Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 68)

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