| RS422 | |
|---|---|
| Standard | EIA RS-422 |
| Physical Media | Twisted Pair |
| Network Topology | Point-to-point, Multi-dropped |
| Maximum Devices | 10 (1 driver & 10 receivers) |
| Maximum Distance | 1200 metres (4000 ft) |
| Mode of Operation | Differential |
| Maximum Baud 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 is a technical standard that specifies electrical characteristics of a digital signalling circuit. Differential-mode signals can be sent at rates as high as 10 million bits per second, or may be sent on cables as long as 1200 metres. 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 are set by other standards.
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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 (which is multi-point instead of multi-drop), EIA-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 megabaud at 12 metres (40 ft). 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.
EIA-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 (DB-25 connector) or EIA-449 (DC-37 connector), however devices exist which have 4 screw-posts to implement the transmit and receive pair only. The maximum cable length is 1200 m. Maximum data rates are 10 Mbit/s at 12 m or 100 kbit/s at 1200 m. EIA-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.
A common use of EIA-422 is for RS-232 extenders. Broadcast automation systems and post-production linear editing facilities use RS-422A to remotely control the players/recorders located in the central apparatus room.
Also, an RS-232-compatible variant of RS-422 using a mini-DIN-8 connector was widely used on Macintosh hardware until it was replaced by Universal Serial Bus on the iMac in 1998.
EIA-422 can interoperate with interfaces designed to MIL-STD-188-114B, but they are not identical. EIA-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 (i.e. with connectors) should be labeled with the specification that defined the signal function and mechanical layout of the connector, such as RS-449.
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
Maxim Integrated Products, Inc., retrieved from [4]
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