S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0
9-1
Cycle and Response Times of the S7-400
9
9.1 Cycle time
Definition of the 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-Sharing Model
Cyclic program scanning, and thus also processing of the user program, is performed in time
slices. So that you can better appreciate these processes, we will assume in the following
that each time slice is exactly 1 ms long.
Process Image
The process signals are read or written prior to program scanning so that a consistent image
of the process signals is available to the CPU for the duration of cyclic program scanning.
Then the CPU does not directly access the signal modules during program scanning when
the address area "inputs" (I) and "outputs" (O) are addressed, but addresses instead the
internal memory area of the CPU on which the image of the inputs and outputs is located.
The Cyclic Program Scanning Process
The following table and figure illustrate the phases of cyclic program scanning.
Table 9-1 Cyclic program processing
Step Process
1 The operating system starts the scan cycle monitoring time.
2 The CPU writes the values from the process-image output table in the output modules.
3 The CPU reads out the status of the inputs at the input modules and updates the
process-image input table.
4 The CPU processes the user program in time slices and performs the operations
specified in the program.
5 At the end of a cycle, the operating system executes pending tasks, such as the loading
and clearing of blocks.
6 The CPU then goes back to the beginning of the cycle after the configured minimum
cycle time, as necessary, and starts cycle time monitoring again.
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