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SFC Basic Operation Section 2-1
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2-1 SFC Basic Operation
This section describes the behavior of an SFC program using a basic step progression
control.
Step Progression Control
Step progression control is a method of program control in which the execution of each
step, consisting of an entire process, is sequential or time-dependant.
SFC is a graphical programming language developed to allow step progression control
to be easily implemented. Step Ladder has been widely used as a language for
describing step progression control, but SFC has visual properties superior to
traditional Step Ladder.
Consider the following example: “Execute Process 1 until Condition A is met, then stop
Process 1 and execute Process 2”. The following shows the behavior of an SFC
program using basic step progression control (Process 1 is executed and, if condition A
is met, process 2 is executed) as an example.
Step 1
Assign Process 1 to Action 1.
Assign Process 2 to Action 2.
Assign Condition A, the transition condition between steps, to
Trans 1.
Step 2
Trans1
Action 1
Action 2
A step is either active or inactive.
An active step executes the actions defined in that step.
If the condition for the step to transfer to the next step is met, then the immediately
preceding step becomes inactive and the immediately succeeding step becomes
active.
The following illustrates that the transition condition, "Trans1" is met and that the active
status has transferred from step 1 to step 2.
The step showing "
z
" is the active step.
Step 1 z
Conditions Met for
Trans 1
Step 2
Trans1
Action 1
Action 2
Step 1
Step 2 z
Trans1
Action 1
Action 2
All the actions within an active step are executed for at least one cycle, even if the
condition for the step to transfer to the next step has been met (Note, however, that the
execution of some actions may be prevented by AQs).