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

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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

Re: What differences between Nullmodem cable and straight-through cable for Serial Port?
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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 ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

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

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Homework?  Always happy to help, please click here:
http://www.nullmodem.com

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

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: What differences between Nullmodem cable and straight-through cable for Serial Port?
CBFalconer said ...

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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

Re: What differences between Nullmodem cable and straight-through cable for Serial Port?
You can also look at

     http://www.cisco.com/warp/public/701/14.html#topic11

Tim

Lee wrote:
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