Lenze PLC Designer PLC Designer (R2-x) - Manual (Page 327)

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DMS 3.2 EN 02/2011 TD29 325
Note!
If you rename a step and perform an Online Change while this step is active, the
program will be stopped in undefined status !
Processing order of elements in a sequence:
First, all Action Control Block flags in the IEC actions that are used in this sequence
are reset (not, however, the flags of IEC actions that are called within actions).
All steps are tested in the order which they assume in the sequence (top to bottom
and left to right) to determine whether the requirement for execution of the output
action is provided, and this is executed if that is the case.
All steps are tested in the order which they assume in the sequence to determine
whether the requirement for the input action is provided, and this is executed if
that is the case.
For all steps , the following is done in the order which they assume in the sequence:
- If applicable, the elapsed time is copied into the corresponding step variable.
- If applicable, any timeout is tested and the SFC error flags are serviced as required.
- For non-IEC steps, the corresponding action is now executed.
IEC actions that are used in the sequence are executed in alphabetical order. This is
done in two passes through the list of actions. In the first pass, all the IEC actions
that are deactivated in the current cycle are executed. In the second pass, all the IEC
actions that are active in the current cycle are executed.
Transitions are evaluated: If the step in the current cycle was active and the
following transition returns TRUE (and if applicable the minimum active time has
already elapsed), then the following step is activated.
The following must be noted concerning implementation of actions:
It can come about that an action is carried out several times in one cycle because it is
associated with multiple sequences. (For example, an SFC could have two IEC actions
A and B, which are both implemented in SFC, and which both call IEC action C; then in
IEC actions A and B can both be active in the same cycle and furthermore in both
actions IEC action C can be active; then C would be called twice).
If the same IEC action is used simultaneously in different levels of an SFC, this could
lead to undesired effects due to the processing sequence described above. For this
reason, an error message is issued in this case. It can possibly arise during processing
of projects created with older versions of »PLC Designer«.