What is Scan Time of PLC?

PLC scan time is the time taken by the PLC to scan the inputs, execute the operation logic, and generate the outputs for one cycle. This process occurs in a continuous loop. Higher the number of steps in a PLC program, the more time it will require to complete each scan. PLC scanning works from left to right and top to bottom in the program’s logic.

“For high accuracy and fast machines, a low scan time is required. Typically, PLC scan times range from 0 to 500ms, with a scan time below 100ms considered good. Higher-end PLCs generally have lower scan times.

lower scan time is indicative of a better-performing PLC, while a higher scan time suggests a less efficient or worse-performing PLC.

Instructions become more complex, their execution takes longer, leading to an increase in the scan time of a PLC.

Steps of Scan Time...

  1. When the PLC is powered ON, it first performs a diagnostic check on module configuration and parameter settings. It verifies that all hardware modules are connected and correctly configured in the program.
  2. After the diagnostic check, the PLC scans all hardware inputs to check their status. Based on the program logic, it then executes the program and updates the output status in the output map of the PLC.
  3. After that, the PLC performs communication-related tasks.
  4. And it performs another diagnostic check.

If PLC In fault state (Diagnostic function of PLC)

  • It puts the outputs into a safe state, turning all of them off.
  • And the PLC waits for the fault to clear.

PLC scanning is performed in four steps.


  • The application program is executed based on the status of inputs.


  • In the Input Scan, the PLC reads the actual inputs and transfers the image to the input image for processing.


  • The PLC reads the output processed by the CPU and updates the actual output.


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