In order to facilitate remote data communication between computers, external devices or intelligent instruments equipped with different standard serial interfaces, standard serial interfaces must be converted to each other. The converter is compatible with RS-232, RS-485 and RS-422 standards and can convert single-ended RS-232 signals into balanced differential RS-485 or RS-422 signals. The converter can extend the RS-232 communication distance to 1.2 kilometers without the need for an external power supply. It adopts a unique "RS-232 charge pump" driver, so it does not need to initialize the RS-232 serial port to obtain power. It has zero-delay automatic transceiver conversion and a unique I/O circuit to automatically control the direction of data flow without any handshake signal (such as RTS, DTR, etc.). The conversion between full-duplex (RS-422) and half-duplex (RS-485) mode can be achieved without the need for jumper settings. Programs written in RS-232 full and half-duplex mode can run in RS-485/422 mode without modification which is suitable for existing operating software and interface hardware. The converter transmission rate is 300 -115.2 Kbps. It can be applied to form a point-to-point or point-to-multipoint remote multi-machine communication network between main control computers, or between main control computers and microcontrollers or peripherals, to achieve multi-machine response communication. It is widely used in industrial automation control systems, all-in-one cards, access control systems, parking systems, self-service banking systems, bus toll collection systems, canteen food sales systems, company employee attendance management systems, highway toll station systems, etc.