Siemens Simatic S7-300 CPU 31xC and CPU 31x S7-300 - Manual (Page 84)

Cycle and reaction times
5.2 Cycle time
CPU 31xC and CPU 31x, Technical data
5-2
Manual, Edition 08/2004, A5E00105475-05
5.2 Cycle time
5.2.1 Overview
Introduction
This section explains what we mean by the term "cycle time", what it consists of, and how
you can calculate it.
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.