RS-422 is half-duplex 4-wire serial communication standard developed in 1994 and completed in 2004 designed to overcome some of the shortfalls of the RS-232 specification, specifically those regarding maximum transmission distance and maximum transmission speed.

Whereas the maximum transmission distance for RS-232 is officially 50ft (100-250ft is possible with good quality equipment, cabling, and some luck), the maximum transmission distance for RS-422 is just under a mile at 4,000. Rs-422 accomplishes this by removing the common ground wire and replacing the RX and TX wires with RX and TX pairs. In RS232 a single bit is transmitted down the line by pulling the TX wire up to 3v relative to the common ground. This is called absolute voltage level because in order for the receiving device to properly detect the signal it must be above 3v. With RS232's elimination of the ground wire, and usage of TX and RX pairs (TX+ and TX-, etc), it need only achieve a 3-5v difference between the + and - pairs. This combined with twisted pair cabling allows for RS422 to hit distances and transmission speeds that are unheard of with RS232. The transmission speeds for RS422 range from 10Mbs at 40ft, to 100kbs at 4,000 ft. Even at its slowest, RS422 is orders of magnitude faster than RS232.

Another benefit of RS422 is that it is a one-to-many connection. One host or transmitter can transmit to up to 10 receiving stations, though these receivers cannot transmit back. An example of this might be a battery system broadcasting its status and availability to a number of attached computers or devices that all might need to safely shut down in a hurry should the battery backup system begin to fail.

RS422 is typically found in industrial environments that require extremely long range transmissions, or very fast baud rates. Examples might include factory milling machines or robots, controlled on the factory floor from computers on the other side of the factory. It is rarely, if ever, found in consumer products.

There is no official connection or plug for RS422 it is commonly found with DB9 ports simply for ease at which cabling can be found, though straight screw down "terminals" are not uncommon.