Whether you’re testing a PLC program with an actual PLC or with a simulator, by default the CPU executes the program in continuous scan mode. However, the PLC simulator in STEP 7 is capable of executing the program in single scan mode as well.
Now, you may be asking, “What is the difference between continuous and single scan modes?” When in continuous scan mode, the CPU repeatedly executes the program from top to bottom as soon as it’s placed in RUN mode. Program execution stops only when the CPU is returned to STOP mode.
When in single scan mode, the CPU executes the program from top to bottom only one time, and then awaits your command to execute the program once more.
In this lesson, I’m going to test the program that I previously wrote for extended timing in single scan mode.