What happens during the PLC scan cycle?

• A. The PLC reads inputs, executes the program, and updates outputs in a cyclic sequence at a fixed rate.
• B. The PLC continuously processes the program without reading inputs.
• C. The PLC only updates outputs while not reading inputs.
• D. The PLC cycles power on and off to refresh its memory.

Answer: (A)

During a PLC scan cycle, the controller follows a fixed sequence: it first reads all input signals and stores their states, then it executes the user program using those input values, and finally it updates the outputs to reflect the results of the program. This cycle repeats continuously at a predictable rate, determined by the PLC’s speed and the size of the program.

This order ensures a coherent snapshot of inputs is used for every run of the logic, so outputs respond to the most recent input states in a timely and deterministic way. If inputs change, they’re captured at the start of the next cycle, the logic processes that snapshot, and the outputs are updated accordingly. The other options imply bypassing input reads, only updating outputs, or refreshing memory in a way that doesn’t match how PLCs actually operate.