Mitsubishi Electric MELSEC-Q Structured Programming Manual page 45

FXCPU Structured Programming Manual
[Device & Common]
2. Latched (backed-up) type
When the power is turned OFF while the PLC is operating, all of output relays and general type auxiliary
relays turn OFF.
Even when the power is restored after that, all of output relays and general type auxiliary relays remain OFF
except those whose input condition is ON. In some output relays and auxiliary relays, however, the ON/OFF
status just before power interruption should be stored and then replicated when the power is restored,
depending on control targets. In such a case, use latched (backed-up) type auxiliary relays.
Latched (backed-up) type devices are backed up by a battery, EEPROM, etc. built in the PLC.
X000 X001
M600
Backup against power interruption
(self-holding circuit)
X000
X001
Backup against power interruption
(set/reset circuit)
1) Application example of latched (backed-up) type auxiliary relays
Limit switch
Left
LS 1(X000)
limit
Table in reciprocating motion
X000 X001
M600
X001 X000
M601
2) Method to use a fixed latched (backed-up) type auxiliary relay as a general type auxiliary relay
When using a fixed latched (backed-up) type auxiliary relay as a general type auxiliary relay, provide a
reset circuit shown in the figure below around the head step in the program.
M8002
Initial pulse
→ For details on the backup method against power interruption, refer to Section 1.2.
M600
PLC
SET
EN ENO
d
M600
RST
EN ENO
d
M600
PLC
Limit switch
Right
LS 2(X001)
limit
Motor
with brake
Rightward
M600
drive command
Leftward
M601
drive command
ZRST
EN ENO
d1
M1024
d2
M1999
The left figure shows an operation example of M600
(latched type device) in a self-holding circuit.
When X000 turns ON and M600 turns ON in this circuit,
M600 holds its operation by itself even if X000 turns OFF.
Because M600 is a latched (backed-up) type device, it
remains activated even when the operation is restarted
after X000 has turned OFF due to power interruption. If a
normally-closed contact of X001 is opened when the
operation is restarted, however, M600 is deactivated.
The left figure shows a circuit using the SET and RST
instructions.
In some cases, the table should be restarted in the same
direction as the direction selected just before power
interruption.
X000 = ON (at the left limit) → M600 = ON → The table is
driven rightward. → The power is turned OFF. → The table
stops in an intermediate position. → The table is restarted
(M600 = ON). → X001 = ON (at the right limit) → M600 =
OFF, M601 = ON → The table is driven leftward.
2 Devices in Detail
2.3 Auxiliary relay [M]
1
2
3
4
5
6
7
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