Signals and PLC · Glossary
What is PLC Scan Time?
milliseconds per cycleSignals and PLC
PLC scan time is the time a PLC takes to read inputs, execute the program and update outputs once.
Advertisement
PLC Scan Time Explained
The PLC repeats this cycle continuously. Longer programs and communication load increase scan time.
Worst case response to an input change is about two scans plus input filter and output delays, which matters for fast events and safety functions.
PLC Scan Time Formula
Response ≈ tinput + 2 × tscan + toutput
| tinput | input filter delay |
| tscan | scan time |
| toutput | output delay |
Example: 3 ms input, 20 ms scan, 10 ms relay output: about 53 ms.
Advertisement
Calculators for PLC Scan Time
Free Downloads
Frequently Asked Questions
What is PLC Scan Time?
PLC scan time is the time a PLC takes to read inputs, execute the program and update outputs once.
How do I catch a pulse shorter than the scan?
Use a high speed counter input or an interrupt.
Is a watchdog related to scan time?
Yes, the watchdog faults the PLC if a scan takes too long.
You May Also Like
Learn more on the blog
Why 24 volts is used in PLC systems
A practical guide on instrumentationblog.in with more detail on PLC scan time and how it is applied on real plants.
Read Full ArticleRelated Terms
Signal termSignal DampingSignal damping is a filter applied in a transmitter or PLC that smooths a noisy measurement by slowing…Signal termPulse OutputA pulse output sends one pulse per fixed quantity of flow, or a frequency proportional to flow, for…Signal termPLC RedundancyPLC redundancy uses duplicate controllers, power supplies or networks so the system keeps running if one…
More Signals and PLC Terms
Signal termSinking and SourcingSinking and sourcing describe the direction of current in digital I/O: a sourcing device supplies…Signal termNAMUR SensorA NAMUR sensor is a two wire proximity sensor that signals by changing its current, used with isolating…Signal termDry ContactA dry contact is a switch or relay contact with no voltage of its own; the receiving circuit supplies the…Signal termInterposing RelayAn interposing relay is a relay driven by a PLC output that switches a load the PLC cannot drive directly.Signal termSignal IsolatorA signal isolator copies an analog signal from input to output with galvanic isolation, often converting…Signal termEncoderAn encoder converts shaft rotation into pulses or a digital position, used to measure speed and position.
All signals and plc terms · Full glossary A to Z
Advertisement