• Parallel Processes can happen separately but must eventually work together. This happens when PLC’s control multistage machines, etc.


• PLC’s are well suited to this problem.


• There is a method (based on Petri nets) that goes by a number of names,

- GRAFCET - developed in france

- IEC848 - the standard developed from GRAFCET

- SFC (Sequential Function Charts) - Allen Bradley version


• The basic elements are,

flowlines - connects steps and transitions (these basically indicate sequence)

transition - causes a shift between steps, acts as a point of coordination


initial step - the first step


step - basically a state of operation. A state often has an associated action


macrostep - a collection of steps (basically a subroutine)


selection branch - an or - only one path is followed


simultaneous branch - an and - both (or more) paths are followed




• The example below shows a SFQ for a a controller that is watching two separate doors. One door requires a two digit entry code, the second door requires a three digit entry code.



• SFC editors are available from many companies.


• SFCs can also be converted to ladder logic the same way state diagrams were.


• Consider the example below,










1.4.1 Implementation with Microcontroller


• We could also write a program for the basic stamp microcontroller,



• For practice,