PLC Scan and Response Time Calculator
Estimate how long a PLC takes to react: from a field input changing to the output switching. The result combines input filter delay, scan time and output module delay for typical and worst cases.
- Typical = input filter + 1 scan + output delay = 24 ms
- Worst = input filter + 2 scans + output delay = 44 ms
- Minimum pulse width = input filter + 1 scan = 23 ms
Response Time Formulas
Worst case = tfilter + 2 × tscan + tout
Minimum detectable pulse = tfilter + tscan
The worst case occurs when the input changes just after the PLC has read its inputs: the change waits almost one full scan to be read, then a second scan to be executed and written to the outputs. Module delays come from the I/O datasheet; network I/O adds its update time.
Worked example
With a 20 ms scan, 3 ms input filter and 1 ms output delay, the PLC typically reacts in 24 ms and in the worst case in 44 ms. A 15 ms relay brings the total to 59 ms.
Frequently Asked Questions
Why can the response take two scans?
Inputs are read at the start of the scan and outputs written at the end. A change just after the input read waits for the next scan.
How do I catch very short pulses?
Use a high speed counter, an interrupt (event) input or a module with pulse catch, or lengthen the pulse in the field device.
What is the input filter?
A debounce delay in the input module that rejects noise and contact bounce. Many modules let you set it from about 0.1 ms to 20 ms.
Learn more on the blog
Why 24 volts is used in PLC systems
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article