Structured languages

The orthogonal step-and-transition language of IEC 61131-3:

Sequential Function Chart (SFC)

State-machine notation: steps + transitions + actions. SFC is not a “fifth equal language” — it is a structuring layer that sits on top of any of the other four (ST, IL, LD, FBD). The action body of each step can be written in whichever of the four languages best fits. SFC is the IEC-blessed way to encode sequential machine logic without inventing a step counter by hand. (Stub.)