Siemens SIMATIC S7-1500R/H Manual page 12

Principle of operation
One of the two CPUs in the redundant system takes on the role of CPU for process control
(primary CPU). The other CPU takes on the role of the following CPU (backup CPU). The
assigned role of the CPUs can change during operation. Synchronization of all relevant data
between primary CPU and backup CPU ensures fast switching between CPUs in the event
of a primary CPU failure. If the primary CPU fails, the backup CPU retains control of the
process as the new primary CPU at the point of interruption.
The redundancy connections are the PROFINET ring with MRP. The CPUs are
synchronized via a PROFINET ring.
Reference
You can find a detailed description of the operation and design of the CPUs in the system
manual Redundant System S7-1500R/H
(https://support.industry.siemens.com/cs/ww/en/view/109754833).
CPU 1515R-2 PN (6ES7515-2RM00-0AB0)
Manual, 10/2018, A5E42009914-AA
2.1 Configuration and operating principle
Product overview
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