What is a PLC program scan cycle?

What is a PLC program scan cycle?

• A. The order of I/O card addressing
• B. The process of programming a PLC with ladder logic
• C. The method for uploading programs from PLC to PC
• D. The sequence: read inputs, execute program logic, write outputs, and update status in a loop.

Answer: (D)

A PLC scan cycle is the repeating sequence the controller uses to manage I/O and logic. It starts by reading the current state of all inputs and storing those values in an input image. Next, the PLC executes the programmed logic using those input values, determining what the outputs should do. Then it writes the resulting results to the outputs, updating the actual hardware. Finally, the PLC updates its internal status and any housekeeping tasks (timers, counters, communications, etc.), and the cycle starts again. This loop happens continuously, so the system continuously responds to changes in inputs. The described sequence—read inputs, run the logic, update outputs, and refresh status in a loop—is exactly what the scan cycle entails.

A PLC scan cycle is the repeating sequence the controller uses to manage I/O and logic. It starts by reading the current state of all inputs and storing those values in an input image. Next, the PLC executes the programmed logic using those input values, determining what the outputs should do. Then it writes the resulting results to the outputs, updating the actual hardware. Finally, the PLC updates its internal status and any housekeeping tasks (timers, counters, communications, etc.), and the cycle starts again. This loop happens continuously, so the system continuously responds to changes in inputs. The described sequence—read inputs, run the logic, update outputs, and refresh status in a loop—is exactly what the scan cycle entails.