23.4 PARALLEL PROCESS FLOWCHARTS

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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,

23.4.1 Implementation with Microcontroller

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

For practice,