Siemens SIMATIC S7-400H System Manual page 199

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 a longer period 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 "Measurement type"
parameter.
Redundant analog input modules with non-redundant encoder
With non-redundant encoders, analog input modules are used in a 1-out-of-2 configuration:
Figure 13-10 Fault-tolerant analog input modules in 1-out-of-2 configuration with one encoder
Remember the following when connecting an encoder to multiple analog input modules:
● Connect the analog input modules in parallel for voltage sensors (left in figure).
● You can convert a current into voltage using an external load to be able to use voltage
● 2-wire transmitters are powered externally to allow you to repair the module online.
The redundancy of the fail-safe analog input modules enhances their availability.
You will find connection examples in Appendix Connection examples for redundant I/Os
(Page 461).
S7-400H
System Manual, 03/2012, A5E00267695-11
analog input modules connected in parallel (center in the figure).
13.4 Connecting redundant I/O to the PROFIBUS DP interface
Using I/Os in S7–400H
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