Siemens CPU 31 Series Technical Data Manual page 88

Cycle and reaction times
5.2 Cycle time
Meaning of the term cycle time
The cycle time represents the time that an operating system needs to execute a program,
that is, one OB 1 cycle, including all program sections and system activities interrupting this
cycle. This time is monitored.
Time slice model
Cyclic program processing, and therefore user program execution, is based on time shares.
To clarify these processes, let us assume that every time share has a length of precisely
1 ms.
Process image
During cyclic program processing, the CPU requires a consistent image of the process
signals. To ensure this, the process signals are read/written prior to program execution.
Subsequently, the CPU does not address input (I) and output (Q) address areas directly at
the signal modules, but rather accesses the system memory area containing the I/O process
image.
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
5-2
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.
CPU 31xC and CPU 31x, Technical Data
Manual, 01/2006 Edition, A5E00105475-06