Using Analog Input Modules As Redundant I/O - Siemens SIMATIC S7-400H Manual

Using analog input modules as redundant I/O

The following parameters are set to configure analog input modules for redundant
operation:
• Tolerance window (configured as a percent of the end value of the measuring
range).
Two analog values are the same when they within the tolerance window.
• Discrepancy time (maximum allowed time in which the redundant input signal
can be outside the tolerance window).
A fault is occurs when there is an input value discrepancy after expiration of the
configured discrepancy time.
If you connect identical sensors to the two analog input modules, the default
value for the discrepancy time is usually sufficient. If you connect different
sensors, especially when they are temperature sensors, you will have to
increase the discrepancy time.
• Applied value
The applied value is the value from the two analog input values that is entered
into the user program.
The system checks if the two read analog values are within the configured
tolerance window. If this is the case, the applied value is written to the lower data
memory area of the input's process image. If there is a discrepancy and it is the
first discrepancy, it is marked and the discrepancy time is started.
When the discrepancy time is running the most recently valid value is written to the
process image of the module with the lower address and made available to the
current process. When the discrepancy time is expired the module with the
configured standard value is declared as valid and the other module 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 is
passivated. If the minimum value is set, this module supplies the data for the
process and the module with the maximum value is passivated. The passivated
module is registered in the diagnostic buffer in any case.
When the discrepancy ceases within discrepancy time, the analysis of the
redundant input signals continues.
Notice
The time that the system actually needs to determine a discrepancy depends on
several factors: bus operating times, cycle and call-up time of the user program,
conversion time, etc. Redundant input signals may therefore be different longer
than the discrepancy time.
Note
You have to deactivate non-switched inputs in HW Config (parameter "Measuring
type").
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Using I/O on the S7-400H
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