Scan Cycle of a PLC

Scan cycle of a PLC determines the cycle within which the PLC fetches inputs , runs the inputs and process the outputs. So as a whole we can say that there are 3 basic steps in scanning and each input, program and output scans acts as separate independent functions
1.Fetching the inputs and checking its status: Data from input modules are taken and placed in an area of PLC memory referred as input image area.
2. Execute the PLC program: Data from input image area is applied to corresponding user program and executed.
3. Updating the output status: Executed data from output image area is sent to the output modules.

Any change made in the input status during program or output scan will not be effective until the next scan, similarly changes made in the output status will take place only during output scan.

Program scanning in PLC takes place usually in two formats:

  1. From left to right across each rung defined as Rung Scanning (eg: AB) 
  2. From top to bottom defined as Column Scanning (eg :Schneider )in which the processor looks at the first contact at the top left corner and reads the first column.

From top to bottom, it next reads the second column from top to bottom, and so on. Both methods are appropriate but one should be aware of it as it has impact on whether a coil gets energized or de-energized.