Siemens SIMATIC S7-400H System Manual page 124

Using I/Os in S7-400H
10.4 Using single-channel switched I/Os
The single-channel switched I/O configuration is recommended for system components
which tolerate the failure of individual modules within the ET 200M.
Figure 10-1
Rule
A single-channel, switched I/O configuration must always be symmetrical, in other words:
● The H CPU and other DP masters must be installed in the same slots in both subsystems
● The DP masters must be connected to the same integrated interface in both subsystems
Single-channel, switched I/O and the user program
In redundant mode, any subsystem can, in principle, access single-channel switched I/O.
The data is automatically transferred via the synchronization link and compared. An identical
value is available to the two subsystems at all times owing to the synchronized access.
The fault-tolerant system uses only one of the interfaces at any given time. The active
interface is indicated by the ACT LED on the corresponding IM 153-2 or IM 157.
The path via the currently active interface (IM 153-2 or IM 157) is called the active channel
and the path via the other interface as the passive channel. The DP cycle is always active on
both channels. However, only the input and output values of the active channel are
processed in the user program or output to the I/O. The same applies to asynchronous
activities, such as interrupt processing and the exchange of data records.
124
Single-channel, switched ET 200M distributed I/O
(for example, slot 4 on both subsystems), or
(for example, to the PROFIBUS DP interfaces of both H CPUs).
System Manual, 09/2007, A5E00267695-03
S7-400H