Time Monitoring - Siemens SIMATIC S7-400H System Manual

Fault-tolerant systems
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9.4 Time monitoring

9.4
Time monitoring
Program execution is interrupted for a certain time during updating. This secton is relevant to
you if this period is critical in your process. If this is the case, configure one of the monitoring
times described below.
During updating, the fault-tolerant system monitors the cycle time extension, communication
delay and inhibit time for priority classes > 15 in order to ensure that their maximum values
are not exceeded, and that the configured minimum I/O retention time is maintained.
NOTICE
If you have not defined any default values for the monitoring times, make allowance for the
update in the cycle monitoring time. If this is the situation, the update is cancelled and the
fault-tolerant system switches to single mode: The previous master CPU remains in RUN,
and the standby CPU goes into STOP.
You can either configure all the monitoring times or none at all.
You made allowances for the technological requirements in your configuration of monitoring
times.
The monitoring times are described in detail below.
● Maximum cycle time extension
● Maximum communication delay
● Maximum inhibit time for priority classes > 15
● Minimum I/O retention time:
106
– Cycle time extension: The cycle time extension is the time during the update in which
neither OB 1 nor any other OBs up to priority class 15 are executed. The "normal"
cycle time monitoring function is disabled within this time span.
– Max. cycle time extension: The maximum cycle time extension represents the
configured and permissible maximum.
– Communication delay: The communication delay represents a time span within the
update during which the CPU does not execute any communication functions.
Note: The master CPU maintains all existing communication links.
– Maximum communication delay: The maximum communication delay represents the
configured and permissible maximum.
– Inhibit time for priority classes > 15: The time span within an update during which the
CPU neither executes any OBs (and so any user program) nor any further I/O updates.
– Maximum inhibit time for priority classes > 15: The maximum inhibit time for priority
classes > 15 represents the configured and permissible maximum.
This represents the interval between copying of the outputs from the master CPU to the
standby CPU and the time of the transition to the redundant system state or
master/standby changeover (time at which the previous master CPU goes into STOP and
the new master CPU goes into RUN). Both CPUs control the outputs within this period, in
order to prevent the I/O from going down when the system performs an update with
master/standby changeover.
The minimum I/O retention time is of particular importance when updating with
master/standby changeover. If you set the minimum I/O retention time to zero, the outputs
could possibly shut down when you modify the system in Run.
System Manual, 09/2007, A5E00267695-03
S7-400H

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