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
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.


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

Standard scope[edit] 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,[1] also known as X.27. These technical standards specify the electrical characteristics of the balanced voltage digital interface circuit.[2] 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
Telecommunications Industry Association
in 2005. Characteristics[edit]

Data Rate / Line Length chart from 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
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 each direction. 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
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. Applications[edit] The most widespread use of 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 RS-422
mode, which changed the behavior of some of the pins while turning others on or off completely. These connectors were used both to support RS-232
devices like modems, as well as AppleTalk networking, RS-422
printers, and other peripherals. Two such ports were part of every Mac until they were replaced, along with ADB ports, by Universal Serial Bus
Universal Serial Bus
on the iMac in 1998. RS-422
is a common solution for RS-232
extenders. These consist of RS-232
ports on either end of an RS-422
connection. Broadcast automation
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 de facto industry standard connector for RS-422
used by many manufacturers. See also[edit]

Electronic Industries Alliance Profibus Fieldbus List of network buses

References[edit] 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.

^ http://www.itu.int/rec/T-REC-V.11/en V.11 ITU Recommendation T-REC-V.11 ^ TIA/EIA STANDARD, Electrical Characteristics of Balanced Voltage Digital Interface Circuits, TIA/EIA-422-B, May 1994

External links[edit]

Wikibooks has a book on the topic of: Programming:Serial Data Communications

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]

v t e

Technical and de facto standards for wired computer buses


System bus Front-side bus Back-side bus Daisy chain Control bus Address bus Bus contention Network on a chip Plug and play List of bus bandwidths


SS-50 bus S-100 bus Multibus Unibus VAXBI MBus STD Bus SMBus Q-Bus Europe Card Bus ISA STEbus Zorro II Zorro III CAMAC FASTBUS LPC HP Precision Bus EISA VME VXI VXS NuBus TURBOchannel MCA SBus VLB PCI PXI HP GSC bus InfiniBand UPA PCI Extended (PCI-X) AGP PCI Express
PCI Express
(PCIe) Direct Media Interface (DMI) RapidIO Intel QuickPath Interconnect NVLink HyperTransport

Infinity Fabric

Intel UltraPath Interconnect


ST-506 ESDI IPI SMD Parallel ATA
Parallel ATA
Serial ATA

Parallel SAS

Fibre Channel SATAe PCI Express
PCI Express
(via AHCI or NVMe logical device interface)


Apple Desktop Bus DCB HP-IL HIL MIDI RS-232 RS-422 RS-423 RS-485 DMX512-A IEEE-488
(GPIB) IEEE-1284 (parallel port) UNI/O ACCESS.bus 1-Wire D²B I²C SPI Parallel SCSI Profibus IEEE 1394
IEEE 1394
(FireWire) USB Camera Link External PCIe Thunderbolt




PC Card ExpressCard


Multidrop bus CoreConnect AMBA Wishbone SLIMbus

Interfaces are listed by their speed in the (roughly) ascending order, so the interface at the end of each section should be the