LG Electronics 120S user manual Operation Processing Method, Cyclic operation

Chapter 5 Power Supply / CPU

5.3Operation Processing

5.3.1Operation Processing Method

1) Cyclic operation

A PLC program is sequentially executed from the first step to the last step, which is called scan. This sequential processing is called cyclic operation. Cyclic operation of the PLC continues as long as conditions do not change for interrupt processing during program execution. This processing is classified into the following stages:

Stages

Operation Start

Initialization

Input image area refresh

Program operation processing

Program starts

~

Program ends

Output image area refresh

END processing

Processing

-

•Stage for the start of a scan processing. it is executed only one time when the power is applied or reset is executed. It executes the following processing..

▶I/O reset

▶Execution of self-diagnosis

▶Data clear

▶ Allocating I/O address and type

Input conditions are read and stored into the input image area before starts processing.

Program is sequentially executed from the first step to the last step Program operation processing

The contents stored in the output image area is output to output part when operation processing of a program is finished.

Stage for return processing after the CPU part has finished 1 scan.

The END processing following processing is executed.

▶Self-diagnosis

▶Change the present values of timer and counter, etc.

▶Processing data communications between computer link module and communications module.

▶Checking the switch for mode setting.

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