Which sequence best describes a PLC's program execution cycle?

Which sequence best describes a PLC's program execution cycle?

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

Answer: (A)

The PLC scan cycle governs how a PLC processes input data and controls outputs. It begins by sampling the current state of all inputs and creating an input image that holds those values. The program logic is then executed using that input image, so the decisions it makes are based on the most recent input state. After the logic runs, the resulting values are written to the outputs, updating the connected devices. Finally, the PLC refreshes internal status and diagnostics before starting the next cycle. This sequence—read inputs, execute logic, write outputs, update status—keeps the controller’s behavior synchronized with the real-world changes it monitors and controls. Other activities described in the other options aren’t part of the runtime cycle. Programming ladder logic is about creating or editing the program, not how it runs. Uploading programs from the PLC to a PC is a data transfer process, not the execution sequence. The order of I/O card addressing is about hardware configuration and addressing, not how the program is executed on each cycle.

The PLC scan cycle governs how a PLC processes input data and controls outputs. It begins by sampling the current state of all inputs and creating an input image that holds those values. The program logic is then executed using that input image, so the decisions it makes are based on the most recent input state. After the logic runs, the resulting values are written to the outputs, updating the connected devices. Finally, the PLC refreshes internal status and diagnostics before starting the next cycle. This sequence—read inputs, execute logic, write outputs, update status—keeps the controller’s behavior synchronized with the real-world changes it monitors and controls.

Other activities described in the other options aren’t part of the runtime cycle. Programming ladder logic is about creating or editing the program, not how it runs. Uploading programs from the PLC to a PC is a data transfer process, not the execution sequence. The order of I/O card addressing is about hardware configuration and addressing, not how the program is executed on each cycle.