Table 16-1 Cyclic Program Execution - Siemens SIMATIC S7-400H System Manual

S7-400 cycle and reaction times
16.1 Cycle time
Process image
The CPU reads and writes the process signals to a process image before it starts cyclic
program execution, in order to obtain a precise image of the process signals. The CPU does
not access the signal modules directly when the I/O operand areas respond during program
execution, but rather addresses its memory area which contains the I/O process image.
Phases in cyclic program execution
The table below shows the various phases in cyclic program execution.
Table 16-1
Step
1
2
3
4
5
6
Elements of the cycle time
Figure 16-1
260
Cyclic program execution
The operating system initiates the cycle monitoring time.
The CPU writes the values of the process image to the outputs of the output modules.
The CPU reads the status of inputs of the input modules, and then updates the process
image of the inputs.
The CPU executes the user program in time slices, and executes the operations
defined in the program.
At the end of the cycle, the operating system performs all pending tasks, such as
loading or deleting blocks.
Finally, on expiration of any given minimum cycle time, the CPU returns to the start of
the cycle and restarts cycle monitoring.
Elements and composition of the cycle time
Sequence
System Manual, 09/2007, A5E00267695-03
S7-400H