Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 85)

Cycle and reaction times
5.2 Cycle time
CPU 31xC and CPU 31x, Technical data
Manual, Edition 08/2004, A5E00105475-05
5-3
Sequence of cyclic program processing
The table and figure below show the phases in cyclic program processing.
Table 5-1 Cyclic program processing
Step Sequence
1 The operating system initiates cycle time monitoring.
2 The CPU copies the values of the process image of outputs to the output modules.
3 The CPU reads the status at the inputs of the input modules and then updates the
process image of inputs.
4 The CPU processes the user program in time shares and executes program instructions.
5 At the end of a cycle, the operating system executes queued tasks, for example, loading
and deleting blocks.
6 The CPU then returns to the start of the cycle, and restarts cycle time monitoring.
Cycle time
Time slices (1 ms each)
Time slice (1 ms)
2
3
4
5
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.