RS-422
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RS-422, also known as TIA/EIA-422, is a technical standard originated by the
Electronic Industries Alliance The Electronic Industries Alliance (EIA; until 1997 Electronic Industries Association) was an American standards organization, standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the ...
, first issued in 1975, that specifies the electrical characteristics of a digital signaling circuit. It was meant to be the foundation of a suite of standards that would replace the older RS-232C standard with standards that offered much higher speed, better immunity from noise, and longer cable lengths. RS-422 systems can transmit data at rates as high as 10
Mbit/s In telecommunications, data transfer rate is the average number of bits (bitrate), characters or symbols (baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are multi ...
, or may be sent on cables as long as at lower rates. It is closely related to RS-423, which uses the same signaling systems but on a different wiring arrangement. RS-422 specifies
differential signaling Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conduc ...
, with every data line paired with a dedicated return line. It is the voltage difference between these two lines that defines the mark and space, rather than, as in RS-232, the difference in voltage between a data line and a local ground. As the ground voltage can differ at either end of the cable, this required RS-232 to use signals with voltage magnitudes greater than 5 volts. Moving to dedicated return lines and always defining ground in reference to the sender allows RS-422 to use 0.4 V, allowing it to run at much higher speeds. RS-423 differs primarily in that it has a single return pin instead of one for each data pin.


Standard scope

RS-422 is the common short form title of the
American National Standards Institute The American National Standards Institute (ANSI ) is a private nonprofit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organiz ...
(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.''TIA/EIA STANDARD, Electrical Characteristics of Balanced Voltage Digital Interface Circuits, TIA/EIA-422-B'', May 1994 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, RS-422/V.11 does not allow multiple drivers but only multiple receivers. The first version of RS-422 was issued in 1975, with revision A issued in December 1978. Revision B, published in May 1994 was reaffirmed by the Telecommunications Industry Association in 2005.


Characteristics

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 In telecommunications, data transfer rate is the average number of bits (bitrate), characters or symbols (baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are multi ...
. 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 vary with the parameters of the cable length, balance, and termination, as well as the individual installation. Figure A.1 shows a maximum length of , 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 24 AWG UTP ( POTS) cable are 10 Mbit/s at to 90 kbit/s at , 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 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 that 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 The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smallest connectors used on computer systems. Description, ...
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 each direction. RS-422 cannot implement a true multi-point communications network, such as with
RS-485 RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. Electrical signaling is balanced, and Telecomm ...
, since there can be only one driver on each pair of wires. However, one driver can fan-out 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 The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
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. RS-423 is a similar specification for unbalanced signaling. When used in relation to communications wiring, RS-422 wiring refers to cable made of 2 sets of
twisted pair Twisted pair cabling is a type of communications cable in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced ...
, 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 The RS-449 specification, also known as EIA-449 or TIA-449, defines the functional and mechanical characteristics of the interface between data terminal equipment, typically a computer, and data communications equipment, typically a modem or te ...
.


Applications

One of the most widespread uses of RS-422 was on the early
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computers. This was implemented in a multi-pin connector that had enough pins to support the majority of the common
RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such as a compu ...
pins; the first models used a 9-pin D connector, but this was quickly replaced by a mini-DIN-8 connector starting with the Macintosh Plus. The ports could be put into either RS-232 or RS-422 mode, which changes the behavior of some of the pins while turning others on or off completely. These connectors are used to support RS-232 devices like
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s,
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networking, RS-422 printers, and other peripherals. Two such ports were part of early Apple Macintosh series designs until they were replaced, along with ADB ports, by
Universal Serial Bus Universal Serial Bus (USB) is an industry standard, developed by USB Implementers Forum (USB-IF), for digital data transmission and power delivery between many types of electronics. It specifies the architecture, in particular the physical ...
on the
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in 1998. RS-422 is a common transport mechanism for RS-232 extenders. These consist of RS-232 ports on either end of an RS-422 connection. Before hard-disk-based playout and editing systems were used,
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systems and
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linear editing facilities used RS-422A to remotely control the players/recorders located in the central apparatus room. In most cases, the Sony 9-pin connection was used, which makes use of a DE-9 connector. This is the de facto industry standard connector for RS-422,Sony 9-Pin Remote Protocol
which is still found on broadcast equipment today.


See also

* Differential TTL *
Electronic Industries Alliance The Electronic Industries Alliance (EIA; until 1997 Electronic Industries Association) was an American standards organization, standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the ...
* Fieldbus * List of network buses * Profibus


References


Further reading

* * * {{Computer-bus EIA standards Serial buses Serial communication interfaces