Siemens SIMATIC S7-400H System Manual page 497

Redundancy, functional
Redundancy with which the additional technical means are not only constantly in operation
but also involved in the scheduled function. Synonym: active redundancy.
Redundant
In redundant system mode of a fault-tolerant system the central processing units are in RUN
mode and are synchronized over the redundant link.
Redundant link
A link between the central processing units of a fault-tolerant system for synchronization and
the exchange of data .
Redundant systems
Redundant systems are characterized by the fact that important automation system
components are available more than once (redundant). When a redundant component fails,
processing of the program is not interrupted.
Reserve CPU
The redundant central processing unit of a fault-tolerant system that is linked to the master
CPU. It goes to STOP mode when the redundancy connection is lost. The user program is
processed identically in both the master and reserve CPUs.
Self-test
In the case of fault-tolerant CPUs defined self-tests are executed during startup, cyclical
processing and when comparison errors occur. They check the contents and the state of the
CPUs and the I/Os.
Single mode
An H system changes to single mode, when it was configured to be redundant and only one
CPU is in RUN. This CPU is then automatically the master CPU.
Stand-alone operation
Stand-alone mode is the use of a fault-tolerant CPU in a standard SIMATIC-400 station.
Stop
With fault-tolerant systems: In the stop system mode of a fault-tolerant system, the central
processing units of the fault-tolerant system are in STOP mode.
S7-400H
System Manual, 03/2012, A5E00267695-11
Glossary
497