1. Decision-Making and Logic Execution: The core function of the CPU is to
execute the user program stored in its memory. It processes the logic (typically ladder logic, function blocks, or structured text) line by line. During this execution, it evaluates the status of various inputs to determine the necessary state of the outputs based on the programmed instructions.
2. Interfacing with I/O Modules: The CPU acts as the central hub for
controlling input and output devices.
• It reads data from the input modules (which are connected to sensors, switches, and transmitters).
• It writes data to the output modules (which drive actuators like motors, solenoid valves, and indicator lights).
3. System Management and Housekeeping: Beyond logic execution, the CPU performs vital internal tasks, including:
• Self-Diagnostics: Continuously checking the PLC hardware and software for faults or errors to ensure safe operation.
• Memory Management: Managing the storage of the operating system, user programs, and data tables.
• Communication Management: Handling data exchange with programming terminals, Human-Machine Interfaces (HMIs), and other networked devices.