Siemens SIMATIC CPU 410 System Manual page 91

Using analog input modules as redundant I/O
You specified the following parameters when you configured the analog input modules for
redundant operation:
● Tolerance window (configured as a percentage of the end value of the measuring range)
Two analog values are considered equal if they are within the tolerance window.
● Discrepancy time (maximum permitted time in which the redundant input signals can be
outside the tolerance window). The specified discrepancy time must be a multiple of the
update time of the process image and therefore also the basic conversion time of the
channels.
An error is generated when there is an input value discrepancy after the configured
discrepancy time has expired.
If you connect identical sensors to both analog input modules, the default value for the
discrepancy time is usually sufficient. If you use different sensors, in particular
temperature sensors, you will have to increase the discrepancy time.
● Applied value
The applied value represents the value of the two analog input values that is applied to
the user program.
The system verifies that the two read-in analog values are within the configured tolerance
window. If they are, the applied value is written to the lower data memory area of the process
input image. If there is a discrepancy and it is the first, it is marked accordingly and the
discrepancy time is started.
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 channel with the configured standard value is declared as valid
and the other 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
channel is passivated. If the minimum value is set, this channel supplies the data to the
process and the channel with the maximum value is passivated. Whichever is the case, the
passivated 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.
Note
The time that the system actually needs to determine a discrepancy depends on various
factors: Bus runtimes, cycle times and call times of the user program, conversion times, etc.
For this reason, it is possible for redundant input signals to 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 channel is passivated immediately.
You should therefore disable all unused inputs in HW Config using the "Measurement type"
parameter.
CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC
6.7 Connection of two-channel I/O to the PROFIBUS DP interface
I/O configuration variants
91