Play a "heart" role. Vertical: When the program input from the programmer is stored in the user program memory, the CPU translates the user program into a user compiler approved by the PLC according to the function assigned by the system (the interpretation program of the system program memory). Horizontal: Input status and input information are input from the input interface, and the CPU stores it in the working data memory or into the input image register. The data and the program are then organically combined by the CPU. The result is stored in the output image register or working data memory, and then output to the output interface to control the external drive. Composition: The CPU consists of a controller, an operator, and registers. These circuits are integrated on one chip. The CPU is connected to the I/O interface circuit through an address bus and a data bus.