Siemens SIMATIC PCS 7 CPU 410-5H System Manual page 68

I/O configuration variants
4.5 Using single-channel switched I/O
Duration of a changeover of the active channel
The maximum changeover time is
DP/PN error detection time + DP/PN changeover time + changeover time of the DP slave
interface/PNIO interface
You can determine the first two values from the bus parameters of your DP master system or
PNIO system in STEP 7. You determine the last two values using the manuals of the
respective DP slave interfaces or PNIO interfaces.
Note
When using fail-safe modules, always set a monitoring time for each fail-safe module that is
longer than the changeover time of the active channel in the fault-tolerant system. If you
ignore this rule, you risk passivation of the fail-safe modules during the changeover of the
active channel.
You can use the Excel file "s7ftimea.xls" to calculate the monitoring and reaction times. The
file is available at the following address:
http://support.automation.siemens.com/WW/view/en/22557362
Note
Note that the CPU can only detect a signal change if the signal duration is greater than the
specified changeover time.
When there is a changeover of the entire DP master system, the changeover time of the
slowest DP component applies to all DP components. A DP/PA link or Y link usually
determines the changeover time and the associated minimum signal duration. We therefore
recommend that you connect DP/PA and Y links to a separate DP master system.
Changeover of the active channel during link-up and updating
During link-up and updating with master/standby changeover (see Chapter Link-up
sequence (Page 287)), a changeover between the active and passive channels occurs for 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 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/PNIO stations and by the DP master system.
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CPU 410-5H Process Automation
System Manual, 09/2014, A5E31622160-AB