In order to facilitate remote data communication between computers, external devices or smart instruments equipped with different standard serial interfaces, it is necessary to convert the standard serial interfaces to each other. The converter is compatible with RS-232 and RS-485 standards and is capable of converting single-ended signals to balanced differential RS-485 signals, and the converter can extend the RS-232 communication distance up to 1.2 km without external power supply. The unique I/O circuitry automatically controls the direction of data flow without any handshaking signals (such as RTS, DTR, etc.), thus ensuring that programs written in RS-232 half-duplex mode can run in RS-485 mode without changes, which makes the device suitable for existing operating software and interface hardware. The converter transmission rate is 300 - 115,200 bps. It can be used to form a point-to-point or point-to-multipoint remote multi-computer communication network between the main controller, or between the main controller and the microcontroller or peripherals to realize multi-computer answering communication. It is widely used in industrial automation control system, one-card, access control system, parking system, self-service banking system, bus fare collection system, canteen vending system, company staff attendance management system, highway toll station system, and etc.