Mitsubishi Electric MELSEC FX Series - Page 104

6 Step Ladder Instructions
6.1 Step Ladder
102
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
This relay automatically turns ON M8046 when any of the state relays S0 to S899 or S1000 to S4095 turn
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 device is driven, the state relays in the ON status on the programming function are
automatically read and displayed.
For details, refer to the manual of each peripheral equipment.
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