What is RS422?

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06 June
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What is RS422?

RS-422, also known as TIA/EIA-422, is a technical standard originated by the Electronic Industries Alliance that specifies electrical characteristics of a digital signaling circuit. Differential signaling can transmit data at rates as high as 10 Mbit/s, or may be sent on cables as long as 1500 meters. Some systems directly interconnect using RS-422 signals, or RS-422 converters may be used to extend the range of RS-232 connections. The standard only defines signal levels; other properties of a serial interface, such as electrical connectors and pin wiring, are set by other standards.

Several key advantages offered by this standard include the differential receiver, a differential driver and data rates as high as 10 Megabits per second at 12 meters (40 ft). Since the signal quality degrades with cable length, the maximum data rate decreases as cable length increases.
The maximum cable length is not specified in the standard, but guidance is given in its annex. (This annex is not a formal part of the standard, but is included for information purposes only.) Limitations on line length and data rate varies with the parameters of the cable length, balance, and termination, as well as the individual installation. Conservative maximum data rates with 24AWG UTP (POTS) cable are 10 Mbit/s at 12 m to 90 kbit/s at 1200 m.

RS-422 specifies the electrical characteristics of a single balanced signal. The standard was written to be referenced by other standards that specify the complete DTE/DCE interface for applications which require a balanced voltage circuit to transmit data. These other standards would define protocols, connectors, pin assignments and functions. Standards such as EIA-530 (DB-25 connector) and EIA-449 (DC-37 connector) use RS-422 electrical signals. Some RS-422 devices have 4 screw terminals for pairs of wire, with one pair used for data in one direction.

RS-422 cannot implement a truly multi-point communications network such as with EIA-485 since there can be only one driver on each pair of wires, however, one driver can be connected to up to ten receivers.
RS-422 can interoperate with interfaces designed to MIL-STD-188-114B, but they are not identical. RS-422 uses a nominal 0 to 5-volt signal while MIL-STD-188-114B uses a signal symmetric about 0 V. However the tolerance for common mode voltage in both specifications allows them to interoperate. Care must be taken with the termination network.
EIA-423 is a similar specification for unbalanced signaling (RS-423).

When used in relation to communications wiring, RS-422 wiring refers to cable made of 2 sets of twisted pair, often with each pair being shielded, and a ground wire. While a double pair cable may be practical for many RS-422 applications, the RS-422 specification only defines one signal path and does not assign any function to it. Any complete cable assembly with connectors should be labeled with the specification that defined the signal function and mechanical layout of the connector, such as RS-449.

 

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