Siemens Simatic PCS 7 Function Manual page 41

The following figure illustrates this configuration. Signals from redundant sensors can be
registered and IO-modules can drive actors.
Availability
The block diagram shows an example configuration with Redundant I/O in ET 200SP HA
without a fault.
If a fault occurs in an input module (SM) the system remains operable. The connected device
continues to supply data to the central device, which remains available. If any other component
in the redundancy chain fails, however, the complete system will fail.
High Availability Process Control Systems (V9.0)
Function Manual, 05/2017, A5E39221836-AA
High availability solutions in PCS 7
4.1 Solutions for the I/O
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