RS232 is the best known industry standard providing asynchronous serial communication using ± 12 Volt logic levels over distances up to 100 feet, at speeds up to 20,000 Baud.
RS422 allows one transmitter and up to 10 receivers with data transmission rates up to 10 Megabits per second for distances up to 40 feet and up to 100 Kilobits per second for distances up to 4000 feet (1219 metres).
To achieve good long distance noise immunity, 2 wires are used to carry each signal, configured as a twisted pair of cables. The TXD pair and RXD pair are used to carry the data whilst the RTS pair and CTS pair lines are used for handshaking. Thus 2 twisted pairs are used without handshaking and 4 twisted pair cable is used with handshaking.
RS485, based on the RS422 standard allows up to 32 driver/receivers pairs on a party line data bus. Whilst only one of these should be transmitting data at any time, the rest can all simultaneously listen to the data. Handshaking is performed by a software protocol. Two twisted pairs form a full duplex system.
Often only one twisted pair cable is used as the TXD and RXD lines are tied together; this is known as half duplex mode.
Traditionally, the RTS signal is used as the party line control to tristate the PC’s transmitter, but in multitasking operating systems the only reliable method is hardware autogating.