Scan Cycle
Quick answer
A PLC scan cycle is the repeated controller process of servicing inputs, executing scheduled logic, updating outputs and performing communications or diagnostics. The exact order and timing depend on the controller, task model and use of immediate or asynchronous I/O.
Key Takeaways
- A PLC scan cycle is the repeated controller process of servicing inputs, executing scheduled logic, updating outputs and...
- Beginner-level topic in PLC Fundamentals
- Commonly used in: Understanding PLC execution order, Diagnosing missed pulses and one-scan events
- Related to: Scan Time, Input Scan, Program Execution
Detailed Definition
In a simple cyclic PLC, the controller copies physical inputs into an input image, executes the program using that image and then transfers the output image to physical outputs. That model explains why a field input can change during a scan without normal logic seeing the change until a later input update.
Modern PLCs can also have periodic or event tasks, asynchronous communications, immediate I/O instructions and distributed modules with independent update rates. Therefore “read all inputs, run all logic, write all outputs” is a useful teaching model, not a universal timing guarantee. Use task monitoring and controller documentation when timing affects the design.
Evidence and review status
The page distinguishes the common cyclic teaching model from controller-specific task and I/O behavior. Verify actual task periods, priorities and module update rates in the target platform.
Technical review date:
Critical behavior to understand
- Scan time and task period are different: a periodic task can execute every 10 ms while using less than 10 ms of CPU time.
- Input and output images may update outside the simple cyclic sequence on modern controllers.
- Later writes to the same output tag can override earlier writes during one execution.
- Online edits, communications, interrupts and higher-priority tasks can add jitter.
Verification checklist
- 1Measure maximum task execution time, not only the average.
- 2Compare the shortest field pulse with task and module update periods.
- 3Search the project for multiple writes to critical output tags.
- 4Test cold start, first scan, communications loss and task overrun behavior.
Worked example
Determine whether a cyclic task can see a short pulse
A photoeye pulse lasts 2 ms. The PLC logic that reads it runs in a 10 ms periodic task.
- 1Compare pulse width with the task period.
- 2Account for the input module update time and phase relative to the task.
- 3Test a worst case where the pulse begins just after the input value was sampled.
- 4Select a hardware latch, faster task or pulse-stretching method if every event must be captured.
Expected result: A 2 ms pulse is not guaranteed to be observed by a 10 ms cyclic task without an appropriate capture strategy.
Common Questions
What is Scan Cycle?
A PLC scan cycle is the repeated controller process of servicing inputs, executing scheduled logic, updating outputs and performing communications or diagnostics. The exact order and timing depend on the controller, task model and use of immediate or asynchronous I/O.
When should I use Scan Cycle?
Scan Cycle is particularly useful in scenarios such as Understanding PLC execution order and Diagnosing missed pulses and one-scan events. Consider implementing it when you need reliable, efficient solutions for these types of applications.
What should I verify before using Scan Cycle?
Measure maximum task execution time, not only the average. Compare the shortest field pulse with task and module update periods. Search the project for multiple writes to critical output tags. Test cold start, first scan, communications loss and task overrun behavior.
What are related concepts I should learn?
To fully understand Scan Cycle, you should also familiarize yourself with Scan Time, Input Scan, and Program Execution. These concepts work together in industrial automation systems.
Continue Learning
Ready to deepen your understanding of Scan Cycle? Here are some recommended resources:
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Quick Info
- Category
- PLC Fundamentals
- Difficulty
- Beginner
- Tier
- Important
About PLC Fundamentals
Core concepts and hardware components of programmable logic controllers