Siemens Simatic S7-300 Technical Data Manual page 85

Sequence of cyclic program processing
The table and figure below show the phases in cyclic program processing.
Table 5-1
Step
1
2
3
4
5
6
Time slice (1 ms)
In contrast to S7-400 CPUs, the S7-300 CPUs data only allow data access from an OP / TP
(monitor and modify functions) at the scan cycle check point (Data consistency, see the
Technical Data). Processing of the user program is not interrupted by the monitor and modify
functions.
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
Cyclic program processing
Sequence
The operating system initiates cycle time monitoring.
The CPU copies the values of the process image of outputs to the output modules.
The CPU reads the status at the inputs of the input modules and then updates the
process image of inputs.
The CPU processes the user program in time shares and executes program instructions.
At the end of a cycle, the operating system executes queued tasks, for example, loading
and deleting blocks.
The CPU then returns to the start of the cycle, and restarts cycle time monitoring.
Time slices (1 ms each)
Cycle and reaction times
5.2 Cycle time
2
3
4
5
5-3