The master computer refers to the computer that can directly send out control commands, also called PC / host computer / upper computer, and the screen displays various signal changes (hydraulic pressure, water level, temperature, etc.). It is the main control part in the system, mainly as the planning control of the system, belongs to the decision-making level.

The slave computer is a computer that directly controls the equipment to obtain the equipment status, generally called PLC and single chip micro computer. It is the specific executive layer mainly completes the tasks assigned by the system planning layer.

Conceptually, the controller and the server are the upper computer; the controlled and the served are the slave computer. It can also be understood as the relationship between the host and the slave. The upper computer and the slave computer generally refer to the PC in the centralized control system and the field industrial embedded PC. The upper computer (PC) is mainly used to send out operation instructions and display result data, while the slave computer (industrial embedded PC) is mainly used to monitor and execute the operation instructions of the upper computer.

The command from the upper computer is given to the slave computer firstly, and then the slave computer interprets it as the corresponding time sequence signal to directly control the corresponding equipment. From time to time, the slave computer reads the equipment status data (general analog quantity) and converts it into a digital signal to feed back to the upper computer. Both the upper and slave computers need to be programmed and have special development systems. But the slave computer generally has a more reliable unique communication protocol. When you buy the slave computer, you will get a lot of manual CDs to tell you how to use the unique protocol communication, which will offer a lot of examples. Generally speaking, for programmers, it's just to use some new APIs.

Multi-language support function modules, generally support several high-level languages for upper computer programming. The slave computer collects data and controls the device by the single-chip microcomputer, while the upper computer processes all kinds of complex data and controls the single-chip microcomputer. In the industrial control system, all kinds of data acquisition and the control of the actuator are done by the slave computer or the detection station.

In addition, in physical layer, the upper computer and the slave computer are computers connected through communication, which is ts relatively. Generally, the slave computer is responsible for the front-end "measurement, control" and other processing; the upper computer is responsible for the "management" processing. The slave computer is the execution unit only when receiving the command from the main equipment. However, the slave computer can also directly and intelligently process the measurement and control execution. The upper computer does not participate in the specific control and only manages (data storage, display, printing, human-machine interface, etc.). Common DCS system, "centralized decentralized (distributed) system" is a system with centralized upper computer and decentralized slave computer.

Generally, the communication between the upper computer and the slave computer adopt different communication protocols, RS232 serial port communication, or RS485 serial communication are available, the communication between PLC and the upper computer can be realized by using packaged program development tools. Of course, you can write your own interface protocol to control the communication between the upper computer and the slave computer.