RS-422
| RS-422 | |
|---|---|
| Standard | TIA/EIA-422 |
| Physical Media | Twisted Pair |
| Network Topology | Point-to-point, Multi-dropped |
| Maximum Devices | 10 (1 driver & 10 receivers) |
| Maximum Distance | 1500 metres (4,900 ft) |
| Mode of Operation | Differential |
| Maximum Binary Rate | 100 kbit/s – 10 Mbit/s |
| Voltage Levels | −6V to +6V (maximum differential Voltage) |
| Mark (1) | Negative Voltages |
| Space (0) | Positive voltages |
| Available Signals | Tx+, Tx-, Rx+, Rx- (Full Duplex) |
| Connector types | Not specified |
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.
RS-422 is the common short form title of American National Standards Institute (ANSI) standard ANSI/TIA/EIA-422-B Electrical Characteristics of Balanced Voltage Differential Interface Circuits and its international equivalent ITU-T Recommendation T-REC-V.11, also known as X.27. These technical standards specify the electrical characteristics of the balanced voltage digital interface circuit. RS-422 provides for data transmission, using balanced, or differential, signaling, with unidirectional/non-reversible, terminated or non-terminated transmission lines, point to point, or multi-drop. In contrast to EIA-485 (which is multi-point instead of multi-drop), RS-422/V.11 does not allow multiple drivers but only multiple receivers.
Revision B, published in May 1994 was reaffirmed by the Telecommunications Industry Association in 2005.
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). The specification is for circuits with a data rate up to 10 Mbit/s, but since the signal quality degrades with cable length, the maximum data rate decreases as cable length increases. Figure A.1 in the annex plotting this stops at 10 Mbit/s.
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. Figure A.1 shows a maximum length of 1200 meters, but this is with a termination and the annex discusses the fact that many applications can tolerate greater timing and amplitude distortion, and that experience has shown that the cable length may be extended to several kilometers. Conservative maximum data rates with 24AWG UTP (POTS) cable are 10 Mbit/s at 12 m to 90 kbit/s at 1200 m as shown in the figure A.1. This figure is a conservative guide based on empirical data, not a limit imposed by the standard.
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