In the PLC program cycle, which sequence best describes how inputs, logic, and outputs flow?

• A. Inputs are sampled, then logic is evaluated, then outputs are refreshed.
• B. Outputs are refreshed before inputs are sampled.
• C. Logic is evaluated before inputs are sampled.
• D. Inputs and outputs are refreshed at the same time.

Answer: (A)

In PLCs, the scan cycle processes signals in a fixed order: inputs are sampled first to capture the current state of all input devices, then the program logic is evaluated using those sampled values, and finally the outputs are refreshed to reflect the results of that logic. This order makes sure decisions are based on a stable snapshot of inputs from the same moment in time, avoiding changes in inputs during the decision process and ensuring predictable actuation. If logic were evaluated before sampling inputs, or outputs were refreshed before sampling, you could end up using outdated data or driving devices based on the previous cycle, which leads to inconsistent or undesirable behavior. Refreshing outputs after the logic runs is what updates the actual hardware to match the computed results.