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Is termination needed for short and slow RS485?

Discussion in 'Electronic Design' started by mrsoft99, Sep 19, 2004.

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  1. mrsoft99

    mrsoft99 Guest

    We are running some led billboard displays using a multi-drop 2-wire
    RS485 network.
    The cable length is 50 feet; the baud rate is 2400 baud.

    Are RS485 terminators really needed for this? I know they are needed
    for 'long' cables and 'fast' baud rates, but does 50 feet at 2400 baud
    qualify for this?

    And yes the reason I am asking is there are some problems with some of
    the displays. (If it was easy to just plug in some terminators at an
    existing customer site with the problem I would; but it's not easy.)

    Also, it may be that the cable being used it not twisted pair. Is
    that important for this setup?

    Thanks very much.
  2. Probably not. However, you may still need pull-up/pull-down resistors.
    If total cable length is less than, say 1/16 of a bit time, you
    probably don't need terminating resistors.
    Depends on how noisy the environment is.
  3. It could work without teminating resistors.
    What is essential is the common more range of some
    7 V or so. So you should always have the GND as
    reference together with the differential signal.
    If there is the chance to exceed the common more range,
    couplers should be inserted and held at the GND
    potential. These 7V may not be much and are quickly
    exceeded, and be it only at burst noise.
    It happened in one building with an inhouse-net,
    that communication failed when the lift was running
    even though the RS485 was held at EARTH potential.
    Yes, the lift made the EARTH jump by more than
    the common more range. (...)


  4. I suggest you start with this article written for Circuit Cellar by Jan

    Bias the signal leads per Jan's recommendations. Also note Rene's comment on
    common mode voltage and the fact that three wires (not two) are required
    (even if one of them is the DC ground wire).

    You'll probably have to provide terminations on one of the signs that is
    giving you problems, just to see if it fixes the problem.

    I designed the electronics hardware and wrote the software for the full-size
    and mini message boards shown here...

    The balanced data bus is RS-485 at 115,200 bps using randomly-dressed
    non-twisted 18-gauge wire. There have been no problems. I don't recommend
    such haphazard wiring without extensive testing.

    Good luck.

  5. Just to give an idea of how noisy some environments can be:
    At a shopping center with cable runs up to 500m, we have picked
    up noise spikes over over 200V that lasted in the ms range. This was
    using good quality twisted pair cable with a defined impedance. This
    was measured directly over a 120 ohm termination resistor.

    Anton Erasmus
  6. Galvanic isolation of the bus may be a good idea.

    Best regards,
    Spehro Pefhany
  7. I read in that Anton Erasmus
    eranews>) about 'Is termination needed for short and slow RS485?', on
    Sun, 19 Sep 2004:
    It can happen, but you have to be extremely careful how you do the
    measurement. 'Balance' in balanced circuits means 'balanced impedances';
    it has nothing to do with signal voltages. The line remains balanced (or
    not) when no signal is present.

    So, you need to make sure that the sending-end impedances from each
    conductor to ground are closely equal and preferably low. At the
    receiving end, the impedances to ground must also be closely equal, but
    in the interests of signal transfer, especially at low frequencies, are
    preferably not equally low. This is because the four impedances form a
    balanced bridge and the error voltage due to the impedances not being
    exactly equal, which is differential and becomes an inextricable part of
    the signal if it's in-band, is smaller if the impedances at the sending
    and receiving end are very different.

    'Closely equal' can mean really close. To get 80 dB common-mode
    rejection, the impedances have to match within 0.01%.

    At higher frequencies, of course, the *differential* impedances at each
    end may well need to match the line impedance.

    Measuring such 'highly balanced' circuits requires extreme care,
    otherwise the measuring instruments upset the balance and produce
    seriously pessimistic results.
  8. John Smith

    John Smith Guest

    Wow! John, that's good info I haven't seen before. Thanks!

  9. It is better to use 3-wire for differential comms
    so that the receivers have at least some idea of
    the common mode voltage. Also a low-pass filter
    on the input of each receiver can be useful in
    avoiding difficulties with line-ringings, (from
    a line that is being driven too hard), or stray

    RS485 lines.
    A B 0V
    | | |
    | | | 2k _
    +--|--|----/\/\---+----| \
    | | | | | \
    | | | 2.2nF === |Rx >--
    | | | | | /
    | +--|----/\/\---+----|_/|
    | | | 2k |
    | | | |
    | | +----/\/\-----------+---0v
    | | | 100R
    \|/ | \|/
    More receivers, as above.

    2400 bits/sec is 417uS per bit.

    2+2k*2.2nF is an 8.8uS RC time constant, which
    will get to 99.9% of where it is supposed to go
    in about 44uS, or about 10% of each bit-width.

    The 100R resistor connecting each receiver 0v to
    the line 0V is there to limit the current in case
    of earth loops. In fact if there is any voltage
    across any 100R (when not transmitting) then you
    know that you probably have an earth loop problem.
  10. At least you should have somewhere a pull-up and a pull-down resistor
    to drive the bus into the idle mark ("1") state when there is no
    active transmitters.
    This is essential.
    This might generate more problems than it solves.

    If the equipment are ground referenced to the electric system neutral
    (directly or through PE connected to neutral), the reason for the
    equipment ground potential differences is the different voltage drops
    in the mains system neutral wiring carrying the whole mains neutral
    current. The drops can be quite with large single phase loads or even
    in a three phase system there can be quite considerable 3rd harmonic
    currents at 150 or 180 Hz if there are a lot of switching mode power
    supplies in each phase.

    Connecting the signal ground in the signal cable between two equipment
    that also are referenced to the mains neutral will effectively connect
    two points in the mains neutral together. If these two points in the
    mains neutral network have a different potential (due to different
    voltage drops), some of the neutral wire load current will flow
    through the signal ground in the signal cable between the two neutral
    connections. This current could be several amperes and in the worst
    case even burn some PCB ground tracks. This current could be mainly at
    the fundamental 50/60 Hz frequency, but at least also in three phase
    electric distribution systems at 150/180 Hz and in addition all kinds
    of high frequency interference from switchers etc.

    If the distance between the two equipment is the same measured on the
    serial cable and along the electric network, the ratio between the
    current flowing in the neutral wire to the current in the signal
    ground is directly proportional to the cross section area ratio
    between the mains neutral and signal ground.

    A thin signal ground between the equipment does not carry a lot of
    current, so it does not reduce a lot the ground potential difference
    (and hence common mode voltage) between the equipment. If a thick
    serial cable shield (with cross section comparable to the mains
    neutral cross section area) is used, about half of the mains neutral
    current will flow through this shield and drop the voltage difference
    between the signal grounds to one half of the original value. However,
    if such system is used, it is essential, that the shield is connected
    directly to the equipment chassis and then to the mains PE and not let
    this large current flow through the PCB of the equipment.

    If the worst case ground potential difference is non-zero but well
    within the common mode range, it should be sufficient to run the
    connection without a separate signal ground and rely on the mains
    Galvanic isolation from the local mains grounds is required, if such
    large variations exists, since a thin signal ground would have no
    effect on the voltage difference in the mains neutral wiring.

    IMO, a separate signal ground in the RS-422/485 cable should only be
    used, if all the equipment are floating relative to the mains
    grounding. In this situation, it simply is supplying the bias currents
    for the receiver input transistors.

    With floating system, the signal ground wire can be eliminated, if the
    transistor bias current is supplied some other way.

    On RS-422 connections, the "fail-safe" termination (a voltage divider
    between local Vcc, the signal Rx wires and the local signal ground)
    should supply the bias current. On multidrop RS-485 the A and B signal
    lines should be terminated at both ends of the bus as usually with
    resistors, however, high resistance (1-10 kohm) pull-up/down resistors
    from _local_ Vcc and ground to the signal line are required to supply
    the bias current for each transceiver. The internal power supplies
    within each station will be floated to approximately the same
    potential without any external ground connections.

  11. You're perfectly right Paul.
    I was somewhat unclearly indicating that the GND
    should be designed into the system, into the connector
    and into the cable. Current flow through it should
    be avoided. Should at some point be decided that isolators
    were required, the GND could be used. The 100 Ohms in
    series with the GND could als be employed to detect
    current flow. Depending on the location, measuring
    GND differences are unpractical.
    I actually run +5V together with the GND too. This was
    isolators can be fully powered through the cable.

  12. CBFalconer

    CBFalconer Guest

    That 100 ohms could function as a fuse. Grounds can have massive
    differences, think start-up currents into elevator motors and the
    ilk. The fault there is the connection to 0v, assuming that is
    connected to local ground. Any long wire can have large voltages
    inductively induced by nearby equipment.
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