Siemens SIMATIC PCS 7 Operating Manual page 28

Configuring S7F/FH hardware
6.1 Adapting CPU parameters (single F-system)
CPU utilization
The cycle utilization of the system should never be above 75%, regardless of whether it
involves a standard AS or an H system.
Note
To help you determine the processing times of the individual cyclic interrupt OBs, please
refer to the FAQ "How can you calculate the cycle load of the automation system (AS)
online?" (https://support.industry.siemens.com/cs/ww/en/view/22000962)
As of PCS 7 V7.0 SP1 and an S7-400H CPU with FW 4.5 and higher, the run times for the
cyclic interrupt OBs and the complete utilization of the AS may also be read at the CPU_RT
block.
As of PCS 7 V7.0 with PCS 7 Library V7.0, the block CPU_RT will be installed in each CPU.
This block prevents a CPU STOP as a result of cycle overload. thanks to a two-stage "load-
shedding" process:
● In the first stage of load shedding, all cyclic interrupt OBs that have not been disabled for
the function by the project engineer are not processed for one cycle.
● In the second stage, none of the cyclic interrupt OBs are processed for one cycle (even
the ones that were excluded previously). If this does not have a steadying effect,
whenever cyclic interrupt OBs are processed their processing will be suspended again for
one cycle.
Note
In order to prevent F-cycle-time monitoring in the CPU and the F-monitoring time on the F-
signal modules from being triggered, and the system response time from being extended,
you must disable the cyclic interrupt OBs with the F-program during the stage 1 of stop
prevention.
Alternatively, you can disable stop prevention altogether by setting the parameter for input
Max_RTRG of block CPU_RT to 0. You can find this block in the @CPU_RT chart.
For more information on the subject of CPU stop prevention, please refer to the help section
for the CPU_RT block.
28
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB