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Powering isolated RS485 transceivers over cable

Discussion in 'Electronic Design' started by markp, Jul 15, 2010.

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  1. markp

    markp Guest

    Hi All,
    I have an application that requires about 12 sensor devices over a RS485
    cable, evenly spaced over about 20m with a controoler at the end.

    I'd like to use galvanically isolated RS485 buffers such as IL3185 on each
    sensor, but would also like to not have to put isolated DC-DC converters to
    supply the power to the cable side of the buffers.

    So can I supply +5V up the cable to all the devices to power their cable
    side buffers? Since it is RS485 only one sensor will be transmitting at a
    time so as long as the input buffers don't take much current it would seem
    practicable (with some capacitance and possible ferrite filtering where the
    power comes in to each sensor).

    First, is this a pratical solution? If so, assuming CAT5 cable with a pair
    dedicated to the 2 wire RS485 signals, a pair to +5V and a pair to ground,
    how long a cable would be possible, or is it best to use a higher voltage
    and use a cheap LDO on the receivers to generate +5V?

  2. markp

    markp Guest

    I guess I'm concernred about the voltage drop across a 20m cable due to the
    resistance of the cable when the worst case of all 12 sensor receivers are
    powered and one sensor's transmitter is on. The caps would smooth things
    locally so it's down to DC losses.

    As a matter of interest, what drop-out voltages were you seeing (I'm
    assuming that's down to cable losses), and how much current were you
    supplying? At the moment I've got a 5V regulated supply available, but the
    chips require 4.5V.

    Also I guess since power has return currents I should use a twisted pair for
    ground and power to reduce noise, maybe tripling up as I have 4 wire pairs
    available and one is taken by the signal pair (?).

  3. While not exactly RS-485, the CAN bus based DeviceNet standard
    transfers the actual data in one pair and the power for the
    transceivers and small loads in an other pair. You might find some
    interesting information about power arrangements etc.

    If you have full control of both the master and slave side, have you
    considered the good old current loop system, which is quite easy to
    optoisolate (no local floating power needed at slaves) ?

    Connect all devices, both master as well as slaves, both receivers
    (photo transistors) and transmitters (optoisolator LEDs) into a single
    20 mA current loop. When idle, all transmitter transistors are
    conducting and the 20 mA current flows through the loop and when the
    active station (master or selected slave) wants to send the "0" bit,
    it will cut the loop current. Since the active station will hear its
    own transmission on the receiver side, it must contain some echo
    cancellation in software.

    The problem with this arrangement, especially with optoisolators with
    bipolar transistors, is the large voltage drop at each station, nearly
    2 V for the receiver LED and 1 V for the Vce of the transmitter. With
    12+1 stations the loop current loop would have to provide nearly 40 V,
    which might be too much for some optoisolator transistors.

    One way around this problem is to use optoisolators with FET output
    stage and hence a low voltage drop across the transmitter, reducing
    the loop voltage requirement.

    An other approach is to use two loops, one containing the master
    transmitter and all the slave receiver LEDs in series, the other
    containing the master receiver LED and the slave transmitter
    transmitters (bipolar or FET) in series. 24 V should be sufficient for
    both loops. In this configuration, echo cancellation is not needed.
  4. Charlie E.

    Charlie E. Guest

    I had a simple three station design, but there were 120 feet between
    stations. I used one pair for power, and individual bucks at the
    stations. I had an old programmable laptop power supply on hand, so
    just used it (I think I had it set to around 12 volts.) Depending on
    the current draw for your individual stations, you shouldn't need too
    high a voltage. Remember, voltage loss is a function of both voltage
    and current!

  5. Jamie

    Jamie Guest

    Funny you would talk about this, we have some Red Lion devices that use
    the current loop and if you don't have more than one slave unit on the
    loop, in hot operating conditions, the crappie current source circuit
    used will bias itself more than 20 ma's and over time burn out the
    optical input on the slave device!. So, what I have done to correct the
    issue for now is to use a R in series. This really should be documented
    in the Red Lion manual but I haven't see it..

    The current source that is being used is your first year 101 basic
    PNP transistor being biased to 20 ma. there is a diode on the base to
    the (+) rail to help out but its not enough, it still gets out of spec..
    I've see it reach up to 35 ma when thing get warm..

  6. Uwe Hercksen

    Uwe Hercksen Guest


    I would look for the cable data, especially the resistance per length of
    one wire and calculate the voltage loss over a 20 m long cable with the
    necessary current for the tranceivers.

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