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RS485 and power on the same cable

Discussion in 'Electronic Design' started by Henrik [6650], Jun 13, 2006.

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  1. Hello group,

    I am trying to figure out how to communicate serially (eg.RS485) and power
    the devices on the string via the same pair of cables.

    I want to be able to transmit data at baudrates of 19200 to 57600 via this
    link. The reason I would like to have data and power on the same cable is
    that I need to use existing cable in a large installation.

    Could somebody advice me on how to get ahead, how to mix power and data? Or
    perhaps provide a link or reference design.

    Product price is an issue.

    Thanking you in advance
  2. I don't think you can do loop power off an RS485 circuit...

    RS485 is voltage differential, and each node turns on its transmitter only
    when it has something to say, so it is possible that there is 0 voltage on
    the line at times.

    OTOH, if anyone can help you, it Mr. RS485:


    ,>/'_ o__
    (_)\(_) ,>/'_ o__
    Yan Seiner, PE (_)\(_) ,>/'_ o__
    Certified Personal Trainer (_)\(_) ,>/'_ o__
    Licensed Professional Engineer (_)\(_) ,>/'_
    Who says engineers have to be pencil necked geeks? (_)\(_)
  3. Guest

    I don't think you can do that with RS485, but there are some options if
    you build something custom.

    If you were willing to go really slow and build hefty drivers, you
    could put a full-wave bridge rectifier at the receiver, as long as the
    communication was one way (or reply periods were short enough that you
    could run off a capacitor).

    If you went to a non-return-zero coding with no DC component, you could
    AC couple the signal on top of a DC offset, using a series inductor to
    add and draw the power and a series capacitor (or a transformer as in
    ethernet) to couple the signals. Your transmitter would be single
    ended, but your receiver could still be differential.
  4. I don't think that you can do that with RS485. However have a look at
    solutions from Echelon (Lonworks & Neuron chips) : data+power (either AC or
    DC) on the same 2-wires cable.

  5. You can "phantom power" the remote the same as using a microphone, add and
    take power using a centre tapped transformer, returning via the screen; make
    sure the transformers can take the additional DC current-
    if you can change the impedance to 600R then lots of telecoms transformers
    will do the job.
  6. Terry Given

    Terry Given Guest

    yuk! expensive, slow, proprietary....

    I've done a lot with 10Mbps RS485 + 48Vdc in an STP cable (two pairs) -
    perhaps 25,000 or so in the last 3 years. I spent some time coming up
    with a way to do it over 2 wires, so I could use coax cable. its not
    hard. In practice, we found cheaper multipole connectors and didnt
    implement it, but I did get one up and running.

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