Siemens SIMATIC S7-400H System Manual page 126

Using I/Os in S7-400H
10.4 Using single-channel switched I/Os
Duration of a failover of the active channel
The maximum failover time is
DP error detection time + DP failover time + failover time of the DP slave interface module
You can determine the first two values from the bus parameters of your DP master system in
STEP 7. You can obtain the last value from the manuals of the relevant DP slave interface
module
NOTICE
When using fail-safe modules, always set a monitoring time for each fail-safe module that is
longer than the failover time of the active channel in the fault-tolerant system. If you ignore
this rule, you risk failure of the fail-safe modules during the failover of the active channel.
NOTICE
The above calculation also includes the processing time in OB 70 or OB 86. Make sure that
the processing time for a DP station does not last longer than 1 ms. In situations requiring
extensive processing, exclude this processing from direct execution of the OBs mentioned.
Note that the CPU can only detect a signal change if the signal duration is greater than the
specified failover time.
When there is a failover of the entire DP master, the failover time of the slowest component
applies to all DP components. A DP/PA Link or Y Link usually determines the failover time
and the associated minimum signal duration. We therefore recommend that you connect
the DP/PA and Y Links to a separate DP master system.
When using fail-safe modules, always set a monitoring time for each fail-safe module that is
longer than the failover time of the active channel in the fault-tolerant system. If you ignore
this, you risk failure of the fail-safe modules during the failover of the active channel.
Changeover of the active channel during link-up and updating
During link-up and update with master/standby changeover (see section Link-up sequence
(Page 99)) the active and passive channels are changed over on all stations of the switched
I/O. At the same time OB 72 is called.
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 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 failures of a DP station are detected during the changeover, the
changeover is monitored by the various DP stations and by the DP master system.
Provided the minimum I/O hold time is set correctly (see section Time monitoring
(Page 106)), 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 failover times to separate chains. This, for
example, simplifies the calculation of monitoring times.
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(Distributed I/O ET 200M DP/PA Bus Link
and ).
System Manual, 09/2007, A5E00267695-03
S7-400H