Siemens SIMATIC S7-400H System Manual page 204

Using I/Os in S7–400H
13.4 Connecting redundant I/O to the PROFIBUS DP interface
Analog output signals
Only analog output modules with current outputs (0 to 20 mA, 4 to 20 mA) can be operated
redundantly.
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 deteremined by the following time
intervals:
 Time interval between the initial occurrence of an interrupt and the interrupt report
 Time interval until the next FB 453 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 a
parameterizable 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 redundant
mode, in the event of a CPU STOP, the response of the current outputs is automatically set
to "zero current and zero voltage" in their configuration. 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 parameterized 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).
NOTICE
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".
204
reaching the CPU.
System Manual, 03/2012, A5E00267695-11
S7-400H