Siemens SIMATIC PCS 7 System Manual page 288

Supplementary information
16.15 Cycle and response times of the CPU 410-5H
Process image
During cyclic program execution, the CPU requires a consistent image of the process
signals. To ensure this, the process signals are read/written prior to program execution.
During the subsequent program execution, the CPU does not access the signal modules
directly when addressing the input (I) and output (O) address areas. It accesses the CPU's
system memory area containing the image of the inputs and outputs.
Sequence of cyclic program processing
The table below shows the various phases in cyclic program execution.
Table 16- 9
Step
1
2
3
4
5
6
288
Cyclic program processing
The operating system initiates the scan cycle monitoring time.
The CPU copies the values from the process output images to the output modules.
The CPU reads the status of inputs of the input modules, and then updates the process
image of the inputs.
The CPU processes the user program in time slices and executes the instructions spec-
ified in the program.
At the end of a cycle, the operating system executes pending tasks, e.g., loading and
deleting of blocks.
Finally, on expiration of any given minimum cycle time, the CPU returns to the start of
the cycle and restarts cycle monitoring.
Sequence
CPU 410-5H Process Automation/CPU 410 SMART
System Manual, 09/2014, A5E32631667-AB