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FX Series Programmable Controlers Applied Instructions 5
5-12
5.1.6 WDT (FNC 07)
Operation:
The WDT instruction refreshes the PLC’s watchdo
g
timer. The watchdo
g
timer checks that the pro
g
ram
scan (operation) time does not exceed an arbitrar
y
time
limit. It is assumed that if this time limit is exceeded
there is an error at some point. The PLC will then cease operation to prevent an
y
further errors
from occurrin
g
. B
y
causin
g
the watchdo
g
timer to refresh (drivin
g
the WDT instruction) the
usable scan (pro
g
ram operation) time is effectivel
y
increased.
Points to note:
a) When the WDT instruction is used it will operate on ever
y
pro
g
ram scan so lon
g
as its input
condition has been made.
To force the WDT instruction to operate for onl
y
ONE scan requires the user to pro
g
ram
some form of interlock. FX users have the additional option of usin
g
the pulse (P) format of
the WDT instruction, i.e. WDTP.
b) The watchdo
g
timer has a default settin
g
of 100
msec for FX PLC’s and 200 msec for FX
0
/FX
0N
/
FX
2N
PLC’s. This time limit ma
y
be customized to
a users own requirement b
y
editin
g
the contents of
data re
g
ister D8000, the watchdo
g
timer re
g
ister.
Mnemonic Function
Operands
Program steps
D
WDT
FNC 07
(Watch do
g
timer refresh)
Used to refresh
the watch do
g
timer durin
g
a
pro
g
ram scan
N/A
Can be driven at an
y
time within the main
pro
g
ram bod
y
WDT, WDTP:
1 step
X0
WDT
END
END
WDT
Main program
Main program pt1
Main program pt2
Program scan
time 120 msec
Program(pt2) scan
time 60 msec
Program(pt1) scan
time 60 msec
X0
WDTP
X0
WDT
X0
Excuted every program scan
FX only
All PCs general
operation
Excuted in the
first program scan
M8000
MOV K150 D8000