Siemens Simatic S7-300 Technical Data Manual page 68

Memory concept
4.1 Memory areas and retentivity
Process image update
The operating system updates the process image periodically. The figure below shows the
sequence of this operation within a cycle.
4-6
Startup
PIO
PII
User program
CCP (OS)
Startup program
Processing the user program (OB 1)
and all programs called inside of it.
Reading the inputs from the modules
and refreshing the data in the process
image of the inputs.
Writing the process image of the outputs
into the modules.
Manual, Edition 08/2004, A5E00105475-05
CPU 31xC and CPU 31x, Technical data