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.
Standard scope RS-422 is the common short form title of American
National Standards Institute standard ANSIEIA-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, 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 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 metres.
The specification itself does not set an upper limit on data rate, but rather shows how signal
rate degrades with cable length.
The figure plotting this stops at 10 Mbit/s.
RS-422 only specifies the electrical signaling characteristics of a single balanced signal.
Protocols and pin assignments are defined in other specifications.
The mechanical connections for this interface are specified by EIA-530 or EIA-449, however
devices exist which have 4 screw-posts to implement the transmit and receive pair only.
The maximum cable length is 1500 m.
Maximum data rates are 10 Mbit/s at 12 m or 100 kbit/s at 1200 m.
RS-422 cannot implement a truly multi-point communications network such as with EIA-485,
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.
Applications A common use of RS-422 is for RS-232 extenders.
An RS-232-compatible variant of RS-422 using a mini-DIN-8 connector was widely used on
Macintosh hardware until it were replaced by Universal Serial Bus on the iMac in 1998.
Broadcast automation systems and post-production linear editing facilities use RS-422A to remotely
control the players/recorders located in the central apparatus room.
In most cases the Sony 9-pin connection is used, which makes use of a standard DE-9 connector.
This is a defacto industry standard connector for RS-422 used by many manufacturers.
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.
See also Electronic Industries Alliance
Profibus Fieldbus
List of network buses References
This article is based on material taken from the Free On-line Dictionary of Computing prior
to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version
1.3 or later.
External links The Telecommunications Industry Association
National Semiconductor Application Note AN-1031 "TIA/EIA-422-B Overview", January 2000, National
Semiconductor Inc., retrieved from [1] National Semiconductor Application Note AN-759
"Comparing EIA-485 and EIA-422-A Line Drivers and Receivers in Multipoint Applications",
February 1991, National Semiconductor Inc., retrieved from [2]
National Semiconductor Application Note AN-214 "Transmission Line Drivers and Receivers or
TIA/EIA Standards RS-422 and RS-423" August 1993, National Semiconductor Inc., retrieved
from [3] Maxim IC Application Note 723 "Selecting and
Using RS-232, RS-422, and RS-485 Serial Data Standards" Dec 2000,
Maxim Integrated Products, Inc., retrieved from [4]
Texas Instruments Application Report "422 and 485 Standards Overview and System Configurations"
June 2002, Texas Instruments, retrieved from [5]
Texas Instruments Application Report SLLA067B "Comparing Bus Solutions" October 2009, Texas
Instruments, retrieved from [6]