769
FX
3U
/FX
3UC
Series Programmable Controllers
Programming Manual - Basic & Applied Instruction Edition
34 SFC Program and Step Ladder
34.2 Step Ladder
31
FN
C
275-FN
C
279
Da
ta
Tr
ans
fe
r 3
32
FN
C
280
-F
N
C
28
9
H
igh Speed
Pr
oces
si
ng 2
33
FN
C
290-FN
C
299
Ext
ensi
on Fi
le
R
egi
st
er
34
SF
C
•S
T
L
Pr
o
g
ra
m
m
in
g
35
In
te
rru
p
t
Funct
ion
36
Sp
e
cia
l De
vic
e
37
E
rro
r C
o
d
e
A
V
e
rs
ion U
p
In
fo
rm
a
tio
n
B
Ex
ec
ut
ion Ti
m
e
s
C
Appl
ic
ab
le
In
st
ru
ct
ion Li
st
Special auxiliary relays
For efficiently creating step ladder programs, it is necessary to use some special auxiliary relays. The table
below shows major ones.
The special auxiliary relays shown below are the same as those available in SFC programs.
*1.
Processed when END instruction is executed.
Block
A step ladder program is created as ladder circuits in the
same way as relay ladder. Accordingly, different from
SFC programs, it is not necessary to divide blocks for
relay ladder parts and SFC parts.
When there are ladder blocks and SFC blocks, put RET
instruction at the end of each step ladder program.
A PLC starts the step ladder processing by STL
instruction, and returns to the relay ladder processing
from the step ladder processing by RET instruction.
However, when consecutively programming a step
ladder in a different flow (when there is no relay ladder
before the step ladder in the different flow), RET
instruction between flows can be omitted, and RET
instruction can be programmed only at the end of the last
flow.
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 of 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 a 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.
SET
S0
Relay ladder
SET
S1
M8002
Y000
X000
X001
Y001
Initial pulse
STL
S0
SET
S20
S20
SET
S30
STL
S30
S0
RET
X002
Y003
S1
S1
RET
X003
Step ladder
Step ladder
Relay ladder
Step ladder is
started by STL
instruction.
It can be
omitted.
Step ladder is
finished by RET
instruction.
Step ladder is
started by STL
instruction.
Step ladder is
finished by RET
instruction.
STL
Y002
END
STL