Which functions are performed by the PLC CPU?

• A. Accept input data from sensing devices; Execute the stored user program; Sense appropriate output commands to control devices
• B. Store data in memory only
• C. Drive outputs directly without a program
• D. Provide power to sensors

Answer: (A)

The PLC CPU’s job is to run the control loop that makes the machine respond: it reads the current input states from sensing devices, executes the stored user program to decide what should happen, and then updates the output signals to control the devices. This happens in a continuous scan: input data is captured, the program logic (including timers, counters, arithmetic, and ladder logic) is executed, and the resulting commands are sent to the output modules. The program itself and the data it uses are stored in memory, but the CPU is what processes that data and makes the decisions in real time. Powering sensors is supplied by the plant’s power system, not the CPU. Outputs are not driven directly without a program—the program determines which outputs to energize based on the inputs.