Siemens SIMATIC S7-400H System Manual page 138

Using I/Os in S7-400H
10.5 Connecting redundant I/Os
Quality levels in the redundant configuration of signal modules
There are three quality levels for reliable operation of a redundant configuration of signal
modules if an error occurs:
● Highest quality with fail-safe signal modules (but without F functionality)
● Medium quality with signal modules capable of diagnostics
● Simple quality with signal modules without diagnostics
Using digital input modules as redundant I/O
The following parameters are set to configure digital input modules for redundant operation:
● Discrepancy time (maximum allowed time in which the redundant input signals may differ)
● Reaction of the redundancy software to a discrepancy in the input values
First, the input signals of the paired redundant modules are checked for consistency. If the
values match, the uniform value is written to the lower 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.
During the discrepancy time, the most recent matching (non-discrepant) value is written to
the process image of the module with the lower address. This procedure is repeated until the
values once again match within the discrepancy time or until the discrepancy time of a bit
has expired.
If the discrepancy continues past the expiration of the configured discrepancy time, an error
has occurred.
The defective side is localized according to the following strategy:
1. During the discrepancy time the most recent matching value is retained as the result.
2. Once the discrepancy time has expired the following error message is displayed:
3. If another signal change now occurs, the module/channel in which the change occurred is
Modules with diagnostic functions are also passivated by calling OB 82.
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When there is still a discrepancy in the input values after the configured discrepancy time
has expired, an error has occurred.
Error code 7960: "Redundant I/O: discrepancy time at digital input expired, error not yet
localized". Passivation is not performed and no entry is made in the static error image.
Until the next signal change occurs, the configured reaction is performed after the
discrepancy time expires.
the intact module/channel and the other module/channel is passivated.
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.
System Manual, 09/2007, A5E00267695-03
S7-400H