Run-time enviroment
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The PLC runtime environment is firmware which provides the operating systemservices and library functions associated with the PLC. In the RUN mode,the PLC firmware runs as real-time executive which processes the (LadderLogic) instructions that have been loaded into the program RAM area. The program runs in a continuous loop which consists of the following major phases:

 

•Input read and output write scan
•Housekeeping
•Program scan (logic solve).