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6 Step Ladder Instructions
6.1 Step Ladder
94
FXCPU Structured Programming Manual
[Basic & Applied Instruction]
• Special auxiliary relays
For efficiently creating step ladder programs, it is necessary to use some special auxiliary relays. The table
below shows major ones.
*1. Processed when END instruction is executed.
•Block
When there are relay ladder blocks and
step ladder 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 (during one operation cycle) 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
Use this relay to prevent simultaneous startup of another flow or as a process ON/OFF flag.
When M8047 is OFF, M8046 is normally OFF.
When M8047 is ON, M8046 operates as follows:
FX3S PLC
When at least one among the state relays S0 to S255 is ON: ON
When all of the state relays S0 to S255 are OFF: OFF
FX3G/FX3GC/FX3U/FX3UC PLCs
When at least one among the state relays S0 to S899 and S1000 to S4095 is ON: ON
When all of the state relays S0 to S899 and S1000 to S4095 are OFF: OFF
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 S255 (FX3S PLC), S0 to S899 and S1000 to S4095 (FX3G/FX3GC/FX3U/FX3UC
PLCs) 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 in registers up to D8047.
Relay ladder
M8002
Initial pulse
Step ladder
Step 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
Y000
S0
STL
EN ENO
s
SET
EN ENO
d S20
X000
SET
EN ENO
d S1
SET
EN ENO
d S0
Y001
S20
STL
EN ENO
s
X001
S0
RET
EN ENO
S1
STL
EN ENO
s
SET
EN ENO
d S30
X002
Y003
S30
STL
EN ENO
s
X003
S1
RET
EN ENO
Y001