Siemens CPU 410-5H Process Automation System Manual page 114

Link-up and update
8.3 Time monitoring
Monitoring time accuracy
Note
The monitoring times determined by STEP 7 or by using formulas merely represent
recommended values.
These times are based on a fault-tolerant system with two communication peers and an
average communication load.
Your system profile may differ considerably from those scenarios, therefore the following
rules must be observed.
● The cycle time extension factor may increase sharply at a high communication load.
● Any modification of the system in operation may lead to a significant increase in cycle
times.
● Any increase in the number of programs executed in priority classes > 15 (in particular
processing of communication blocks) automatically increases the communication delay
and cycle time extension.
● You can even undercut the calculated monitoring times in small plants with high-
performance requirements.
Configuration of the monitoring times
When configuring monitoring times, always make allowances for the following dependencies;
conformity is checked by STEP 7:
Maximum cycle time extension
> maximum communication delay
> (maximum inhibit time for priority classes > 15)
> minimum I/O retention time
If you have configured different monitoring times in the CPUs and perform a link-up and
update operation with master/standby changeover, the system always applies the higher of
the two values.
Calculating the minimum I/O retention time (T
The following applies to the calculation of the minimum I/O retention time:
● With central I/O: T
● For distributed I/O (PROFIBUS DP): T
Where T
● For distributed I/O (PROFINET IO): T
switched device in all IO subsystems of the fault-tolerant station
When using central and distributed I/O, the resultant minimum I/O retention time is:
T
PH
114
= 30 ms
PH
= maximum target rotation time
TRmax
of all DP master systems of the fault-tolerant station
with T = maximum cyclic interrupt time (product of WD factor and update time) of a
wd_max
= MAX (30 ms, 3 x T
TRmax
)
PH
= 3 x T
PH TRmax
= T
PH wd_max
, T )
wd_max
CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 10/2013, A5E32631667-AA