Siemens CPU 410-5H Process Automation System Manual page 82

I/O configuration variants
4.7 Connecting redundant I/O to the PROFIBUS DP interface
The output value is divided by 2, and each of the two modules outputs half. If one of the
modules fails, the failure is detected and the remaining module outputs the full value. As a
result, the surge at the output module in the event of an error is not as high.
Note
The output value drops briefly to half, and after the reaction in the program it is returned to
the proper value. The duration of the output value drop is determined by the following time
intervals:
• Time interval between the initial occurrence of an interrupt and the interrupt report
reaching the CPU.
• Time interval until the next RED_OUT (FB 451) call.
• Time interval until the intact analog output module has doubled the output value.
In the case of passivation or a CPU STOP, redundant analog outputs output an assignable
minimum current of approximately 120-1000 μA per module (or 240-1000 μA for HART
analog output modules), i.e., a total of approximately 240-2000 µA (or 480-2000 μA for
HART analog output modules). Considering the tolerance, this means that the output value
is always positive.
A configured substitute value of 0 mA will produce at least these output values. In a
redundant configuration of analog outputs, the substitute value of the current outputs is
automatically set permanently to "zero current and zero voltage". You can also specify a
configurable compensation current of 0-400 µA for an output range of 4-20 mA.
This means you have the option of matching the minimum/compensation current to the
connected I/O.
To minimize the error of the total current at the summing point in case of one-sided
passivation, the assigned compensation current is subtracted in this case from the current of
the depassivated (i.e., active) channel with a pre-set value of 4 mA (range +-20 µA).
Note
If both channels of a channel pair were passivated (e.g., by OB 85), the respective half of the
current value is still output to both storage locations in the process image of outputs. If one
channel is depassivated, then the full value is output on the available channel. If this is not
required, a substitute value must be written to the lower channels of both modules prior to
executing FB 451 "RED_OUT".
Depassivation of modules
Passivated modules are depassivated by the following events:
● When the fault-tolerant system starts up
● When the fault-tolerant system changes over to "redundant" operating state
● After system modifications during operation
● If you call FC 451 "RED_DEPA" and at least one redundant channel or module is
passivated.
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CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 10/2013, A5E32631667-AA