This subject is presented as a Guideline for Languages in Section 6 of 88.00.02. The graphic display of procedural control as presented in IEC 60848 (circa 1985) is popular. It seemed to be a good fit for displaying procedural elements and their order of execution. So, SP88 proceeded to see what needed modifying, but any committee starts thinking about the worst case and loses the simplicity.

IEC 60848 describes a simple structure in which each step is followed by a transition condition. If the transition condition expression below an active step becomes true then the active step turns off and the following step becomes active. There are rules for choosing paths or splitting into parallel paths and later rejoining to one path. Any path through a set of procedural elements can be described. The rules are easily learned. The result is called a sequential function chart (SFC), which is one of the languages of IEC 61131-3.

The first problem is that procedural elements don't have transition conditions until you get down to the steps below a phase. The procedural element simply does what its procedure tells it to do until it is done. The procedural function chart (PFC) could have used transition conditions labeled Done. They aren't needed at all on simple paths, but they are needed for parallel splits and choices. You might think you want to see the names of the choices by the transition conditions when there is a choice, but the names of...