RS-232, also known as the standard serial port, is the most commonly used serial communication interface. It was developed in 1970 by the American Electronics Industry Association (EIA) in collaboration with Bell Systems, modem manufacturers, and computer terminal manufacturers for serial communications. Its full name is \"Technical Standard for Serial Binary Data Exchange Interface between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE).\" The traditional RS-232-C interface standard has 22 wires, and uses the standard 25-core D-type plug holder (DB25), which is later simplified to a 9-core D-type socket (DB9), and now the 25-core plug holder is rarely used in applications.
RS-232 adopts unbalanced transmission mode, which is called single-ended communication. Because the difference between the send level and the receive level is only about 2V to 3V, its common mode rejection ability is poor, coupled with the distributed capacitance on the twisted pair, its transmission distance is up to about 15 meters, and the maximum speed is 20kb\/s. RS-232 is designed for point-to-point (that is, only one pair of receiving and sending devices) communication with a driver load of 3 to 7kΩ. So RS-232 is suitable for communication between local devices.