Figure 10-11 Fault-Tolerant Analog Output Modules In 1-Out-Of-2 Configuration - Siemens SIMATIC S7-400H System Manual

Redundant analog output modules
You implement fault-tolerant control of a final control element by wiring two outputs of two
analog output modules in parallel (1-out-of-2 structure).

Figure 10-11 Fault-tolerant analog output modules in 1-out-of-2 configuration

The following applies to the wiring of analog output modules:
● Wire the ground connections in a star structure to avoid output errors (limited common-
Information on wiring the diode circuit
● Suitable diodes are, for example, those of the series 1N4003 ... 1N4007, or any other
● It is advisable to separate the chassis ground of the module and load ground. There must
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 then recovers
to the proper value.
Redundant analog outputs output a minimum current of approximately 120 μA per module,
meaning a total current of approximately 240 µA. Allowing for 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, the current outputs are automatically set to
"off current and off voltage".
S7-400H
System Manual, 09/2007, A5E00267695-03
mode suppression of the analog output module).
diode with U >= 200 V and I_
_r
be equipotential bonding between both.
>= 1 A.
F
Using I/Os in S7-400H
10.5 Connecting redundant I/Os
147