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PLC Scan Time Calculator
PLC scan time is the time a controller takes to read inputs, execute the program, update outputs and handle housekeeping, then repeat. Enter your program size, average instruction time, I/O update times and overhead to estimate scan time, scans per second, worst-case response time and the shortest input pulse the PLC can reliably detect.
HOW IT IS CALCULATED
The formulas
- Program execution time = instructions × average time per instruction ÷ 1,000 (µs to ms)
- Scan time = input update + program execution + output update + overhead
- Scan rate = 1,000 ÷ scan time (ms)
- Worst-case response ≈ 2 × scan time (an input changes just after it was read, is processed on the next scan, and reaches the output at the end of that scan)
- Shortest reliably detected input pulse ≈ 1 × scan time, unless a fast interrupt or high-speed input is used
Worked example
With the default values (5,000 instructions at 0.2 µs, 0.5 ms input update, 0.5 ms output update, 1 ms overhead) program execution takes 1 ms and the scan time is 3 ms, or about 333 scans per second. Worst-case response is about 6 ms.
USING THE RESULT
What to do with the estimate
Scan time matters most when a control loop, a fast pulse or a safety-related interlock has a response requirement. If the estimate is too slow, reduce program size, move fast tasks into periodic or interrupt tasks, use hardware high-speed inputs, or choose a faster CPU. Always confirm the final figure on the real controller.
COMMON QUESTIONS
Frequently asked questions
What is a PLC scan cycle?
A PLC scan cycle is the repeating sequence of reading inputs, executing the control program, updating outputs and performing communications and housekeeping tasks.
What is a typical PLC scan time?
Small programs on modern controllers can scan in a few milliseconds or less, while large programs with heavy communications can take tens of milliseconds. The right value depends on the process; check the controller’s manual and diagnostics.
Why is the worst-case response about two scans?
If an input changes immediately after it has been sampled, the change is seen at the start of the next scan and reaches the outputs at the end of that scan, so the response can approach two scan times before adding input and output hardware delays.
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