Differences Between RS232, RS485 And RS422

Published: 15th December 2009
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RS232, RS485 and RS422 mainly defines the electrical signal characteristics of drivers and receivers like, signaling rate, voltage levels, short circuit behavior, maximum load capacity and voltage withstand levels. But there are some differences between each of them. Any engineer or specialist in electronic data communications can differentiate between RS232, RS485 and RS422 easily but it might not be easy for all. For them, the differences between the three are enlisted below.

1. RS232 is the oldest standard of the Electronics Industries Associations (EIA) which was followed later by RS422 and RS485. The origin of RS232 can be traced back to the days when remote teletype machines were connected to main frames via modems. Communications between DCE (Data Circuit-terminating equipment) and DTE (Data Terminal Equipment) used to happen sequentially. The entire process was very slow and very mechanical. With the help of RS232, EIA made a standard which minimized incompatibilities to make life easier for those involved with the information technology department and data communication. But shortcomings and limitations in speed, flow control and reliability posed a question on the performance of RS232. This is where RS422 came in. RS422 provided a solution to the problem by using two sets of negative and positive pairs of voltages. RS422 has speed up to 10 megabits per second with a connection distance of about 4000 feet.

2. RS422 has two twisted pairs carrying negative and positive voltages which help it to communicate in both directions at the same time. In case of RS232, communication takes place one after another. RS422 provides data transmission using differential or balanced signaling. It has unidirectional and non reversible transmission lines. Unlike EIA 485, it allows multiple receivers but not multiple drivers. It has the advantage of differential receiver, differential driver and high data rates (10 megabaud at 12 meters). EIA 422 on the other hand only specifies the characteristics of electrical signaling of a single balanced signal.

RS485 also uses twisted pairs of negative and positive voltage differentials but for serial binary data communication. This is also known as EIA-485, TIA 485 because this was published by the ANSI Telecommunication Industry Association (TIA) and Electronic Industries Alliance (EIA). They can be effectively used over long distances and in environments with electrical noises. Multiple receivers can be connected to these in a multi-drop configuration.

3. RS232 and RS422 are point to point protocols but RS485 devices are addressable and have the capacity to communicate with multiple nodes, up to 32. However, RS 485 works almost like RS422 in matters of speed and distance.

4. RS485 is appropriate for variety of master or slave networks and architecture connections. Each of them can be connected bi-directionally as line termination requires special consideration. But as RS232 and RS422 are point to point, they are not suitable for this purpose.

5. As RS422 has a single driver circuit, it cannot be switched off, but EIA 485 or RS485 drivers are required to be put in the transmit mode by placing a signal to the driver.

These are the basic differences between RS232, RS422 and RS485 largely used in various industrial and commercial systems.

Mordy Naftaly author of this article. RS232, RS422 and RS485 mainly define the electrical signal characteristics, of drivers and receivers. But still, they differ from each other and have individual characteristics and advantages.

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