What are the primary functions of the CPU in a PLC?

• A. Execute the operating system, manage memory, monitor inputs and energize appropriate outputs.
• B. Process analog signals only.
• C. Store programs on external disks.
• D. Control network routing.

Answer: (A)

The CPU in a PLC is the brain that runs the control program and keeps the system responsive in real time. In each scan cycle, it reads all input signals, executes the control logic using the PLC’s memory (where variables, timers, and counters live), and then updates the outputs to reflect the results. Beyond just processing logic, it also handles the PLC’s firmware and manages the data it stores, ensuring the program runs smoothly and the system stays in sync with the physical world.

This is why the primary function isn’t just about processing signals or storing programs. Analog processing is handled by dedicated input/output modules, while storing programs on external disks isn’t the CPU’s main job in most PLCs. Routing network traffic isn’t the PLC’s focus either; its real role is executing logic, managing memory, and driving outputs based on inputs.