While to an electronic data communications engineer or specialist, the difference between RS232, RS422 and RS485 is clear as day, to the rest of us they are just some ancient industry standards that have probably all been replaced by USB anyway. Or at least you can use a converter to deal with them. The latter is true, but the former really isn't: those standards are very much alive and continue to be used in a wide variety of applications. But let's take a look at the backgrounds of those standards.
RS-232, a standard of the Electronics Industries Association (EIA), has been around for over four decades. It goes back to the days where remote Teletype machines were connected to mainframes via modems. Communication between those DTE (Data Terminal Equipment) machines and DCE (Data Circuit-terminating Equipment) happened sequentially, one bit at a time. It was all slow and quite mechanical, with different voltage levels compared to ground, representing binary system ones and zeros. Serial communication is quite simple, but early on manufacturers experimented with various control signals, pin layouts and proprietary methods. With RS232, the EIA established a standard that minimized incompatibilities and made life a lot easier for information technology departments and anyone else involved with data communication.
RS232 turned out to be a very enduring standard that remained a primary means of data communication until the mid-1990s, and it is still being used today. There are, however, some inherent shortcomings and limitations to RS232, primarily in the areas of speed, reliability and flow control. The RS422 standard addresses some of those issues with two sets of twisted pairs that carry negative and positive voltages. The RS-422 standard offers speeds up to 10 megabits per second and the maximum connection distance is 4,000 feet. In addition, since it uses two twisted pairs, with RS-422 you can communicate in both directions simultaneously. However, RS-422 also has an important limitation: it is still a point-to-point protocol and therefore unsuitable for many multi-drop applications. This is where RS-485 comes in.
RS-485 works very much like RS422 in terms of speed and distance, also using twisted pairs and voltage differentials for serial binary data communication. However, unlike with RS-422, with RS485 devices are addressable (like in Ethernet) and therefore can communicate with multiple nodes (up to 32). RS-485 is therefore suitable for a variety of master/slave architecture connections and networks. With more addressable devices and each device being able to communicate bidirectionally, line termination becomes an issue and requires special consideration.
What it all boils down to is that these older serial communication standards are still being used. USB may have largely replaced RS232 serial connectivity in consumer products, but in many industrial and commercial systems, serial data communication remains very much alive. Which means that anyone dealing with serial communication products and systems will need not only an understanding of the standards and principles behind the standards, but also a tool chest of serial data converter products. That includes RS-232, RS-422 and RS-485, but also TTL and fiber optics converters and adapters.
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Chris Robertson is an author of Majon International, one of the worlds MOST popular internet marketing companies.
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