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4 SFC PROGRAM CONFIGURATION
MELSEC-Q
POINTS
The following gives the precautions to be taken when SM328 is turned ON
(1) When there is only the held step left at arrival at the end step, that held step is deactivated
if SM328 is ON.
When the user does not want to turn OFF the coil output of the held step suddenly, it can
be prevented by turning ON SM327.
(2) If a block is started at the block START step when SM328 is ON, execution returns to the
source as soon as there are no non-held active steps in the block.
(3) Do not describe an always satisfied transition condition immediately after the operation
HOLD step (with transition check).
Step n
Step (n+1)
Block n
Block m
Step m
Step (m+1)
3)
2)
ST
1)
aSM400
aM0
1) Since the transition condition is always
satisfied, step (m+1) remains an active
step (non-held active status).
2) If M0 turns ON and the transition
condition is satisfied, block m cannot
be ended.
3) Since block m is not ended, execution
cannot proceed to step (n+1).
(a) When the transition condition immediately after the operation HOLD step (with transition
check) is always satisfied, the next step is kept in a "non-held active status". Therefore,
the block cannot be ended when SM328 is ON.
Further, if this block has been started at the block START step (with END check),
processing cannot be returned to the START source step.
(b) When it is desired to describe an always satisfied transition condition immediately after
the operation HOLD step (with transition check), make provision so that the block can
be forcibly ended from outside.
(3) After end step execution, a restart is performed as described below.
Block No. Restarting Method
START condition of block 0 is
set to "Auto START ON" in the
SFC setting of the PLC
parameter dialog box
• Execution automatically returns to the initial step again, and
processing is executed repeatedly.
Block 0
START condition of block 0 is
set to "Auto START OFF" in the
SFC setting of the PLC
parameter dialog box
All blocks other than block 0
• A restart is made when any of the following is executed.
1) When another START request is received from another block
(when the block START step is activated)
2) When the block START instruction of the SFC control
instructions is executed
3) When the block START/END bit of the block information
devices is forcibly turned ON
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