The CPU serves as the core of the PLC, functioning as its central nervous system. Each PLC system includes at least one CPU. It receives and stores user programs and data based on the system program of the PLC. It scans and collects states or data from field input devices, storing them in designated registers. At the same time, it diagnoses the working status of the power supply and internal circuits of the PLC, as well as syntax errors in the programming process.

During operation, the CPU sequentially reads instructions from the user program memory. After analysis, it generates corresponding control signals according to the instructions’ tasks. These signals are then used to command relevant control circuits.

CPU mainly consists of operators, controllers, registers and achieve the link between their data, control and status bus composition, CPU unit also includes peripheral chips, bus interfaces and related circuits. Memory block is mainly used to store procedures and data, is an indispensable component of PLC.

It is not necessary to analyse the internal circuitry of the CPU in detail for the user, but it is still important to have a good understanding of the mechanism of each part of the CPU, which is controlled by the controller, which reads, interprets, and executes the instructions. However, the working rhythm is controlled by oscillating signals. Operators are used to perform digital or logical operations and work under the command of the controller. Registers are involved in the operations and store the intermediate results of the operations, which also work under the command of the controller.

CPU speed and memory capacity are important parameters of a PLC, they determine the PLC’s operating speed, I/Os number and software capacity, etc. and therefore limit the control scale.