About the PLC Scan Time Estimator
What is the PLC Scan Time Estimator?
A PLC repeatedly reads inputs, executes the program and updates outputs. The PLC scan time sets how fast the machine can react. This estimator adds logic execution, I/O update, communication and operating-system overheads.
Enter instruction counts from your program and the instruction times from your PLC's manual.
How to use the PLC Scan Time Estimator
- Bit Instructions (contacts, coils) (sample: 2,000)
- Word / Maths Instructions (sample: 400)
- Time per Bit Instruction (µs) (sample: 0.085)
- Time per Word Instruction (µs) (sample: 1.7)
- I/O Points (sample: 138)
- I/O Update Time per Point (µs) (sample: 2)
- Communication Overhead (ms) (sample: 2)
- Operating System Overhead (ms) (sample: 1)
- Input Filter Delay (ms) (sample: 6.4)
- Press Estimate Scan Time (results also refresh as you type). The result card shows the Estimated Scan Time, along with Logic execution, I/O update, Scans per second and Worst-case input-to-output response.
Formula used
Scan time = Bit instructions × t_bit + Word instructions × t_word + I/O points × t_io + Comms + OS overhead
- Worst-case response = 2 × scan time + input filter delay
Default instruction times resemble a compact modern PLC; older or smaller CPUs are much slower.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Bit Instructions (contacts, coils) | 2,000 |
| Word / Maths Instructions | 400 |
| Time per Bit Instruction (µs) | 0.085 |
| Time per Word Instruction (µs) | 1.7 |
| I/O Points | 138 |
| I/O Update Time per Point (µs) | 2 |
| Communication Overhead (ms) | 2 |
| Operating System Overhead (ms) | 1 |
| Result | Value |
|---|---|
| Estimated Scan Time | 4.13 ms |
| Logic execution | 0.85 ms |
| I/O update | 0.276 ms |
| Scans per second | 242 |
| Worst-case input-to-output response | 14.65 ms |
| Tip | Check the instruction timings in your PLC's manual; they vary widely between models |
Tips for using the PLC Scan Time Estimator
- Move rarely needed logic to timed or conditional blocks to shorten the main scan.
- Use hardware interrupts or high-speed counters for signals faster than the scan.