| RS-422 | |
|---|---|
| Standard | TIA/EIA-422 |
| Protocol information | |
| Physical media | Twisted pair |
| Network topology | Point-to-point, unidirectional multidrop bus |
| Maximum devices | 10 (1 driver and 10 receivers) |
| Maximum distance | 1,200 meters (3,900 ft) |
| Mode of operation | Differential |
| Maximum binary rate | 100 kbit/s – 10 Mbit/s |
| Voltage | −6 to +6 V (maximum voltage for each line) |
| Mark(1) | Negative difference |
| Space(0) | Positive difference |
| Available signals | Tx+, Tx−, Rx+, Rx− (full duplex) |
| Connector types | Not specified |
RS-422, also known as TIA/EIA-422, is a technical standard originated by the Electronic Industries Alliance, first issued in 1975, that specifies 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, or may be sent on cables as long as 1,200 meters (3,900 ft) 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, 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.[1]