Siemens SIMATIC S7-400 Configuration And Use Manual page 31

Self-test
Disruptions or errors have to be detected, located and reported as quickly as
possible. Consequently, wide-ranging 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:
• Interconnection of the central controllers
• 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.
Single-channel, One-way I/O
In the single-channel, one-way configuration, there are single input/output modules
(single-channel). The input/output modules are located in just one of the
subsystems and are only addressed by that subsystem.
The single-channel, one-way I/O configuration is recommended for individual
input/output channels for which normal availability of the I/O is sufficient.
Single-channel, Switched I/O
In the single-channel, switched configuration, there are single input/output modules
(single-channel). In redundant mode, they can be addressed by both subsystems.
The single-channel, switched I/O configuration is recommended for devices that
tolerate the failure of individual modules.
The single-channel, switched I/O configuration is possible with the ET 200M und
ET 200 iSP distributed I/O devices or redundant standard slaves, for example.
PROFIBUS PA can be connected to a redundant system using DP/PA Link. A
single-channel DP master system can be connected to a redundant system using a
Y-coupler.
Redundant I/O
I/O modules are considered redundant when there are two of each and they are
configured and operated as redundant pairs. The use of redundant I/O provides
the highest degree of availability since it means that failure of a CPU failure and
failure of a signal module are both tolerated.
The implementation of redundant I/O is supported by the software blocks
integrated in STEP 7.
Redundant Input Modules
With redundant input modules, the measured values or input signals are read by
both modules and the results are compared. When the results are the same or
within a configured tolerance window, they are further processed.
Automation System S7-400 Configuration and Use
A5E00442711-02
CPUs of the S7-400
2-15