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What differences between Nullmodem cable and straight-through cable for Serial Port?

Discussion in 'Embedded' started by Lee, Oct 27, 2004.

  1. Lee

    Lee Guest

    What is the difference between Nullmodem cable and straight-through
    cable for Serial Port?Why do we need them two, not one only?


    Lee, Oct 27, 2004
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  2. For a combination of two reasons: first, because RS232 is
    bi-directional and full-duplex, but its signalling method on the
    individual wire is uni-directional. This is what necessitates
    _having_ two wires to be crossed (or not) in the first place. It also
    implies the two signals have to be crossed (TX(device 1) --> RX(device
    2) and vice versa)

    The second reason is that the designers of RS232 differentiated two
    classes of devices (from a current-day perspective, you'ld recognize
    them as "PC" and "modem"), and one of the difference between those is
    which pin on a standard connector carries which signal. The idea is
    to allow a straight-through connection (think of press-on connectors
    and ribbon cables) between such devices.

    This strategy implies that if you deviate from it, by connecting two
    devices from the same class (e.g. PC <--> PC), you have to actually
    cross wires to achieve the necessary signal crossing. Hence the need
    for null-modem cables.

    The design choice was between *always* needing crossed cables, and
    only needing them in what was likely considered an "exotic"
    application at the time. They may even have simply overlooked the
    Hans-Bernhard Broeker, Oct 27, 2004
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  3. Homework? Always happy to help, please click here:
    Vadim Borshchev, Oct 27, 2004
  4. Lee

    CBFalconer Guest

    The nullmodem cable interchanges the send and receive data lines
    (pins 2 & 3), and also the supervision lines (forget, include 4, 5
    22 IIRC). Then you have to worry about male/female connectors. It
    is usually better to have clearly labelled adaptors and keep all
    cables straight through male to female.
    CBFalconer, Oct 27, 2004
  5. Lee

    Casey Guest

    CBFalconer said ...
    Don't forget the usual confusion when using PC-style DB25 to DB9 serial
    cable since the meanings of pins 2 & 3 are reversed.

    Casey, Oct 27, 2004
  6. Lee

    Tim Ames Guest

    Tim Ames, Nov 5, 2004
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