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

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
can differ).
When there is still a discrepancy in the input values after the configured
discrepancy time has expired, a fault has occurred.
• Reaction of the H system to discrepancy in the input values
First, the input signals of the paired redundant modules are checked to see if they
match. If the values match the uniform 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.
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,
a fault has occurred.
The localization of the defective page is performed according to the following
strategy:
1. During the discrepancy time the most recent matching value is retained as a
result.
2. Once the discrepancy time has expired the following error message is
displayed:
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 transition occurs, the configured reaction
is performed after the discrepancy time expires.
3. If another signal transition occurs, the module in which the transition takes
place is the intact module and the other module is passivated.
If both paired modules are completely passivated, "zeroes" are entered at the
passivated memory location of the process image.
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.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Using I/O on the S7-400H
7-17