Siemens SIMATIC S7-1500 System Manual page 265

In general, the two CPUs have equal priority; each CPU can be either primary or backup.
Figure 11-6 System and operating states
The following table provides you with an overview of how the redundant system starts and at
the same time runs through the various operating modes and system states. The following
initial situation and steps are an example.
The operating and system states are described in detail in the following sections.
Table 11-3 Redundant system startup
No. in dia­ Primary CPU
gram
Initial situation: Both CPUs are in STOP operating state. The mode selectors are also in the STOP position.
Step 1: Switch the mode selector of the CPU that is to be primary CPU from STOP to RUN.
The CPU switches to STARTUP and
①
executes startup OB 100 and other
available startup OBs.
Following successful STARTUP, the
②
CPU switches to RUN.
The CPU runs like a standard CPU
in RUN and executes the user pro­
gram.
Step 2: Switch the mode selector on the backup CPU from STOP to RUN.
S7-1500R/H redundant system
System Manual, 11/2022, A5E41814787-AD
System state
STOP → STARTUP
STARTUP → RUN-Solo
Commissioning
11.4 Operating and system states
Backup CPU
The CPU remains in STOP mode.
The CPU remains in STOP mode.
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