RS232 Serial communication


Thread Starter

yash verma

i was communicating a micrologix 1200 with HMI (PANEL VIEW 550) through RS232 DB9 connector. i found that the 9 pins of the connector cable is actually having only 3 wires inside connected to pin 2,5&7. can some 1 clear me on why other poins have not been used and when should we use the same.

thanx in advance
yash verma
yash.arya11 @

Michael Griffin

On an IBM compatible PC, the 9 pin D-shell serial port connection uses the following pins: 2 and 3 are receive and transmit. 5 is signal ground. The
rest are handshake lines.

The handshake lines may or may not be used, depending upon the application. When they are used, they act to throttle the transmission speed to whatever the receiving end can keep up with. In many applications they are not used (or connected) either because there is no chance that the receiving end cannot keep up, or because the throttling is done via software protocol handshake.

Since you can't hook up receive to receive or transmit to transmit and hope to get a signal through, either the cable must cross over the connections (e.g. connect pin 2 at one end to pin 3 at the other), or the device must be designed with the pins already swapped around.

A device manufacturer of course can make any pin do whatever they want, provided they don't intend to follow normal convention. This means the 2, 3,
and 5 convention noted above does not apply to all devices. If you want to make your own cables, you need to read up more on how RS-232 works, and be
prepared to figure out what connection will work between particular pairs of devices.