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null modem cable vs serial to serial connection

Posted on 2012-04-09
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Last Modified: 2012-04-24
How do I see the difference between null modem cable and serial connection cable?
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Question by:mokkan
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by:Callandor
Callandor earned 1332 total points
ID: 37823237
The send and receive signal pins are cross connected on each end of a null modem cable (pin 2 goes to pin 3), whereas a straight cable has matching signal pins (pin 2 goes to pin 2, pin 3 goes to pin 3) on each end.  There are also crossover of a few other signals: http://www.nullmodem.com/NullModem.htm
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by:mokkan
ID: 37823430
but,  can we differentiate from the outside look?
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n2fc earned 668 total points
ID: 37823460
Simplest way to test these externally is with a VOM or multi(ohm)meter and test continuity between the pins 2 and 3 at each end... Some crossover cables also cross the CTS/RTS signals as well...

Any simple continuity tester (even a battery and lightbulb or buzzer) can test if the pins are "straight thru" or "crossed"...

Just touch the 2 leads to the the 2 pin 2's (1 at each end) and if they are connected it is not a crossover!

Visual verification is only available if the connector hood can be easily removed (not molded) and color coded cable is used between the 2 ends.
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LVL 69

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by:Callandor
Callandor earned 1332 total points
ID: 37824238
>but,  can we differentiate from the outside look?

No, they typically look exactly the same on the outside; only the wiring is different.  Vendors typically stick a label on them to distinguish them.
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