Rules of evolution

IntroductionThe initial situation of a SFC network is characterized by the initial step which is in the active state upon initialization of the program or function block containing the network.Evolutions of the active states of steps take place along the directed links when caused by the clearing of one or more transitions.A transition is enabled when all the preceding steps, connected to the corresponding transition symbol by directed links, are active. The clearing of a transition occurs when the transition is enabled and when the associated transition condition is true.The clearing of a transition causes the deactivation (or “resetting”) of all the immediately preceding steps connected to the corresponding transition symbol by directed links, followed by the activation of all the immediately following steps.The alternation Step/Transition and Transition/Step is always maintained in SFC element connections, that is:

• two steps are never directly linked; they are always separated by a transition;
• two transitions are never directly linked; they are always separated by a step.

When the clearing of a transition leads to the activation of several steps at the same time, the sequences to which these steps belong are called simultaneous sequences. After their simultaneous activation, the evolution of each of these sequences becomes independent. In order to emphasize the special nature of such constructs, the divergence and convergence of simultaneous sequences is indicated by a double horizontal line.The clearing time of a transition may theoretically be considered as short as one may wish, but it can never be zero. In practice, the clearing time will be imposed by the PLC implementation: several transitions which can be cleared simultaneously will be cleared simultaneously, within the timing constraints of the particular PLC implementation and the priority constraints defined in the sequence evolution table. For the same reason, the duration of a step activity can never be considered to be zero. Testing of the successor transition condition(s) of an active step shall not be performed until the effects of the step activation have propagated throughout the POU in which the step is declared.Sequence Evolution TableThis table defines the syntax and semantics of the allowed combinations of steps and transitions.

Example	Rule
	Normal transitionAn evolution from step S3 to step S4 takes place if and only if step S3 is in the active state and the transition condition c is TRUE.
	Divergent transitionAn evolution takes place from S5 to S6 if and only if S5 is active and the transition condition e is TRUE, or from S5 to S8 only if S5 is active and f is TRUE and e is FALSE.
	Convergent transitionAn evolution takes place from S7 to S10 only if S7 is active and the transition condition h is TRUE, or from S9 to S10 only if S9 is active and j is TRUE.
	Simultaneous divergent transitionAn evolution takes place from S11 to S12, S14,… only if S11 is active and the transition condition b associated to the common transition is TRUE.After the simultaneous activation of S12, S14, etc., the evolution of each sequence proceeds independently.
	Simultaneous convergent transitionAn evolution takes place from S13, S15,… to S16 only if all steps above and connected to the double horizontal line are active and the transition condition d associated to the common transition is TRUE.

Examples

Invalid scheme	Equivalent allowed scheme	Note
		Expected behaviour: an evolution takes place from S30 to S33 if a is FALSE and d is TRUE.The scheme in the leftmost column is invalid because conditions d and TRUE are directly linked.
		Expected behaviour: an evolution takes place from S32 to S31 if c is FALSE and d is TRUE.The scheme in the leftmost column is invalid because direct links flow only downwards. Upward transitions can be performed via jump blocks.