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The Info List - RS-422



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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
RS-422
signals, or RS-422
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.

CONTENTS

* 1 Standard scope * 2 Characteristics * 3 Applications * 4 See also * 5 References * 6 External links

STANDARD SCOPE

RS-422
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
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.

CHARACTERISTICS

Data Rate / Line Length chart from RS-422
RS-422
Annex A

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. 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.

RS-422
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
RS-422
electrical signals. Some RS-422
RS-422
devices have 4 screw terminals for pairs of wire, with one pair used for data in each direction.

RS-422
RS-422
cannot implement a true 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
RS-422
can interoperate with interfaces designed to MIL-STD-188-114B , but they are not identical. RS-422
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
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
RS-422
applications, the RS-422
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 .

APPLICATIONS

The most widespread use RS-422
RS-422
was on the early Macintosh computers. This was implemented in a multi-pin connector that had enough pins to support the majority of the common RS-232 pins; the first models used a 9-pin D connector , but this was quickly replaced by a mini-DIN -8 connector. The ports could be put into either RS-232 or