740
FX
3U
/FX
3UC
Series Programmable Controllers
Programming Manual - Basic & Applied Instruction Edition
34 SFC Program and Step Ladder
34.1 SFC Program
Special auxiliary relays
For efficiently creating SFC programs, it is necessary to use some special auxiliary relays. The table below
shows major ones.
*1.
Processed when END instruction is executed.
Operation of state relays and use of an output
twice or more
• In different state relays, a same output device (Y002 in
this example) can be programmed as shown in the right
figure.
In this case, when S21 or S22 is ON, Y002 is output.
However, if the same device as an output coil (Y002) in
a state relay is programmed in a ladder block program
or if a same output coil is programmed twice in one state
relay, it is handled in the same way as general double
coil.
Interlock of outputs
• In the state relay ON status transfer process, both
states turn ON only instantaneously (during one
operation cycle) at the same time.
Accordingly, between a pair of outputs which should not
be set to ON at the same time, provide an interlock
outside the PLC in conformance to the handy manual of
the PLC so that simultaneous ON can be prevented.
In addition, provide interlock in the program as shown in
the right figure.
Device
number
Name
Function and application
M8000
RUN monitor
This relay is normally ON while the PLC is in the RUN mode.
Use this relay as the program input condition requiring the normally driven status or for
indicating the PLC operation status.
M8002
Initial pulse
This relay turns ON and remains ON only instantaneously when the PLC mode is changed from
STOP to RUN.
Use this relay for the initial setting of a program or for setting the initial state relay.
M8040
STL transfer
disable
When this relay is set to ON, transfer to the ON status is disabled among all state relays.
Because programs in state relays are operating even in the transfer disabled status, output coils
do not turn OFF automatically.
M8046
*1
STL state ON
When any state relay turns ON, this relay automatically turns ON.
Use this relay to prevent simultaneous start up of another float or as process ON/OFF flag.
M8047
*1
Enable STL
monitoring
When this relay is driven, the device number of a state relay in the ON status having the
smallest device number among S0 to S899 and S1000 to S4095 is stored to D8040, and the
state relay number in the ON status having the next smallest device number is stored to D8041.
In this way, up to eight state relays in the ON status are stored up to D8047.
•
In the FX-PCS/WIN(-E), FX-20P(-E), and FX-10P(-E), when this relay is driven, the state
relays in the ON status are automatically read and displayed.
For the details, refer to the manual of each peripheral equipment.
•
In the SFC monitor in GX Developer, the automatic scroll monitoring function is valid even if
this relay is not driven.
21
22
20
Y001
Y002
Y002
When S21 or
S22 is ON,
Y002 is output.
21
20
Y001
Y002
Y001
Y002
Forward
rotation
Backward rotation
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