Siemens SIMATIC S7 System Manual page 73

The following figure depicts the flow of a typical scan including the Local memory usage and
two interrupt events, one during the program-execution phase and another during the
communications phase of the scan cycle. Subroutines are called by the next higher level,
and are executed when called. Interrupt routines are not called; they are a result of an
occurrence of the associated interrupt event.
Figure 4-1
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Typical scan flow
PLC concepts
4.1 Execution of the control logic
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