Siemens S7-300 Sequence of cyclic program processing, Time slices 1 ms each, Step Sequence

Cycle and reaction times 5.2 Cycle time

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.

Time slices (1 ms each)

Time slice (1 ms)

2
3	Cycle time
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.

CPU 31xC and CPU 31x, Technical data	5-3
Manual, Edition 08/2004, A5E00105475-05