PLC Scan Time Calculator

Estimate the total scan time of a PLC program and the maximum reliable input frequency (Nyquist of the scan) from instruction count, time per instruction and system overhead.

Number of instructions

Time per instruction (µs)

System overhead (ms)

How to Use the PLC Scan Time Calculator

Enter the number of instructions in the program.
Enter the time per instruction (default 0.1 µs).
Enter the system overhead in ms.
Click Calculate.

Casi d'Uso

•Sizing control loops.
•Estimating digital input polling limits.
•Comparing PLC CPU performance.

Formula

T_scan = N_instructions · t_per_instruction + T_overhead. f_max_input ≈ 1 / (2 · T_scan).

Domande Frequenti

What is a typical time per instruction?

Modern PLCs range from 0.01 µs (Siemens S7-1500) to ~0.3 µs (S7-1200, AB CompactLogix) per boolean instruction. Floating point takes several times longer.

Why divide by 2?

To catch an input change reliably you need to scan at least twice per event — the Nyquist criterion applied to the PLC cycle.