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The primary CPU switches from the RUN-Redundant operating state to RUN (1)
or
● Primary-backup switchover: The backup CPU becomes the primary CPU and switches
from the RUN-Redundant operating state to RUN (2).
The primary CPU continues to execute the user program in the RUN operating state and
exchanges process data with the IO devices.
Causes of redundancy loss
The redundant system switches from the RUN-Redundant system state to RUN-Solo if:
● You POWER OFF one of the CPUs.
● You set one of the two CPUs to STOP with the PG/PC, the display or the mode selector.
● A CPU detects an error that prevents continued processing, for example:
– Failure of a CPU, for example as a result of a hardware defect.
– Cycle time exceededInitial situation: The R/H redundant system is in the RUN-Redundant system state.
The backup CPU fails because of a hardware defect.
① The CPU switches from RUNRedundant
to RUN. The CPU behaves
like a standard CPU and
continues to process the user program.
RUN-Redundant → RUN-Solo Following the error, the CPU switches
from the RUN-Redundant operating state
to STOP or POWER OFF.
Error elimination procedure for redundant operation
1. Eliminate the error.
2. Switch the backup CPU to the RUN operating state.
The backup CPU switches from the STOP operating state to the SYNCUP operating state.
Synchronization starts as detailed in the section SYNCUP system state (Eliminate the error.
2. Switch CPU 1 to the RUN operating state.
CPU 1 becomes the backup CPU and switches from the STOP operating state to the
SYNCUP operating state.
Synchronization starts as detailed in the section SYNCUP system state (PageFor commissioning and service, you require information on the system state of the redundant
system. Examples:
● The redundant system does not switch to the RUN-Redundant system state upon initial
commissioning.
● The primary CPU has failed due to a fault.
Options
You have the following options for displaying and changing the system state of the -
R/H redundant system:
● In STEP 7, for example when the R/H-CPUs are far apart
● Using the mode selectors on the CPUs, you can change the operating states of the CPUs
and therefore the system state
Display of the primary and backup CPU

Displaying the operating state:
The operating state of the primary and backup CPU is shown in the status information in the
top section of the displays. The two operating states define the system state.
Examples:
● Status information for the primary and backup CPUs: RUN-Redundant, RUN-Redundant.
Result: The system state is RUN-Redundant.
● Status information for the primary and backup CPUs: RUN, STOP. Result: The system
state is RUN-Solo.
Changing the operating state:
Switch the CPU to the required operating state in the "Settings > RUN / STOP" menu of the
display.
Note
Please note that you can only implement the system states RUN-Redundant and STOP
through the displays by switching both CPUs to the operating state RUN or STOP .
SIMATIC Display Simulator
A simulation of the display of the available menu commands is available in the
SIMATIC Display SimulatorThe system state is displayed on the R/H system operating panel (Online & diagnostics).
Changing the system state:
On the R/H system control panel (Online & diagnostics):
● STOP system state: Press the STOP R/H-System button.Please note that you cannot switch the R/H system to the RUN-Redundant
system state over the R/H system control panel. You implement the RUN-Redundant
system state by switching each CPU on its control panel to RUN.Memory resets can be performed for the primary and for the backup CPU. Memory resets
are generally only useful for the primary CPU.
Reason: Following a primary CPU memory reset, you need to trigger synchronization for
redundant operation. In SYNCUP, the backup CPU is synchronized with the retentive data
from the primary CPU. Following SYNCUP, the backup CPU processes the same user
program as the primary CPU.
The memory reset process for R/H-CPUs is identical to that for the standard CPUs.
The CPU must be in the STOP operating state for a memory reset.
A memory reset returns the CPU to its "initial state