02.02 Execution of Controller software

2.2. Execution of Controller software

All Siemens Simatic Controllers (S7-1500 and S7-1200) are event driven devices, the CPU only ever responds to certain specific events (or interrupts). The CPU responds to a specific event by executing a particular organisation block (OB).

For example, if a CPU is started (either by applying power or switching the device from STOP to RUN) it will execute the start-up organisation block (OB 100). If OB 100 were to call any functions or function blocks, these would also be executed.

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2.2.1 Cyclic programme execution

The principal event for running the PAL software is the main cyclic event interrupt.

The Controller triggers a main cyclic event that cause the Controller to write output data to the output cards, read input information from the input cards and then execute the main program cycle by calling organisation block 1 (OB 1), any user programme, and any blocks (FCs or FBs) configured by the user and called from within OB1 will also be executed.

When the end of OB1 is reached, the Controller retriggers the main cyclic event and the process is repeated indefinitely (see the figure below):


Figure 2.1 — Cyclic event interrupt (OB 1)

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