Mitsubishi Electric MELSEC iQ-R Series Programming Manual page 141

Step execution sequence
In the SFC program, the actions of all active steps are processed within one scan.
When the action of each step is finished, whether the transition to the next step becomes TRUE or not is checked.
• When the transition has not become TRUE: The action of the same step is executed again in the next scan.
• When the transition has become TRUE: The outputs of the executed actions by using the OUT instruction are all turned off.
When the next scan is executed, the action of the next step is executed. The step executed previously is deactivated and
the action becomes inactive.
Even when the transition becomes TRUE, if coil HOLD step [SC] is set in the step attribute, the step is not deactivated but
performs processing according to the attribute. ( Page 97 Coil HOLD step [SC])
STOP→RUN
(SM321=ON)
(1) (2) (3) (4)
1 scan
Ex.
The continuous transition bit of an SFC information device is not specified.
(1) (2)
(3) (4)
(5) (6)
(7) (8)
(9)
Scan
Scan 1
Scan 2
Scan 3
Scan 4
Scan 5 and after
(1) (2) (5) (4) (1)
1 scan
Description
Step (1) is activated and the action (2) is executed.
Step (3) is activated and the action (4) is executed.
Step (5) is activated and the action (6) is executed.
Step (7) is activated and the action (8) is executed.
Step (7) is active until the transition (9) becomes TRUE, and the action (8) is executed.
All the active steps in the
block are executed within
a single scan.
(1) Execution of sequence program
(2) Execution of action
(3) Checking the transition to the next step
(FALSE)
(6)
(4) END processing
(5) Checking the transition to the next step (TRUE)
(6) The next action is executed.
1 scan
8.6 SFC Program Execution Order
8 SFC PROGRAM
139
8