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RS-485

Discussion in 'Electronic Design' started by joble, Dec 8, 2004.

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  1. joble

    joble Guest

    Hi,

    I need to transport an RS-485 signal over a "long" cable (100 meter).
    The cable will receive lots of noise (large motors, ...) I'm planning
    to use the SN75176 as a driver and receiver.


    Is it ok to use UTP cable, or should i use STP?
    What kind of grounding topology i have to use in the transmitter and
    in the receiver?

    Can somebody help me with this, or give some URL where i can find more
    info?

    Thanks,
    Joble
     
  2. The question you have to ask, is "what happens if I get this wrong"? If
    your neck is on the line, or comms failure cannot be tolerated, do it right
    first time.

    With big current around and motors starting, ground movement and common mode
    pickup is going to be pushing the common mode limits of the receivers - and
    you can get damaged receiver ICs too. Also if you connect the shield to
    ground at both ends, current is going to go up and down the shield.

    I recommend opto coupling at least one end or even better both ends. That
    way, no common mode voltage worries and no shield current.

    The $$ for shielded vs unshielded is probably insignificant compared with
    the investment elswhere in the plant.

    Roger
     
  3. I read in sci.electronics.design that Roger Lascelles
    I agree: this will cost more initially but it will work first time and
    continue to work reliably. Copper without isolation over that distance
    and in that environment is seriously bad news. You might even consider a
    complete fibre-optic connection.
     
  4. John Smith

    John Smith Guest


    Here is a link to a supplier of optically coupled RS485 devices...

    http://www.bb-elec.com/

    They also have some tech notes.

    I have no affiliation with these people. Good luck.

    John
     
  5. As mentioned : have the GND and Vcc on the cable too and
    use it to optoisolate the other side.
    A shielded cable reduced e-filed coupling. And the twisted
    pair reduces the magnetic coupling. Yes, take STP.

    Rene
     
  6. Maxim has RS485 chips that include opto-isolation. You can also roll
    your own with opto or some other kind of isolation, but you also need
    power on the other side of the isolation, so an isolated DC-DC
    converter or perhaps a spare winding on the PS transformer could be
    used.


    Best regards,
    Spehro Pefhany
     
  7. Anders F

    Anders F Guest

    Maxims devices are very nice. You pay for the ease of implementation - but
    they're nice =)

    /Anders
     
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