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3 Types of Industrial Serial Communication
An industrial communication network is the backbone of any industrial automation system. The network can be as simple as a sensor that trips an emergency stop, or complex as a system that controls articulated industrial robots.
Industrial communication networks are designed to coordinate real-time control, and maintain data integrity, over large installations in harsh environments. The needs of industrial communication networks are distinct from those of consumer-side networks, although there is still some crossover.
The three types of serial communication most commonly used in manufacturing, energy, and similar industries are: RS-232, RS-422, and RS-485.
RS standards do not define data encoding or transmission protocol, instead they are concerned with the physical characteristics of data transmission and wiring.
In general, RS standards define signal characteristics, mechanical interface characteristics, the functions of each circuit in the interface connector, and standard subsets of interface circuits for selected telecom applications. These standards do not, by contrast, define character encoding, character framing, transmission order, error detection, or bit rate.
RS-232, defined in 1960, is the oldest and most popular interface, but as technology changes, it is slowly losing ground. One reason for its ubiquity is that, until fairly recently, PCs almost universally came equipped with RS-232 COM ports— often called simply "serial ports—" making it easy to establish RS-232 communication with readily available hardware. RS-232 does come with some significant drawbacks. In terms of industrial serial communication, one of the biggest is RS-232's transmission distance, which is limited to 50 feet or less, unless one takes advantage of an industrial RS-232 repeater. Furthermore, RS-232 is single-ended, making it susceptible to electrical noise. Finally, RS-232 allows only one device connection per serial port without putting a serial port sharer to use.
RS-422 is similar to RS-232, and can be programmed in the same way. The main disadvantage of RS-422 is accessibility. Many PCs come standard with RS-232 ports, but it is harder to find a 422 port. Luckily, it is easy to use an industrial serial converter to get controllers to communicate over RS-422. Compared to RS-232, the advantages are apparent: RS-422 allows for a transmission distance of up to 500 feet. RS-422 is multi-drop, up to 32 devices can be connected per controlling port, with a unique address assigned to each device. RS-422 is more resistant to electrical noise since it uses separate transmit and receive wire pairs.
RS-485 is similar to RS-422. The two are so similar, in fact, that there can be some confusion. Both provide the advantages of multiple device connections and long transmission distance— especially as compared to RS-232. The main difference is wiring, RS-485 is a two-wire system, compared to 422's four-wire. This makes programming for RS-485 slightly more complicated than for 422. RS-485 also uses tri-state logic to effectively remove the transmitter from the circuit.
In summary: RS-232 is the oldest and still the most common, but comes with a relatively short transmission distance, and only one device connection per serial port. RS-422 is a 4-wire system that allows for transmissions of up to 500 feet, and up to 32 slave devices per controller. RS-485 offers all the advantages of 422 in a two or four-wire configuration with three-state logic.
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