Siemens SIMATIC S7 F Manual page 133

Programming
Changing the Time Conditions or Monitoring Times
This is possible, but you must ensure that such changes don't initiate any cyclic
measures. For example:
•
Changing the OB cycle time: All monitoring times (F_CYC_CO, F module
driver, F communication) must be greater than the new OB cycle time. If this
isn't the case, you must increase these times beforehand and download them
before the new OB cycle time is brought in. Only in the second step can the
parameter assignment of the execution time of the cyclic interrupt OBs be
adapted in the S7 FH System. The monitoring times of F-I/Os cannot be
changed during operation (see "Impermissible Changes").
Note: If the MAX_CYC parameter of the F_CYC_CO is invalid, a new value will be
requested at compile time.
•
Moving run-time groups: This corresponds to changing the OB cycle time for
the run-time group to be moved (see above).
•
Direct changing of monitoring times for F-Blocks: The monitoring times must fit
the OB cycle time. In the case of F-driver blocks, it is not possible to make
changes during operation (see "Impermissible Changes").
First Call and Restart Characteristics
Newly inserted F-Blocks behave for a first call or a warm restart as for a cold
restart. For example:
•
Module drivers or communication blocks output substitute values.
•
The F_START block indicates a cold restart in the first cycle.
It may be necessary in such cases to place these blocks initially without
interconnecting them and to download them to the CPU by means of change
downloading. These blocks can only be interconnected and then downloaded to
the CPU as changes in the second step.
Fail-Safe Systems
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