RS232, RS422 and RS485 are all serial data interface standards. RS422 is developed from RS232, it is proposed to make up for the shortcomings of RS232. In order to improve the shortcomings of RS232 communication distance and low rate, RS422 defines a balanced communication interface, which increases the transmission rate to 10Mb/s and the transmission distance to 4000 feet (when the rate is lower than 100kb/s), and allows one Up to 10 receivers can be connected to the balanced bus. RS422 is a one-way, balanced transmission specification for single machine sending and multiple machine receiving, and it is named TIA/EIA-422-A standard. In order to expand the scope of application, EIA formulated the RS485 standard on the basis of RS422 in 1983, adding multipoint and two-way communication capabilities, that is, allowing multiple transmitters to be connected to the same bus, and at the same time increasing the transmitter's drive capability and Conflict protection feature expanded the common mode range of the bus, and was later named TIA/EIA-485-A standard.
Serial interface refers to the sequential transmission of data bit by bit. Its characteristic is that the communication line is simple, and two-way communication can be realized as long as a pair of transmission lines (the telephone line can be used directly as the transmission line), which greatly reduces the cost, and is especially suitable for remote Distance communication, but the transmission speed is slower. The characteristics of serial communication are: The transmission of data bits is carried out in bit sequence, and at least one transmission line is required to complete; the cost is low but the transmission speed is slow. The distance of serial communication can be from several meters to several kilometers; according to the transmission direction of information, serial communication can be further divided into three types: simplex, half-duplex and full-duplex.