What is PLC scan and it’s working

What is program scan and Scan time?

A PLC operates by executing programs in PLC memory repeatedly. The instructions contained in the program are executed sequentially from beginning to end and continuously, and when the last instruction is executed, the operation will return and starting from the initial instructions, this process will continue as long as the PLC is still running.

The above action is called the program scan. And the decoder of a loop is called a program scan time. As for each scan period the length of time depends on the number of PLC programs and the processor speed of the PLC, but in general, it is 2-5 mS for each kilo program, overall the average scan time takes 10 – 50 mS.

How process scanning is done in PLC?


If the PLC operates where the program is scanned, the output status will not change immediately. A memory area in the PLC associated with the output is changed by the program, then the output status is simultaneously updated at the end of the scan process. In brief, the action can be explained that the process of scanning is reading input, scanning programs and improving output.

This program scan will limit the signal speed where a PLC can respond to the signal. Figure (a) is an illustration where the PLC will be used to calculate high-speed pulses where the pulse speed is lower than the scan speed, here it will be found that the PLC will calculate the pulses correctly. Figure (b) the speed of the pulse is higher than the speed of the scan, here it will be seen that an error occurs while calculating the number of pulses.


The figure (a) shows that the input occurs (status change) just before the scan starts, this will result in output energization occurring in the next scan period. While in Figure (b) shows the input occurs (status change) after the input is read, so that at that time the read input status is OFF and the new ON status will be read on the second scan before the output is ON (energized). Thus it can be concluded that the response can occur or be given to varying between one or two scan periods.

In most applications shifting (time delay) equipment operations takes place in tens of milli-seconds, and this will not be too problematic if the PLC responds to a plan that is operated using a pushbutton. Problems will arise if the input given to the PLC is in the form of input equipment with fast action like a photocell.

The layout of the PLC program itself can produce unwanted time delays if the program flow is not in accordance with the PLC program scan path.

The PLC works from the first instruction for each scan, then sequentially continues on the next instruction until the end of the program.


Instrumentation Engineer

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