• 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
flowlines - connects steps and transitions (these basically indicate sequence)
transition - causes a shift between steps, acts as a point of coordination
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.
