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Unity Pro: Cyclic and Periodic Execution

CYCLIC execution:
  Processes inputs, solves logic program, processes outputs,  same thing on the next scan.
Sequences the task cycles, one after another.
After outputs are updated, the system performs its own specific processing then starts another scan,
without pausing.
   The cycle is checked by the watchdog timer.

PERIODIC execution:
   Processing of inputs, logic program and updating of outpts is carried out periodically over a defined period
of 1 to 255 ms.
At the start of the scan, a timeout whose current value is initialized to the defined period starts the countdown.
The PLC scan must be completed before this timeout expired and launches a new cycle.
If the period is not yet over, the PLC completes its operating cyle until the end of the period by performing internal processing.
If the operating time is longer than the defined period, the PLC signals an overrun by setting %S19 to 1.
Processing continues and is fully executed (but must not exceed watchdog limit).
The next cycle is started after the outputs have been written for the current cycle.

Period overrun and WDT are checked.

A periodic overrun does not cause a PLC HALT if it remains less than the WDT value.
%S19 indicates period overflow. It is set to 1 when the scan time > than the task period.
%SW0 contains the value of the period in ms. (%SW0 = 0 means the task is in cyclic mode).

If WDT overflow occurs, the application is declared in error and the PLC stops (HALT state).
%S11 indicates WDT overflow.   It is set to 1 when scan time > WDT.
%SW11 contains WDT value in ms. (read only).

Once the PLC is in HALT, the task has to be reactivated, analyzed, corrected, PLC initialized and switched
to RUN.
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