Siemens SIMATIC S7-400H, Системное руководство

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Using I/Os in S7–400H
13.4 Connecting redundant I/O to the PROFIBUS DP interface
S7-400H
System Manual, 03/2012, A5E00267695-11
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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:
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 response is performed after the
discrepancy time expires.
3. If another signal change occurs, the module/channel in which the signal change occurred
is 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 a longer period than the
configured discrepancy time.
Modules with diagnostics capability are also passivated by calling OB 82.
Using redundant digital input modules with non-redundant encoders
With non-redundant encoders, you use digital input modules in a 1-out-of-2 configuration:
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Figure 13-7 Fault-tolerant digital input module in 1-out-of-2 configuration with one encoder
The use of redundant digital input modules increases their availability.
Discrepancy analysis detects "Continuous 1" and "Continuous 0" errors of the digital input
modules. A "Continuous 1" error means the value 1 is applied permanently at the input, a
"Continuous 0" error means that the input is not energized. This can be caused, for example,
by a short-circuit to L+ or M.
The current flow over the chassis ground connection between the modules and the encoder
should be the minimum possible.
When connecting an encoder to several digital input modules, the redundant modules must
operate at the same reference potential.
If you want to replace a module during operation and are not using redundant encoders, you
will need to use decoupling diodes.