Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

23.4 PARALLEL PROCESS FLOWCHARTS

 

· 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,

  1. - GRAFCET - developed in france
  2. - IEC848 - the standard developed from GRAFCET
  3. - SFC (Sequential Function Charts) - Allen Bradley version

 

· The basic elements are,

  1. flowlines - connects steps and transitions (these basically indicate sequence)
  2. transition - causes a shift between steps, acts as a point of coordination
  3.  
  1. initial step - the first step
  2.  
  1. step - basically a state of operation. A state often has an associated action
  2.  
  1. macrostep - a collection of steps (basically a subroutine)
  2.  
  1. selection branch - an or - only one path is followed
  2.  
  1. simultaneous branch - an and - both (or more) paths are followed
  2.  
  1.  

 

· 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,

 

 

 

 

 

 

 

 

 

23.4.1 Implementation with Microcontroller