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RS232 Serial Port Pin Numbering

December 10th, 2012

Many embedded engineers have to work on Serial Ports. In fact serial port still is a dominant mode of communication between devices in embedded systems even after the advent of USB and many other media.

The purpose of this blog is to mention a very simple way to memorise the pin numbering of the Serial Port pin numbering in the DB9 Connector of the Serial Port. I am not going to mention what all the 9 pins of the serial port connector do that. There are other places including wiki where you can get that information. I am also not goint to mention what all the serial port protocol is.

All I am going to mention is that most serial port communication consist of just three pins - A ground, a Transmitter and a Receiver. So Pin 5 in the DB9 is the ground. And if you do not remember it you should. You will often need this pin to be able to measure and check your signal on your scope. That leaves us with two more Pins - A transmitter and a Receiver. The Transmitter is Pin 3 and Receiver is Pin 2.

In your computer the Transmitter is Pin 3. And that is all you need to remember. And here is an easy way to remember - both the words Transmitter and Three has the alphabets ‘T’ and ‘r’. And with this fact memorized you know that the receiver is going to be Pin 2.

And of course, if you still have confusion, hook up a Oscilloscope and attach the ground of the Oscilloscope to Pin 5. Now connect the tip of the oscilloscope probe to Pin 3. If you now open a Hyperterminal and write anything in your Hyperterminal you should see some data on that Pin 3 of the DB9 Connector.


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