Question:
What is the execution sequence of input action, output action and 'cyclic' action in the language SL?
Answer:
The first step within a sequencer cannot have an input action. Consequently, after starting up the PLC first processes the logic within the 'cyclic' action. At the end of the PLC cycle the transition following the active step is verified.
1. If the transition condition has not been fulfilled (FALSE), the step remains active and the 'cyclic' action is processed again.
2. If this transition is TRUE, the following step becomes active in the next PLC cycle. This means that first of all it is the logic from the output action of the last active step which is executed. It is followed by execution of the logic of the input action of the now active step and then the logic of the 'cyclic' action is executed. Only then is the following transition verified and the PLC cycle terminated.
The processing described in item 1 or 2 above is repeated depending on whether the transition condition is fulfilled or not.
