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PLC Scan Cycle

A plc executes an initialization step when placed in run mode then repeatedly executes scan cycle sequence. Basic PLC has three steps.

1.       An input scan

2.       A user program scan

3.       An output scan.

1.       Input Scan :

During this process data is taken from all input modules in system and placed into an area of PLC memory referred to as input image area.

In this scan, input terminals are read and the input image area is updated accordingly.

2.       User program Scan:

During program scan, Data in input image area is applied to user program, the user program is executed and output image area is updated.

In this scan,

·         Data in input image area is applied to user program

·         The program executed

·         The output image area updated

3.       Output Scan:

During the output scan, data is taken from the output image area and sent to all output modules in the system.

In this scan data associated with output image area is transferred to output terminals.

We should understand that input, program, and output scans are separate and independent functions. Any changes in status of input devices during the program or output scan are not recognized until the next input.

Program (Ladder logic) scanning typically takes place left to right across each rung and from the top to bottom rungs, in order known as rung scanning. This method used in plc like Allen Bradley.

Another method used by Modicon/Schneider Automation is known as column scanning.

In this processor looks at first contact at Top left corner and reads the first column from top to bottom. Next reads the second column from top to bottom and so on.

We use rung or column method according to PLC we use.

Rung Scanning:

Column Scanning:

For example we have simple operation below:

Two rung ladder logic program:

Program in logix pro simulator:

Program with energized input ( Here after first rung input and output process it automatically selects another rung from left side.)