Taking the Frustration out of Serial Data Communication with USB, RS232, RS485, RS422 Converters

Anyone dealing with serial data communication protocols like RS232, RS422, RS485, and USB knows how frustrating it can be to get a connection to work. In the days before plug-and-play, getting a connection to work was often trial and error, and sometimes it's still that way today. Converters and adapters are making it a lot easier now, but it's still good to remember what we're dealing with here and why things can be tricky.

Classic serial connectors have either 25 or 9 pins. Back in the days when RS232 was a primary means of data communication, most serial ports used the large 25-pin connectors. These days, most serial connectors are of the smaller 9-pin, or DB9, variety. In essence, a DB9 connection pin 1 is used to detect a signal, pins 2 and 3 transmit and receive data, pin 4 indicates the data terminal side is ready to be connected, pin 5 is used for ground, pin 6 indicates the data communication side is ready to receive, pins 7 and 8 are used for request-to-send and clear-to-send signals, and pin 9 is the ring indicator signal.

So what were (and sometimes still are) the remaining 16 pins in a DB25 connector used for? The answer is that the additional pins in the original DB25 connectors were used for a variety of purposes, and not always consistently. Two pins can be used for a secondary set of send/received leads. That may require secondary request-to-send and clear-to-send signals. There are also three pins used for transmitter and receiver signal timing, three for testing purposes, one as an additional shield, one each for local and remote loopback, signal detects for two receiving lines, and one that was entirely unassigned.

Unfortunately, while the 22 signal lines of the initial RS-232 standard were fairly clearly defined, almost no one followed the standard, at least as far as pin assignment went. As a result, knowing the exact pinouts for each piece of equipment was essential, and connecting peripherals often required specially made cables. In order to cut down on cable complexity, a lot of vendors began dropping non-essential lines (most were only needed for modems anyway). This made the smaller and less complex DB9 connector possible. Many serial cables used little more than the send and receive lines.

Another endless source of confusion was whether a device was considered Data Communication Equipment (DCE) or Data Terminal Equipment (DTE). DCE device usually had female connectors whereas DTE devices generally had male connectors, but that is not always the case. As a result, a lot of time was spent making, testing and trying to figure out serial cables. All of this is a lot easier today thanks to smart RS232, RS422, RS485, TTL and USB converters and adapters. Thanks to these converters, it's now possible to connect and convert to and from RS-232, RS-422, RS-485 with ease, and they can also handle USB to serial, fiber optics and Ethernet to fiber optics.

Serial data communication has always been a simple concept with a good deal of added complexity due to the innovative, but non-standard use of signals and pins. Its simplicity and logic account for its continued use today, but thanks to a new generation of inexpensive, reliable converters and adapters, dealing with serial interfacing issues is a whole lot less frustrating than it used to be.

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About the Author

+Chris Robertson is a published author of Majon International. All Rights Reserved. Copyright 2011 (Sun Dec 06 2009) 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