PLC - CPU
The CPU (Central Processing Unit) is the brain of a PLC system, managing all processing tasks and controlling the operation of the entire system. Here’s a breakdown of its key functions:
1. Program Execution
The CPU reads the control program (usually written in ladder logic, function block diagrams, or structured text) stored in the memory and executes it sequentially. This program dictates how the PLC interacts with input and output devices, such as sensors, switches, motors, and actuators, to automate processes.
2. Input Processing
The CPU scans the input signals from field devices (e.g., sensors, push buttons) connected to the PLC's input modules. It monitors the current state of these inputs (e.g., ON or OFF) and stores this information in the input status table. The CPU uses this data to make decisions based on the program logic.
3. Logic Processing
The CPU performs logical operations (e.g., AND, OR, NOT) and arithmetic calculations (e.g., addition, subtraction) as instructed by the control program. It compares the input data against preset conditions and performs the necessary control actions.
4. Output Control
Based on the program’s logic, the CPU sends commands to output devices (e.g., motors, solenoids, valves) through the output modules. These outputs control the machinery or process being automated. The CPU updates the output status table accordingly.
5. Communication
The CPU handles communication between the PLC and other devices or systems, such as human-machine interfaces (HMIs), other PLCs, SCADA systems, or external sensors. It ensures smooth data exchange, whether via industrial communication protocols (Modbus, Ethernet/IP, Profibus, etc.) or proprietary ones.
6. Diagnostics and Monitoring
The CPU continuously monitors the system's health and status, checking for errors such as hardware faults, communication issues, or abnormal conditions in inputs/outputs. If any faults occur, the CPU triggers alarms or takes corrective action as programmed.
7. Memory Management
The CPU manages memory allocation for the program, data, and system diagnostics. It stores the control program in non-volatile memory (to retain the program during power loss) and uses volatile memory (RAM) for processing data and storing temporary results during operation.
8. Scanning Cycle (Scan Time)
The CPU operates in a loop, performing the following sequence in each scan cycle:
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Input Scan: Reads the state of all input devices.
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Program Execution: Processes the logic based on the input status and the control program.
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Output Update: Sends commands to output devices based on the processed logic.
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Housekeeping: Handles communication, diagnostics, and internal checks. This scan cycle happens continuously, with scan times usually measured in milliseconds, ensuring real-time control.