Operating Modes Of The Cpu - Siemens S7-1200 System Manual

4.1.1

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
● In RUN mode, the program cycle OBs execute repeatedly. Interrupt events can occur and
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.
You can configure the "startup after POWER ON" setting of the CPU. This configuration item
appears under the "Device configuration" for the CPU under "Startup". Upon powering up,
the CPU performs a sequence of power-up diagnostic checks and system initialization.
During system initialization the CPU deletes all non-retentive bit (M) memory and resets all
non-retentive DB contents to the initial values from load memory. The CPU retains retentive
bit (M) memory and retentive DB contents and then enters the appropriate operating mode.
Certain detected errors prevent the CPU from entering the RUN mode. The CPU supports
the following configuration choices:
● No restart (stay in STOP mode)
● Warm restart - RUN
● Warm restart - mode prior to POWER OFF
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
process interrupt events during the startup mode.
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 803).
PLC concepts
4.1 Execution of the user program
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