Mitsubishi Electric MELSEC-Q Series Programming Manual page 58

Precautions to be taken when SM328 is turned ON
The following gives the precautions to be taken when SM328 is turned ON
• 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.
• 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.
• Do not describe an always satisfied transition condition immediately after the operation HOLD step (with transition check).
• 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.
• 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.
Restart after end step execution
After end step execution, a restart is performed as described below.
Block No.
Block 0 START condition of block 0 is set to "Auto
START ON" in the SFC setting of the PLC
parameter dialog box
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
4 SFC PROGRAM CONFIGURATION
56
4.2 Steps
(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).
aSM400
aM0
Restarting Method
Execution automatically returns to the initial step again, and processing is executed
repeatedly.
A restart is made when any of the following is executed.
• When another START request is received from another block (when the block START step
is activated)
• When the block START instruction of the SFC control instructions is executed
• When the block START/END bit of the block information devices is forcibly turned ON