Siemens SIMATIC S7-400H System Manual page 142

Using I/Os in S7-400H
10.5 Connecting redundant I/Os
When the discrepancy time is running, the most recent valid value is written to the process
image of the module with the lower address and made available to the current process. If the
discrepancy time expires, the module/channel with the configured standard value is declared
as valid and the other module/channel is passivated. If the maximum value from both modules
is configured as the standard value, this value is then taken for further program execution and
the other module/channel is passivated. If the minimum value is set, this module supplies the
data to the process and the module with the maximum value is passivated. Whichever is the
case, the passivated modules/channels are entered in the diagnostic buffer.
If the discrepancy is eliminated within the discrepancy time, analysis of the redundant input
signals is still carried out.
NOTICE
The time that the system actually needs to determine a discrepancy depends on various
factors: Bus delay times, cycle and call times in the user program, conversion times etc.
Redundant input signals can therefore be different for longer than the configured
discrepancy time.
Note
There is no discrepancy analysis when a channel reports an overflow with 16#7FFF or an
underflow with 16#8000. The relevant module/channel is passivated immediately.
You should therefore disable all unused inputs in HW Config using the "Measuring type"
parameter.
Redundant analog input modules with non-redundant encoders
With non-redundant encoders, analog input modules are used in a 1-out-of-2 configuration:
Figure 10-9
142
Fault-tolerant analog input modules in 1-out-of-2 configuration with one encoder
System Manual, 09/2007, A5E00267695-03
S7-400H