page_title: RS-422
section_title: Characteristics
hierarchical_section_title: RS-422 / Characteristics
caption_attribution_description: English: Data Rate / Line Length chart from RS-422 Annex A (This chart is not in RS-485) This chart is not an absolute limit, but a conservative guide based on empirical data of 24AWG telephone cable. Cable length may be extended from this chart. Chart created with Excel from following data: Cable Length in Meters Data Signaling Rate in kBits/Sec 10000 1200 90000 1200 10000000 14 .
context_page_description: 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. It was intended to replace the older RS-232C standard with a standard 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, or may be sent on cables as long as 1,500 meters at lower rates. It is closely related to RS-423, which used the same signaling systems but on a different wiring arrangement.
  RS-422 specifies differential signaling, with every data line paired with a dedicated return line. It is the voltage difference between these two lines that define 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 large +5 and -5 voltages. Moving to dedicated return lines and always defining ground in reference to the sender allowed RS-422 to use 0.4 V, allowing it to run at much higher speeds.
context_section_description: 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 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 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.
caption: Data Rate / Line Length chart from RS-422 Annex A
exif: {}