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RS485 resistors

Discussion in 'Electronic Design' started by Marco Trapanese, Jan 11, 2008.

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  1. Hello,

    I'm talking about the usual RS485 front-end: MAX485, series resistors
    (if any), pull-up/pull-down and termination resistor.

    The system I'm working on has two RS485 transceiver at both ends of a
    twisted-pair cable. Its length is between 250 m and 500 m.

    I'm wondering how to dimension the resistors' values. With "standard"
    values (4k7 pull-up/down, 220 termination, 10 series) I get a distorted
    signal from side to side, due to the cable length I guess.

    Thanks for any suggestion,
    Marco / iw2nzm
     
  2. James Beck

    James Beck Guest

    Get rid of the 4.7K and series 10 keep the terminator.
    Make sure the terminator should be 220.
    Make sure there is a decent ground for the signals.
    RS-485 is really a 3 wire system 2 signal one ground.
     
  3. Aren't most wires more like 100 ohms?
     
  4. Usually, for UTP connections, the termination resistors are more like
    120R, which matches the typical characteristic impedance of a twisted
    pair.

    Biasing to define the idle state (assuming the reciever does not
    pre-empt this by having a controlled built-in offset voltage) is
    typically done with two resistors, one from A (let's say) to Vdd, and
    one from B (let's say) to GND. In order to get a couple hundred mV
    you need a fairly large current given the termination resistors (60R
    for two in parallel, usually), so values like 680R or 620R are common.

    Since the two bias resistors are effectively in series with each other
    and in parallel with the termination resistor from the pov of the UTP,
    ideally you'd like to increase the termination resistor a bit, maybe
    to 133R, but often people don't bother.

    For short runs, and low speeds, termination is not very important;
    almost anything works. You do need to establish a common ground
    between transmitter and receiver or you'll have problems.


    Best regards,
    Spehro Pefhany
     
  5. Don Bowey

    Don Bowey Guest

    From a transmission POV, 135 Ohms is usually used at and below 56 kbit/s.
    From there to 1.544 Mbit/s about 110 Ohms is good, so your 100 Ohms should
    be fine.
     
  6. Yes, so the OP's 220+10+10 is likely too high.
     
  7. Don Bowey

    Don Bowey Guest

    That's how I see it.
     

  8. I write here, but my answer is for all the others, whom I thanks too of
    course.

    I'm not sure the ground is actually *common*. The cable may have a
    length of more 450 m. Both transceivers have a DCDC converter to isolate
    the ground: digital lines are optocoupled.

    This is the previous design, working now. I have to redesign the whole
    system so if I can improve the serial communication it will be appreciated.

    Let's say the maximum speed I can reach with this configuration is only
    19200 bps. I need more.

    Furthermore, the cable is very complex it carries several coaxial
    cables, optical fibers, ecc... So I don't know if its impedance is about
    100 ohm.

    Marco / iw2nzm
     
  9. Pieter

    Pieter Guest

    The maximum speed and distance depend on:
    - the losses of the cable, Belden has high quality low-loss cables
    - the length of the cable or used speed
    - the allowed losses
    - termination (prevents reflections)

    19200 is very low. At 400 meter you can go much faster with RS485. I
    know a place where there is a 2 km cable running at 62,5 kbit.

    You can check for information about cables and terminations in
    networks like Bitbus (I worked lot with it in the past, see
    www.bitbus.org), Profibus etc. Also use good transceivers like the
    SN75LBC176.

    Make sure you termination impedance is the same as the cable
    characteristic impedance.

    Regards,
    Pieter
     
  10. I'm looking at it. Does the SN75LBC176 give better performances than
    MAX485? What are the critical parameters I must check on the datasheet?

    I'm afraid I don't know the impedance of the cable. Perhaps I might try
    to measure it directly using a VNA.

    Thanks for your answer,
    Marco / iw2nzm
     
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