Calculator
Example data table
These sample inputs demonstrate a typical mid-size program with modest communications.
| Scenario | Instructions | µs / Instr | I/O (ms) | Comms (ms) | Housekeeping (ms) | Other (ms) | ISR Budget (ms) | Margin (%) |
|---|---|---|---|---|---|---|---|---|
| Packaging line | 25,000 | 2.0 | 1.5 | 0.8 | 0.7 | 0.5 | 0.4 | 15 |
| High-speed indexing | 18,000 | 1.2 | 1.0 | 0.5 | 0.6 | 0.3 | 0.6 | 10 |
| Process batching | 55,000 | 2.5 | 2.0 | 1.6 | 1.0 | 1.2 | 0.7 | 20 |
Formula used
The estimate models a single PLC scan as the sum of major time components, then applies a safety margin:
- Logic execution (ms) =
InstructionCount × AvgInstructionTime(µs) ÷ 1000 - Base scan (ms) =
Logic + I/O + Comms + Housekeeping + OtherTasks + InterruptBudget - Estimated scan (ms) =
BaseScan × (1 + Margin% ÷ 100) - Scan frequency (Hz) =
1000 ÷ EstimatedScan(ms)
Note: real PLCs may schedule comms and background tasks differently. Use this calculator as a planning and comparison tool, then confirm by measuring scan time on your controller.
How to use this calculator
- Enter instruction count and an average execution time per instruction.
- Add typical I/O update and communication overhead for your setup.
- Include housekeeping, other cyclic tasks, and a worst-case interrupt budget.
- Set a safety margin to cover peaks and scheduling jitter.
- Click Calculate Scan Time to view results above this form.
- Use the CSV/PDF buttons to export the report for documentation.
FAQs
1) What is PLC scan time?
Scan time is the duration for one controller cycle: read inputs, execute logic, handle comms/background work, and update outputs. It impacts response time and control stability.
2) Why add a safety margin?
Real scan time varies due to interrupt bursts, communication spikes, and conditional logic paths. A margin helps you plan for worst-case behavior and avoid watchdog trips.
3) How do I estimate average instruction time?
Use controller documentation, vendor benchmarks, or built-in performance tools. If you can measure scan time and instruction count, you can back-calculate an average value for planning.
4) What counts as communication overhead?
Fieldbus exchanges, HMI polling, messaging, and protocol stacks can consume CPU time. Some platforms distribute this over multiple scans, so use typical or worst-case values.
5) How can I reduce scan time?
Optimize logic, remove unused rungs, simplify math, reduce polling, and move noncritical tasks to slower timed routines. Also consider faster CPUs or distributing I/O and networks.
6) What is interrupt budget and why include it?
Interrupts and high-priority tasks can preempt the main scan. Budgeting their worst-case time helps prevent underestimating scan time during fast I/O, motion, or event-driven activity.
7) What does the scan frequency mean?
Scan frequency is an approximation of cycles per second. It is simply the inverse of scan time. Use it to compare designs, not as a precise scheduling guarantee.
8) Is this result the same as vendor “task period”?
Not always. Some PLCs run multiple tasks with configured periods and priorities. This calculator estimates one cyclic workload. Validate by checking the controller’s measured task times and jitter.