In terms of structure, PLC is divided into two types: fixed type and combined type (modular type). Fixed PLC includes CPU board, I/O board, display panel, memory block, power supply, etc. These elements are combined into a non-removable whole. Modular PLC includes CPU module, I/O module, memory, power module, backplane or rack, and these modules can be combined and configured according to certain rules.
The composition of the CPU
The CPU is the core of the PLC and acts as the nerve center. Each PLC has at least one CPU. It receives and stores user programs and data according to the functions assigned by the PLC system program. The state or data is stored in the specified register, and at the same time, the working state of the power supply and the PLC internal circuit and the syntax error in the programming process are diagnosed. After entering the operation, read the instructions one by one from the user program memory, and then generate the corresponding control signal according to the task specified by the instruction after analysis to command the relevant control circuit.
CPU is mainly composed of arithmetic unit, controller, register and data, control and status bus to realize the connection between them. The CPU unit also includes peripheral chip, bus interface and related circuits. Memory is mainly used to store programs and data, and is an indispensable component of PLC.
From the user’s point of view, it is not necessary to analyze the internal circuit of the CPU in detail, but it is necessary to have a sufficient understanding of the working mechanism of each part. The controller of the CPU controls the work of the CPU, which reads the instructions, interprets the instructions and executes the instructions. But the working rhythm is controlled by the oscillating signal. The calculator is used to perform digital or logical operations and work under the command of the controller. The register participates in the operation and stores the intermediate result of the operation, and it also works under the command of the controller.
CPU speed and memory capacity are important parameters of PLC. They determine the working speed of PLC, the number of IO and software capacity, etc., thus limiting the control scale.
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