Basis of a PLC function is continual scanning of a program. Under scanning we mean running through all conditions within a guaranteed period. Scanning process has three basic steps:
Step 1. Testing input status.
First, a PLC checks each of the inputs with intention to see which one of them has status ON or OFF. In other words, it checks whether a sensor, or a switch etc. connected with an input is activated or not. Information that processor thus obtains through this step is stored in memory in order to be used in the following step.
Step 2. Program execution. Here a PLC executes a program, instruction by instruction.
Based on a program and based on the status of that input as obtained in the preceding step, an appropriate action is taken. This reaction can be defined as activation of a certain output, or results can be put off and stored in memory to be retrieved later in the following step.
Step 3. Checkup and correction of output status. Finally, a PLC checks up output status and adjusts it as needed. Change is performed based on the input status that had been read during the first step, and based on the results of program execution in step two.
Following the execution of step 3 PLC returns to the beginning of this cycle and continually repeats these steps. Scanning time is defined by the time needed to perform these three steps, and sometimes it is an important program feature.