Siemens Simatic PCS 7 Function Manual page 200

Failure, switchover and reintegration of high availability components
7.1 I/O
Failure scenarios
The following faults may occur in a module:
● Hardware or power failure in the module
● Detected signal interference (e.g. wire break, discrepancy)
● Fault on the assigned bus line to an interface module
The driver blocks detect a disturbance:
● At the input signals:
The disturbed input module or, when channel selectivity is configured, the disturbed channel
is passivated and only the signal of the redundant modules is evaluated. A module or
channel is passivated when the function blocks can no longer access the respective module
or channel.
● At analog output modules:
Only analog output modules with power outputs can be operated redundantly (0 to 20 mA,
4 to 20 mA). The value to be output is halved and each module outputs one half of the
value. If one module fails, the redundant module outputs the entire value.
Discrepancy with input modules
A discrepancy error at the input value occurs when there is a non-tolerated difference between
the input values after the configured discrepancy time has expired. The following parameters
should be set to configure the discrepancy:
● For digital input modules:
– Discrepancy time (maximum allowed time that the redundant input signals can differ)
● For analog input modules:
– Tolerance window (configured by the percent of the end value of the measuring range)
– Discrepancy time (maximum allowed time that the redundant input signals are outside
– Value applied
With discrepancy, information is entered in the diagnostics buffer and a corresponding
message is generated.
Depassivation
Passivated modules or, when channel selectivity is configured, passivated channels are
depassivated with the following events:
● When the H system starts up
● When operating state of the H system changes to "Redundant"
● Following a system modification in runtime
● Following depassivation via the maintenance station
200
Two analog values are the same if they are within the tolerance window.
the tolerance windows)
The value applied is one of the two analog input values that is transferred in the user
program.
High Availability Process Control Systems (V9.0)
Function Manual, 05/2017, A5E39221836-AA