Operating Modes Of The Cpu - Siemens SIMATIC S7 System Manual

Note
The CPU always performs a restart after a download
Whenever you download an element of your project (such as a program block, data block, or
hardware configuration), the CPU performs a restart on the next transition to RUN mode. In
addition to clearing the inputs, initializing the outputs and initializing the non-retentive
memory, the restart also initializes the retentive memory areas.
After the restart that follows a download, all subsequent STOP-to-RUN transitions perform a
warm restart (that does not initialize the retentive memory).
3.1.1

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, and you can download a project.
● In STARTUP mode, the startup OBs (if present) are executed once. Interrupt events are
● In RUN mode, the scan cycle is executed repeatedly. Interrupt events can occur and be
The CPU supports a warm restart for entering the RUN mode. Warm restart does not include
a memory reset. All non-retentive system and user data are initialized at warm restart.
Retentive user data is retained.
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 values of the IP address.
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
● Go to previous mode after warm restart
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
not processed during the startup phase of RUN mode.
processed at any point within the program cycle phase.
You cannot download a project while in RUN mode.
PLC concepts
3.1 Execution of the user program
39