Siemens SIMATIC S7 System Manual page 70

PLC concepts
4.1 Execution of the control logic
Tasks in a scan cycle
The CPU executes a series of tasks repetitively. This cyclical execution of tasks is called the
scan cycle. The execution of the user program is dependent upon whether the CPU is in
STOP mode or in RUN mode. In RUN mode, your program is executed; in STOP mode, your
program is not executed.
Table 4- 1 Tasks performed by the CPU in a scan cycle
Scan cycle
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Description
Reading the inputs: The CPU copies the state of the physi-
cal inputs to the process image input register.
Executing the control logic in the program: The CPU exe-
cutes the instructions of the program and stores the values
in the various memory areas.
Processing any communications requests: The CPU per-
forms any tasks required for communications.
Executing the CPU self-test diagnostics: The CPU ensures
that the firmware, the program memory, and any expansion
modules are working properly.
Writing to the outputs: The values stored in the process
image output register are written to the physical outputs.
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART