Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 85
Updating the Process Image
During cyclic program processing by the CPU, the process image is updated automatically. With
the S7-400 CPUs and the CPU 318 you can deselect the update of the process image if you want
to:
• Access the I/O directly instead or
• Update one or more process image input or output sections at a different point in the program
using system functions SFC26 UPDAT_PI and SFC27 UPDAT_PO.
Communication Load
You can use the CPU parameter "Scan Cycle Load from Communication" to control within a given
framework the duration of communication processes that always increase the scan cycle time.
Examples of communication processes include transmitting data to another CPU by means of MPI
or loading blocks by means of a programming device.
Test functions with a programming device are barely influenced by this parameter. However, you
can increase the scan cycle time considerably. In the process mode, you can limit the time set for
test functions (S7-300 only).