TABLE OF CONTENTS The definition of phase needs to be clarified. "The lowest level of procedural element in the procedural control model" defines where it is, not what it is.
Many years ago, an elderly woman confronted a lecturer in astronomy with the assertion that the world was flat and supported on the back of a very large turtle. The lecturer asked the lady what the turtle was standing on. "Oh no," she said. "You can't fool me. It's turtles all the way down." And so it is with the humble sequential function chart (SFC). An SFC may express a procedure from the top level all the way on down to the smallest sequence in basic control.
The difference between an SFC in a control module and one in an equipment module is the method of execution. A control module uses sequential control, which is one of the many methods of basic control. Sequential control requires that the end of the sequence wrap around and return to the initial step. An equipment module uses procedural control, which executes an SFC from beginning to end but does not wrap around. It just stops, until the next time that procedural control runs the SFC again.
Both the control and equipment modules may run sequences. A sequence is a phase if it is executed by a procedural controller and it simply ends when it is done. The difference between a control and an equipment module hangs by that slender...
