Siemens SIMATIC PCS 7 Function Manual page 58

Fault-tolerant Solutions in PCS 7
3.3 Solutions for communication
The following automation systems can be used:
● AS 41xH
Availability - ring structure
In this system, one CP 443-1 may fail in each subsystem of the AS without this affecting the
complete system.
The plant bus indicated by the asterisk ( * ) is set up with switches so that it is fault tolerant
and will tolerate a break on the bus cable at any point. One of the two switches to which the
OS servers are connected may fail without this affecting the complete system. If one switch
fails, the redundant OS partner server can continue to communicate using the operational
switch. The same scenario applies to the switches that each have a CP of a subsystem of the
H system connected.
To guard against the failure of all switches, however, the redundant double ring described in
the following section can be used.
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Fault-tolerant Process Control Systems (V8.0)
Function Manual, 03/2012, A5E02779471-02