Siemens SIMATIC PCS 7 Operating Manual page 37

Run time of the cyclic interrupt concerned
If a cyclic interrupt OB with special handling has already been configured, half its execution
interval will be entered as the default setting for "Runtime of the watchdog interrupt
concerned".
Here, enter the actual runtime of the cyclic interrupt with special handling, plus some reserve
time (10 to 20%).
Note
The runtime can be calculated using the TIME_BEG and TIME_END blocks.
You can find information on this in the "How do I set the runtime and time scale of a cyclic
interrupt OB?" (https://support.industry.siemens.com/cs/ww/en/view/1023077) FAQ.
From PCS 7 V7.0 and higher, and an S7-400H CPU with FW V4.5 and higher, the runtimes
for the cyclic interrupt OBs may also be read at the CPU_RT block.
Work memory allocation (data memory)
The work memory allocation comprises all data blocks; in other words, it also includes DBs
generated dynamically. A value of 1024 KB is entered here by default. This value should be
modified to reflect the actual data memory requirements of the user program. We
recommend adding an expansion reserve of approximately 10%.
The allocation of work memory for data blocks can be read in SIMATIC Manager by
selecting the block container with the menu command "Edit > Object Properties".
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB
6.2 Adapting CPU parameters (fault-tolerant F system)
Configuring S7F/FH hardware
37