PLC concepts
5.1 Execution of the user program
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
87
Operating modes of the CPU
The CPU has three modes of operation: STOP mode, STARTUP mode, and RUN mode.
Status LEDs on the front of the CPU indicate the current mode of operation.
● In STOP mode, the CPU is not executing the program. You can download a project.
● In STARTUP mode, the startup OBs (if present) execute once. The CPU does not
process interrupt events during the startup mode.
● In RUN mode, the program cycle OBs execute repeatedly. Interrupt events can occur and
the corresponding interrupt event OBs can execute at any point within the RUN mode.
You can download some parts of a project in RUN mode (Page 1335).
The CPU supports a warm restart for entering the RUN mode. Warm restart does not include
a memory reset. The CPU initializes all non-retentive system and user data at warm restart,
and retains the values of all retentive user data.
A memory reset clears all work memory, clears retentive and non-retentive memory areas,
copies load memory to work memory, and sets outputs to the configured "Reaction to CPU
STOP". A memory reset does not clear the diagnostics buffer or the permanently saved
values of the IP address.
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