Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 73)

Planning Your System Programs
Chapter 6
6-2
Each step corresponds to a control task (displayed as a box); each step is
related to a program file that contains the logic for the associated control
task. Each transition (displayed as a horizontal line) examines conditions,
specified in an associated program file, that determines when the processor
can continue to the next task.
Deciding How to Use an SFC
After you identify the major areas of machine operation, convert the
logical paths and steps that you labeled in your design specification to SFC
building blocks. Table 6.A helps explain when to use which SFC
building blocks.
Important: At this point, do not worry about the actual logic for each step
and transition. After you complete the SFC, you can develop the logic.
Table 6.A
Deciding When to Use the SFC Structures
If you have: Then draw: Using these rules:
An independent machine state A step with its transition A step must always be followed by a transition.
A clearly defined chain of events that
occur sequentially
For example, in one heatĆtreating
area, the temperature must ramp up
at a particular rate; maintain the
temperature for a certain duration,
then cool at a particular rate.
A simple path of steps
and transitions
For design purposes, number steps and transitions
consecutively from 2.
Start the path with a step; end the path with a transition.
Two or more alternative paths where
only one is selected
For example, depending on a build
code, one station must either drill or
polish.
A selection branch The transitions beginning each path are scanned from left
to right. The first true transition determines the path taken.
Two or more parallel paths that must
be scanned simultaneously at
least once
For example, communications and
block transfers must occur while
control logic is executing.
A simultaneous branch All paths are active in the structure.
You can define up to 7 parallel paths.
002
Initial
Step
003
010
013
008
012
006
011
009
007
004 005
014
015