System and Operating Modes of the S7-400H
5-3
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Synchronizing the subsystems
The master and standby CPUs are linked by means of fiber-optic cables. The two
CPUs maintain event-driven synchronous program scanning over this link.
Subsystem
(CPU0)
Synchronization
Subsystem
(CPU1)
Figure 5-1 Synchronizing the subsystems
Synchronization is performed automatically by the operating system and has no
effect on the user program. You create your program in the manner in which you
are accustomed for standard CPUs on the S7-400.
Event-driven synchronization procedure
The “event-driven synchronization” procedure patented by Siemens has been used
on the S7-400H. This procedure has proved itself in practice and has already been
used for the S5-115H and S5-155H PLCs.
Event-driven synchronization means that data synchronization between the master
and the standby is performed for any event which may result in a different internal
mode of the subsystems.
The master and standby CPUs are synchronized upon:
• direct access to the I/O
• interrupts
• updating of user times – for example, S7 timers
• modification of data by communication functions
Continued operation – free from discontinuities – even in the event of loss of
redundancy of a CPU
The event-driven synchronization procedure ensures continued operation at all
times, and free from discontinuities, by the standby CPU even in the event of a
master CPU failure.
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