Let's face facts: there's still a need for serial ports in this world. It could be that a device doesn't have USB or needs something USB cannot provide, or that the project simply doesn't warrant the added expense and complexity of USB. Whatever the case, serial remains a viable answer for many projects and systems. Add to that the fact that serial allows for longer cables and that RS232/RS422/RS485 are incredibly flexible data communication protocols thanks to the flexibility of the standards themselves and also to the ease with which you can communicate between protocols via converters, and it's no surprise that serial ports are here to stay.
That said, there are a few areas in serial communication where a bit more standardization wouldn't have hurt. How many times have you tackled a seemingly simple serial communication project just to find that you didn't have the right cable and connector? That can happen with USB, too, with its Mini-USB, USB-B, Mini-A and other connector variants, but it's really worst with serial connections where you often have no idea hat to expect. So let's take a look at the various connectors used in RS-232/RS-422/RS-485.
By far the most common serial plugs are the 9-pin and 25-pin D-sub, or subminiature "D" connectors. Each comes in a male and female variety, and each has two rows of pins, 5 and 4 for the DB-9, and 13 and 12 for the DB-25. Older equipment with serial ports usually has the large DB-25 connectors but those have mostly given way to the smaller DB-9. That's because the significantly narrower DB-9 saves valuable real estate, and also because only very few serial connections actually use more than nine leads.
Other commonly used jacks and plugs include the RJ-11 and RJ-45 formats. RJ-11, of course, is what's commonly known as a modem jack, and RJ-45 is generally associated with Ethernet connections. However, RJ-11 has six wires and RJ-45 8, and that is often enough even for complex serial connection projects. RJ-11 and RJ-45 jacks and plugs are inexpensive, reliable, and they use much less space than the DB-9 and DB-25 connectors, so keep them in mind.
One neat thing about all those different serial connectors is that it's very easy to convert from one to the other. Adapters and converters are readily available and if you keep some of the common ones in your toolbox, there won't be any unpleasant surprises when you tackle a project in the field. Oh, and also make sure to carry along some gender changers. While DTEs (Data Terminal Equipment) should use male connectors and DCEs (Data Communication Equipment) female connectors, that's not always the case, and then it's so much easier to just pop in a gender changer than to make or buy a new cable.
The one characteristic of serial communication that can throw a real monkey wrench into a project is when it's not clear which device is configured as a DTE and which as a DCE. That's usually the problem when you seem to have the right cable with the right genders, and yet, no communication. Sometimes both sides are configured as DTE or DCE. That's when you need a null modem, a special adapter that connects the proper outputs to the proper inputs.
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About the Author
Chris Robertson is a published author of Majon International. All Rights Reserved. Copyright 2011 (Thu Jan 28 2010) Majon International. Majon International is one of the worlds MOST popular internet marketing and internet advertising companies on the web. Visit their main business resource internet marketing web site at: http://www.majon.com
