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

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

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.

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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

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

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Tim

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