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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?

    Thanks,

    Adrian
     
    Lee, Oct 27, 2004
    #1
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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
    possibility.
     
    Hans-Bernhard Broeker, Oct 27, 2004
    #2
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  3. Homework? Always happy to help, please click here:
    http://www.nullmodem.com
     
    Vadim Borshchev, Oct 27, 2004
    #3
  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
    #4
  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
     
    Casey, Oct 27, 2004
    #5
  6. Lee

    Tim Ames Guest

    Tim Ames, Nov 5, 2004
    #6
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