Siemens SIMATIC S7-400 Configuration And Use Manual page 30

CPUs of the S7-400
Linking and Synchronizing
There are two methods for linking and synchronizing:
• In a "normal" link and synchronization procedure, the fault-tolerant system
should change from single mode to redundant system mode. The two CPUs
then process the same program synchronously.
• In the case of a link and synchronization with master/standby switch-over, the
second CPU with modified components may take over process control. Either
the hardware configuration or the memory configuration can be modified.
In order to return to redundant system mode, a "normal" link and
synchronization must be subsequently performed.
Programming an H System
When you configure and program a fault-tolerant automation system with H CPUs,
certain differences to the standard S7-400 CPUs become apparent. On the one
hand, compared to a standard S7-400 CPU, a fault-tolerant CPU has additional
functions, while on the other hand an H CPU does not support certain other
functions. This has to be taken in account particularly if you wish to run a program
that was created for a standard S7-400 CPU on an H CPU.
Fault-tolerant Connections
The controller and the I/O can be redundantly configured for a fault-tolerant
controller. With growing demands on the availability of an overall system, it is
necessary to increase the fault tolerance of the communication. To do this,
configure redundant communication and use fault-tolerant S7 connections.
Unlike the S7 connection, a fault-tolerant S7 connection consists of at least two
lower-level partial connections. From the point of view of the user program, the
configuration and the connection diagnostics, the fault-tolerant S7 connection with
its subordinate partial connections is represented by exactly one ID (like a
standard S7 connection). Depending on the configuration, it can consist of up to
four partial connections, of which two are always established (active). This
maintains communications in the event of an error. The number of partial
connections depends on possible alternative paths and is determined
automatically.
If the active partial connection fails, a previously established second partial
connection assumes responsibility for communications.
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Automation System S7-400 Configuration and Use
A5E00442711-02