Cycle Time - Siemens SIMATIC S7-400H System Manual

S7-400 cycle and response times

19.1 Cycle time

19.1 Cycle time
This section describes the decisive factors in the cycle time, and how to calculate it.
Definition of cycle time
The cycle time is the time the operating system requires to execute a program, i.e. to
execute OB 1, including all interrupt times required by program parts and for system
activities.
This time is monitored.
Time slice model
Cyclic program processing, and therefore also user program processing, is based on time
slices. To demonstrate the processes, let us presume a global time slice length of exactly
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, during program processing the CPU does not access the signal modules
directly when addressing the input (I) and output (O) address areas, but rather it accesses
the CPU's internal memory area containing the I/O process image.
Sequence of cyclic program processing
The table below shows the various phases in cyclic program execution.
Table 19- 1
Step
1
2
3
4
5
6
356
Cyclic program processing
The operating system initiates the scan cycle monitoring time.
The CPU copies the values from the process output images to 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 processes the user program in time slices and executes the instructions
specified in the program.
At the end of a cycle, the operating system executes pending tasks, e.g. loading and
deleting of blocks.
Finally, on expiration of any given minimum cycle time, the CPU returns to the start of
the cycle and restarts cycle monitoring.
Sequence
System Manual, 03/2012, A5E00267695-11
S7-400H