12.8 Creating a Null-Modem

Often during testing of software which utilizes the serial port, it is useful to have your machine "talk to itself." In other words, the receive and send lines on the serial port are in some fashion connected to one another. This means that whatever your computer sends out will be immediately received again--although for all intents and purposes, your computer does not know that the received data originated from itself. One quick way to accomplish this task is to use a so-called turnaround plug on the serial port (the shop often carries these under the part # DE 9S). When viewed from the front, the plug's pins will look like the following:

Also shown are the numbers assigned to each of the nine pins. Pins 2 and 3 must be connected together (a little solder and a short piece of wire will do the trick) as follows:

This is the most rudimentary form of a null-modem. It may not operate correctly for some applications which utilize more of the pins, but will work for any course MP's assigned.