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
177
Bumpless changeover of the active channel
To prevent the I/O failing temporarily or outputting substitute values during the changeover
between the active and passive channel, the DP or PNIO stations of the switched I/O put
their outputs on hold until the changeover is completed and the new active channel has
taken over.
To ensure that total failure of a DP or PNIO station is also detected during the changeover,
the changeover is monitored by the various DP stations and by the DP master system.
Provided the minimum I/O retention time is set correctly (see section Time monitoring
(Page 155)), no interrupts or data records will be lost due to a changeover. There is an
automatic repetition when necessary.
System configuration and project engineering
You should allocate switched I/O with different changeover times to separate chains. This,
for example, simplifies the calculation of monitoring times.
13.4 Connecting redundant I/O to the PROFIBUS DP interface
What is redundant I/O?
Input/output modules are termed redundant when they exist twice and they are configured
and operated as redundant pairs. The use of redundant I/O provides the highest degree of
availability, because the system tolerates the failure of a CPU or of a signal module.
Note
PROFINET
The use of redundant I/O at the PROFINET interface is not possible.