Siemens Simatic S7-1500R System Manual page 200

Overview of system and operating states
The figure below shows the possible operating states of the CPUs and the resulting system
states.
In general, the two CPUs have equal priority; each CPU can be either primary or backup.
The CPU that you switch from STOP to RUN first becomes the primary CPU.
Figure 9-6 System and operating states
S7-1500R/H redundant system
System Manual, 10/2018, A5E41814787-AA
Commissioning
9.4 Operating and system states
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