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Termination Resistor - RS-485 line.

Discussion in 'Electronic Design' started by Franco, Apr 3, 2007.

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  1. Franco

    Franco Guest

    Hey guys, fast question: Is it possible to use as a termination
    resistor (in a RS-485 bus) a combination of L and C in series instead?

    I am just trying to find a way to set a termination resistor that does
    not imply such a big load as a pure 120ohm resistor.


  2. John Larkin

    John Larkin Guest

    Sounds bad to me. If the data rate is low, just don't terminate it, or
    use a series R-C. At high rates (relative to line length) any
    non-resistive termination can cause pattern-dependent distortion,
    "deterministic jitter" and possible errors.

  3. John,
    the idea of the poster was AC termination. Meaning a 100
    Ohms in series with perhaps a 100nF.

  4. John Larkin

    John Larkin Guest

    I mentioned series R-C termination. Again, that can cause
    pattern-dependent timing errors and may snarf the data. If the data
    has no DC or low-frequency components (like NRZI or Manchester), the
    series R-C will work, but saves no power!

  5. Franco

    Franco Guest

    Nice answers.

    I think both are right, and I meant an RC termination implicitly. Now
    I am
    realizing is the most convenient termination for my network.

    One more question to John, why you said "saves no power"?


  6. Noway2

    Noway2 Guest

    I guess I really don't see why you would want to use such a termination
    in an RS485 system. Would you not want to use a terminating resistor
    that closely matches the characteristic impedance of the cable?

    The cable and terminating resistor would form a divider circuit. By
    matching the terminating resistor to the impedance of the cable, you
    would eliminate or at least greatly dampen any reflections on the lines.

    You mention not wanting such a big load as a 120 ohm resistor. I am
    curious as to what your line of thinking is here.
  7. John  Larkin

    John Larkin Guest

    If the data is balanced (NRZI, Manchester) the DC voltage across the
    cap is zero, so it may as well be a short; so all the differential
    signal swing appears across the resistor.

    If the data is unbalanced, like async serial ASCII or anything that
    pauses, the cap can charge up to almost the full driver swing in
    either direction, which can severely skew the receive timing when
    transitions start again.

    Why not use a 120 ohm resistor?

  8. Franco

    Franco Guest

    Ok. The point is than I am going to try to join (by using analog
    switches) two or more
    "little" RS-485 buses with a transceiver of 54ohms max drivers load.
    If I use two
    terminator resistors of 120ohm on each bus, I think I cannot drive
    more than two
    of these "little" buses. All of this, not considering the load of the
    receivers (1/8).

    Maybe is not possible, or maybe the way to do it is with bidirectional
    kind of circuitry, but, so far, I am just trying to get just an
    overall view.

    Comments, please...

    Best Regards...

  9. John  Larkin

    John Larkin Guest

    What sort of data? What rates? Are drivers gated?

  10. mkaras

    mkaras Guest

    As you switch in different bus configurations you want to make sure
    that the "little" busses do not join into non-linear structures. You
    want two distinct ends (and those two places are where your two 120
    ohm terminators go) and have a straight path to the other end going
    past all intermediate nodes without side branches and stubs. If the
    switching topology cannot be done with fixed end terminated nodes then
    you may have to also put switches to turn the termiminators on and off
    at some nodes that swap back and forth between end and intermediate
    type nodes.

    - mkaras
  11. Noway2

    Noway2 Guest

    Double check the data sheets. When you say that the drivers can handle
    a max load of 54 ohms are you certain that this means that it can't
    handle a larger impedance rather than a smaller one? The smaller (less
    ohms) the load on the drivers, the more current they will have to pump
    out to cause a signal to reach full height as it propagates on the
    transmission line.

  12. The Texas Instruments application note SLLA070C discusses
    the various terminations and list the 100 Ohms plus 1nF
    betwwen the two lines as feasible alternative termination
    for baudrates below 100kbit. It saves power as soon as the
    bus is idle in either state.

  13. John  Larkin

    John Larkin Guest

    That's a 100 ns tau for 10 microsecond bits. In that case, you may as
    well not terminate at all. There might be some corner cases where such
    a termination will reduce edge ringing.

    But why not terminate with a resistor?

  14. Franco

    Franco Guest

    The system is battery powered. I think pure termination resistor
    (parallel termination) is not an option.
    Moreover, I want to join little buses, to expand a kind of global bus.
    As far as I understand, if one driver
    is going to send the signal towards two little buses, both terminated
    at 120 ohm (multipoint buses), I am
    over-clocking the transceivers (let said, min 54 load - not less). The
    transcievers are: SP3078E.

    I want high rates, as high as I can. The micro-controllers driving
    each transceiver attached to the bus are fast
    enough to all the span of rates (up to 10Mbs).

    It is a distributed system.

    I know I am going to break the guidelines of linear buses, when
    joining two little buses. I cannot
    do anything about that, more than just analyze the effects of that. In
    this moment I am
    trying to figure out what happen when placing AC termination at both
    end of the buses.


  15. more stuff here about termination

  16. Franco

    Franco Guest

  17. Nico Coesel

    Nico Coesel Guest

    This is true if the length of the cable is greater than (roughly) 1
    fifth of the wavelength of the signal. It might be easier to use
    slew-rate controlled (read: bandwidth limited) RS485 drivers because
    they allow longer unterminated transmission lines.
  18. Well, the swing is the differential voltage across
    the resistor of 3V with either sign. With 100 Ohms,
    this is a loss of 90mW. For a battery operated device,
    these 30mA are a lot. The reflection of the signal
    is a reflection of the slope, and it is the slope
    that dissipates in the AC termination. I'd possibly
    go for a tau in the order of a tenth of a bit.

  19. jasen

    jasen Guest

    not really.
    if the 120 ohms is hurting go to a lower voltage or use a different cable.
    with a higher characteristic impedance.

    or go to a lower bitrate - low enough for one bit to flood the whole bus.
  20. jasen

    jasen Guest

    you ran data wires but forgot to run power wires?

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