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

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4.2 Operating modes of the CPU

This process provides consistent logic through the execution of the user instructions for a
given cycle and prevents the flickering of physical output points that might change state
multiple times in the process image output area.
You can change the default behavior for a module by removing it from this automatic update
of I/O. You can also immediately read and write digital and analog I/O values to the modules
when an instruction executes. Immediate reads of physical inputs do not update the process
image input area. Immediate writes to physical outputs update both the process image
output area and the physical output point.
4.2 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, and you can download a project.
● In STARTUP mode, the CPU executes any startup logic (if present). Interrupt events are
● In RUN mode, the scan cycle is executed repeatedly. Interrupt events can occur and be
Note
You cannot download a project while the CPU is in RUN mode. You can download your
project only when the CPU is in STOP mode.
The CPU supports the warm restart method for entering the RUN mode. Warm restart does
not include a memory reset, but a memory reset can be commanded from the programming
software. A memory reset clears all work memory, clears retentive and non-retentive
memory areas, and copies load memory to work memory. A memory reset does not clear the
diagnostics buffer or the permanently saved IP address. All non-retentive system and user
data are initialized at warm restart.
You can specify the power-up mode of the CPU complete with restart method using the
programming software. This configuration item appears under the Device Configuration for
the CPU under Startup. When power is applied, the CPU performs a sequence of power-up
diagnostic checks and system initialization. The CPU then enters the appropriate power-up
mode. Certain detected errors will prevent the CPU from entering the RUN mode. The CPU
supports the following power-up modes: STOP mode, "Go to RUN mode after warm restart",
and "Go to previous mode after warm restart".
You can also include a STP instruction in your program to change the CPU to STOP mode.
This allows you to stop the execution of your program based on the program logic.
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not processed during the startup mode.
processed at any point within the program cycle phase.
The CPU does not provide a physical switch for changing the
operating mode. Use the CPU operator panel in the online tools of
STEP 7 Basic to change the operating mode (STOP or RUN).
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Manual, 11/2009, A5E02486774-01