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