Skip to content
ENExplore Calculators

PLC Scan Time Calculator

Work out plc scan time instantly with clear inputs, formula shown and shareable results.

About PLC Scan Time Calculator

Scan time is logic execution plus IO update plus housekeeping overhead. Worst-case response to an input is about two scans, because an input arriving just after the input scan waits a full cycle to be read and then another to be acted on — which is why fast events need interrupt inputs.

Formula

PLC scan

scan = (instructions/1000) x speed + modules x IO update time + overhead; worst-case response = 2 x scan

Worked example

With the default values below, PLC Scan Time Calculator returns Total scan time of 1.74. Change any input above and every figure here is recalculated in your browser.

Inputs used

Program instructions
4,200 instructions
Execution speed per 1000 instructions
0.08 ms
IO modules to update
8 modules
Update time per module
0.15 ms

Results

Total scan time
1.74 ms
Logic execution time
0.34 ms
IO update time
1.20 ms
Worst-case response time
3.47 ms

Step by step

  1. 1Values used
  2. 2PLC scan
  3. 3Total scan time= 1.74 ms
  4. 4Logic execution time= 0.34 ms
  5. 5IO update time= 1.20 ms
  6. 6Worst-case response time= 3.47 ms

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is worst-case response two scans?

An input changing immediately after the input read is not seen until the next scan, and the output resulting from it is not written until the end of that scan.

How do I catch a fast pulse?

Use a high-speed counter or interrupt input, or a latching input module. Normal scanned IO cannot reliably see a pulse shorter than one scan.

Related Calculators

PLC Scan Time Calculator – Calculate Total scan time