Serial data communication was once the primary method of interconnectivity between computers. In fact, it is still being used today, typically to hook up peripherals and instruments. That's why almost anyone who has even a rudimentary knowledge and understanding of computers knows what RS232 is, or has at least heard of it. Ask, however, for a description and explanation of RS422, and you'll mostly get blank stares. That's peculiar as RS-422 is a logical progression of RS-232 and one that made a whole lot of sense.
In essence, RS422 (which should really be called by its proper current name, EIA-422) happened due to a number of shortcomings and limitations inherent to the RS232 standard, primarily in the areas of speed, reliability and flow control. If RS232's limitations make it unfeasible for a particular project, RS422 can be a solution as it was designed to handle higher speed (up to 100kbps) and longer distances (up to 4,000 feet) than RS232. In fact, the highest data rate of RS-422 is up to 10 mbps over short distances, which means it rivals USB and Ethernet. Add to it that RS422 can use the same DB-25 connectors used by RS232, and you can see how RS422 is often used as an RS232 replacement or extender. If you remember the little round mini-DIN-8 connectors on older Macs, those used an RS422-style of communication,
How can RS-422 connections be faster and operate error-free over much longer distances? That's because instead of using different voltage levels with respect to ground as RS232 does, RS422 uses a "balanced" signal with negative and positive voltages (ranging from -6 to +6 Volts) on twisted pair wires that is far more immune to interference. This is referred to as "differential" data communication because it relies on the voltage difference between the two wires as opposed to a voltage level compared to ground, a technology that makes it much easier for the receiver to recognize ground shifts and noise and reject them before they can introduce errors into the signals.
In addition to speeds up to 10 megabits per second (albeit only over a fairly short distance of 40 feet) and a maximum connection distance of 4,000 feet, RS-422 can communicate in both directions simultaneously. And while the standard only allows for a single driver, there can be multiple receivers (up to 10). RS-422 systems need a dedicated pair of wires for transmitting and receiving, and also one for control signals, if required. Depending on speed, distance and the characteristics of nodes, termination (the matching of impedance between nodes and the line) may or may not be an issue in RS-422 systems.
However, while RS-422 can be an excellent solution for many data communication projects, it also has an important limitation: it is still a point-to-point protocol and therefore unsuitable for many multi-drop applications. In other words, in a RS-422 system, there's always a single master or driver that issues signals to receiver nodes on a network. The individual nodes, however, cannot communicate with one another. This is where RS-485 comes in, a topic we'll cover in another article.
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Chris Robertson is a published author of Majon International. All Rights Reserved. Copyright 2011 (Fri Jan 01 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
