Figure 8-1 Synchronizing The Subsystems - Siemens SIMATIC S7-400H System Manual

Synchronizing the subsystems
The master and standby CPUs are linked by fiber-optic cables. The redundant CPUs
maintain event-driven synchronous program execution via this coupling.
Figure 8-1
Synchronization is performed automatically by the operating system and has no effect on the
user program. You create your program in the same way as for standard S7-400 CPUs.
Event-driven synchronization
The "event-driven synchronization" procedure patented by Siemens was used for the S7-400H.
This procedure has proved itself in practice and has already been used for the S5-115H and
S5-155H controllers.
Event-driven synchronization means that the master and standby always synchronize their
data when an event occurs which may lead to different internal states of the subsystems.
The master and standby CPUs are synchronized when:
● There is direct access to the I/O
● Interrupts occur
● User timers - for example, S7 timers are updated
● Data is modified by communication functions
Continued bumpless operation if CPU redundancy is lost
The event-driven synchronization method ensures bumpless continuation of operation by the
standby CPU even if the master CPU fails.
Self-test
Malfunctions or errors must be detected, localized and reported as quickly as possible.
Consequently, extensive self-test functions have been implemented in the S7-400H that run
automatically and entirely in the background.
The following components and functions are tested:
● Linking of the central modules
● Processor
● Internal memory of the CPU
● I/O bus
If the self-test detects an error, the fault-tolerant system tries to eliminate it or to suppress its
effects.
For detailed information on the self-test, refer to section Self-test (Page 89).
S7-400H
System Manual, 09/2007, A5E00267695-03
Synchronizing the subsystems
System and operating states of the S7-400H
8.2 Introduction
81