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Programming Tips for Timers and Counters
4.6.2
Switch-off delay
By default, all the timers in MELSEC PLCs are delayed make timers, i.e. the output is switched
ON after the defined delay period. However, you will often also want to program a delayed
break operation (switch OFF after a delay). A typical example of this is a ventilation fan in a
bathroom that needs to continue running for several minutes after the lights are switched off.
Program version 1 (latching)
Ladder Diagram
X001
0
Y000
X001
5
As long as input X1 (e.g. a light switch) is on output Y0 (fan) is also on. However, the latching
function ensures that Y0 also remains on after X1 has been switched off, because timer T0 is
still running. T0 is started when X1 is switched off. At the end of the delay period (300 x 0.1s =
30s in the example) T0 interrupts the Y0 latch and switches the output off.
Signal sequence
X1
T0
Y0
Program version 2 (set/reset)
Ladder Diagram
X001
0
X001
2
T0
6
When X1 is switched on output Y0 is set (switched on). When X1 is switched off timer T0 is
started. After the delay period T0 then resets output Y0. The resulting signal sequence is iden-
tical with that produced by program version 1.
4 – 18
Y000
T0
K300
T0
SET Y000
K300
T0
RST Y000
Instruction List
0
LD
1
LD
2
ANI
3
ORB
4
OUT
5
LDI
6
OUT
30 s
t
Instruction List
0
LD
1
SET
2
LDI
3
OUT
6
LD
7
RST
Devices in Detail
X001
Y000
T0
Y000
X001
T0
K300
X001
Y000
X001
T0
K300
T0
Y000
MITSUBISHI ELECTRIC
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