Siemens SIMATIC S7-1500 System Manual page 175

Application planning
6.4 Redundancy scenarios
Sequence of events
1. The primary CPU of the redundant system fails in the RUN-Redundant system state.
2. The redundant system switches to the backup CPU. The backup CPU becomes the new
primary CPU. The redundant system switches to the RUN-Solo system state. You can find
additional information on the RUN-Solo system state in the section Operating and system
states (Page 380).
3. The new primary CPU exchanges process data with the IO devices.
Note
Temporary separation of standard IO devices in the event of failure of the primary
CPU
If the primary CPU fails, the standard IO devices ("switched S1 devices") are temporarily
separated from the S7-1500R/H redundant system. During this time, the configured
substitute value behavior applies to the modules of the standard IO devices.
The new primary CPU establishes the connections to the standard IO devices again. After a
short time, the primary CPU resumes exchanging process data with the standard IO
devices.
4. The redundancy of the system is restricted. The restriction has no impact on the process.
If another system component or another network segment fails, this can result in the failure
of the S7-1500R/H redundant system. You can find additional information in the section
Failure scenarios (Page 198).
Diagnostics
System state, operating states and error displays after primary-backup switchover:
• Redundant system → RUN-Solo system state
• Primary CPU (previously backup CPU) → RUN operating state
– MAINT LED → yellow light: The R/H system is not in the RUN-Redundant system state.
• Backup CPU (previously primary CPU) → failed
Note
To get detailed diagnostics information, evaluate the diagnostics buffer.
Solution
Replace the defective CPU. You can find additional information on the procedure in the
section Replacing defective R/H CPUs (Page 446).
174
No partner CPU has been found in the redundant system. The PROFINET ring is open.
S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF