Scan Cycle Time
Key Points
- Scan Cycle Time is the duration of one controller input-read, logic-execute, output-update cycle
- Used in PLCs, industrial controllers, and automation systems
- Affects controller responsiveness and the maximum rate of control updates
- Constraints include program size, input and output count, communication delays, and process requirements
- Failure modes include slow response, missed timing requirements, jitter, and unstable control if the cycle is too slow for process dynamics
Definition
Scan Cycle Time is the time required for a controller to read inputs, execute logic, and update outputs once in a control loop iteration.
Concept
Scan Cycle Time is an industrial term describing the time a controller needs to complete one full control loop iteration. It exists to characterize how quickly the controller can sample inputs, run logic, and refresh outputs. The cycle time is used in PLCs, industrial controllers, and automation systems to define operational responsiveness and the maximum rate of control updates.
Explainer
Scan Cycle Time operates as the period of the controller's repeated operation cycle, determining how frequently the controller can observe and react to the process. It is fundamental to PLCs, industrial controllers, and automation systems. Tradeoffs exist between richer control programs and longer cycle time, and between more logic per cycle and lower update frequency. Scan Cycle Time matters because the controller cannot react faster than its execution cycle allows, making it critical for stable and responsive process control.