The RS485 standard defines a balanced two-wire transmission line, which may be shared as a bus line by up to 32 driver/receiver pairs. Many characteristics of the transmitters and receivers are the same as RS422. The main differences between RS422 and RS485 are
•Two-wire (half duplex) transmission instead of four-wire transmission
•Balanced line drivers with tristate capability. The RS485 line driver has an additional "enable" signal which is used to connect and disconnect the driver to its output terminal. The term "tristate" refers to the three different states possible at the output terminal: mark (logical 1), space (logical 0) or "disconnected"
•Extended Common Mode Voltage (Vcm) range from -7V to +12V.
The EIA Standard RS485 "Standard for electrical characteristics of generators and receivers for use in balanced digital multipoint systems" defines the characteristics of an RS485 system.