Cyclic program execution
3.2 Cycle time
Cycle and response times
26 Function Manual, 10/2018, A5E03461504-AD
In STEP 7, you can set a minimum cycle time for a CPU. The default for the minimum cycle
time is one millisecond. It is advisable to increase this setting in the following cases:
● To reduce the cycle time's fluctuation range.
● To make remaining computing time available for communication tasks. The CPU then
processes these communication tasks until the minimum cycle time has expired.
Making the remaining computing time available to communication tasks offers the
following advantages:
– Longer minimum cycle times prevent that process images are updated unnecessarily
often and thus lead to less load on the backplane bus.
– Longer minimum cycle times result in an increase in communication performance.
The maximum cycle time is a configurable high limit of the cyclic program runtime. The task
of the cycle time is to monitor the response time required for the respective process.
The maximum cycle time of non-redundant CPUs is set to 150 ms by default. You can set
this value from 1 ms to 6000 ms when assigning parameters to the CPU. When the cycle
time is longer than the maximum cycle time, the time error OB (OB 80) is called.
You have the option of restarting the maximum cycle time using the "RE_TRIGR" instruction,
and thus extending it.
You specify how the CPU responds to the time error with the user program in OB 80. The
CPU switches to STOP under the following conditions:
● If you have not loaded an OB 80.
● If the cycle is still not completed after an additional maximum cycle time.
Note
Behavior of the CPU on the second cycle timeout
Note that you can only extend the cycle time a maximum of once. On the second
consecutive cycle timeout, the CPU always goes into STOP state.
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