ISOLATION==>Transceiver==>Connecter to the outside world?
    ^
    Isolated DC-DC

    Thanks again
     
  4. James Beck

    James Beck Guest

    Yep, the isolated DC runs the transceiver and its' side of the isolation
    barrier. I do a very similar thing for an RS-232 to isolated RS-485
    converted we make as a support part for one of our network systems.
    In fact we use the MAX253 (if you're gonna' use one Maxim part two ain't
    gonna' kill you any more dead) and a Coil Craft Q4470C transformer.

    Jim
     

  5. That's the usual way. You will probably need 3 signals (Tx, Rx and
    TxEn) opto-isolated (2 in one direction and one in the other), plus
    the DC-DC.

    Best regards,
    Spehro Pefhany
     
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