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Bidirectional LVDS

Discussion in 'Electronic Design' started by Richard Henry, Jun 27, 2007.

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  1. I need to extend a memory-mapped bus into another enclosure and
    thought that a bidirectional LVDS implementation with serial/
    deserializer pairs at each end might work. Does anyone have any
    experience or guidance on such a setup?
  2. Andy

    Andy Guest

    LVDS has a really low common mode voltage tolerance, so if you use it
    between enclosures, make very sure you have an excellent grounding
    scheme and control of return currents between enclosures. Generally
    not a good application of LVDS without some means of improving CMV

  3. The second device draws its power from the first, so they share a
    common ground.
  4. Rich Grise

    Rich Grise Guest

    How far? With proper terminations, depending on the bus speed, you could
    use ordinary ribbon cable but with twice as many leads as signals, and
    make every other line ground.

    I've made this work with 3 feet (~1m) of ribbon cable, but at fairly
    low bus speeds.

    YMMV. :)

    Good Luck!
  5. Erm... one (single-ended) application of LVDS I know of shoves a 25MHz
    clock down some ribbon cable... 10m of it. Seems to work. LVDS, being a
    current loop, tolerates this kind of (ab)use better than most.

    To the OP re bidirectional: it's not clear to me whether you expect to
    run two loops (one outgoing, one incoming), or shove data down one pair
    of wires in boith directions. If the latter, I'm not clear on where
    you'd put the receiver load - at both ends? If so, expect to see half
    the voltage across each receiver.

  6. I was contemplating the former as shown in National's application (Fig

    It also has the precaution about lower voltages, but it predicts a 10m
    limit (my app is much shorter) and cautions against a noisy
    environment (one enclosure includes an rf transmitter).
  7. Further to earlier response: re-reading this carefully, I'd agree if we
    were talking about normal (TTL-esque) digital signals. But with LVDS, it
    *is* a differential current loop. A twisted pair is ideal, otherwise I'd
    put the two conductors side by side in a ribbon with a guard ground
    either side.

    YMMV ;)

  8. Gabor

    Gabor Guest

    If you're going to use SERDES on the LVDS lines, I would assume your
    bit rate is fairly high. In this case I would advise against the
    bidirectional wire idea. Save yourself the headaches and use one
    set of wires for each direction. Certainly it is possible to use
    the circuit of figure 2.4, but you won't be able to change direction
    quickly, in addition to the reduced signal strength mentioned. The
    point of SERDES is to reduce the number of pairs significantly, so
    it shouldn't cost too much to assign the additional pairs to run
    independent one-way data paths.

    Just my 2 cents,
  9. Uwe Bonnes

    Uwe Bonnes Guest

    That's what Bus-LVDS for.

    Called also M-LVDS, LVDM or ...
  10. Rich Grise

    Rich Grise Guest
    page 9. :)

  11. Rich Grise

    Rich Grise Guest

    Thanks! :)

    Sometimes I'm kinda psycho^Hic. ;-)

  12. How about a hybrid solution? I don't want to run so many wires (I
    don't have room for such a big connector). Could I gang up 4 signals
    per pair without a lot of overhead, and still keep it bidirectional?
  13. Uwe Bonnes

    Uwe Bonnes Guest

    You can use any protocoll and multiplex the lines as you like. But that will
    need protocoll overhead.

    Look e.g. at TI or National, what can be done with LVDS.

    Bus LVDS is anyways a good choice, as is has stronger send levels and more
    sensitive receive levels.
  14. John Larkin

    John Larkin Guest

    Most of the LVDS receivers that we've tested, including the Xilinx
    Spartan3 parts, seem to behave like excellent, very fast rail-to-rail
    comparators, at a fraction of the price of an officialy-designated
    comparator of similar performance. They do generally have a deliberate
    input offset, in the 10's of millivolts sort of range. Most of them
    make good zero-crossing detectors, signal on one input pin and ground
    on the other.

    Still, one wouldn't want to drive any input hard much beyond its local

    Return currents?

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