Mitsubishi Electric MELSEC iQ-F Series Programming Manual page 105

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 79 Coil HOLD step [SC])
Ex.
Example of transition operation (without continuous transition)
STOPRUN (SM321=ON)
(1) (2) (3) (4) (1)
1 scan
■Execution of action
The action performs the following operation for the instruction input condition described in the action depending on the status
of the step to be executed.
Execution of action
Not executed
Execution in the contact ON status
Execution in the contact OFF status
When the step is activated, the action is executed in the contact ON status every scan. When the step is deactivated, the
action is executed in the contact OFF status and will not be executed until next time the step is activated. The action is
executed in the contact OFF status in the following cases. Execution in the contact OFF status is enabled only in active steps.
• When the transition just after a normal step becomes TRUE.
• When the end step is executed (other than steps that hold operations when SM327 is on and remaining active steps that do
not hold operations when SM328 is on)
• When the block is forcibly terminated by using the SFC control instruction (Ending a block).
• When the step is forcibly terminated by the SFC control instruction (Ending a step).
• When SM321 (Start/stop SFC program) is turned off
• When the reset step [R] that has been set to reset the running step is activated
• When the reset step [R] that has been set to reset the steps that hold operations is activated and S999 has been specified
for the reset step [R]
(2) (5) (4) (1) (6)
1 scan 1 scan
Description
The input condition is not reflected in the output.
The operation is performed according to the input condition.
The input condition is set to off regardless of the actual input condition, and the operation is performed.
(1) All the active steps in the block are executed
within a single scan.
(1)
(1) Execution of sequence program
(2) Execution of action
(3) Checking the transition to the next step (FALSE)
(4) END processing
(5) Checking the transition to the next step (TRUE)
(6) The next action is executed.
8.5 SFC Program Execution Order
8 SFC PROGRAM
103
8