4.
A2ccPu
/MELSEC-A
4.1.8
Watch
dog
timer
(WDT)
The watch dog timer is an internal timer used to detect errors
in
the PC's repeated operation function.
It
also monitors the time of
one scan of
a
sequence program.
(1) Watch dog timer setting
The watch dog timer default value is 200 ms.
This setting can be changed to between 10
and
2000 ms (in
intervals of 10 ms) using
a
parameter.
(2) Watch dog timer operation
(a)
The watch dog timer is reset after the execution of
the
(END]
instruction as long as
PC
operation is normal (scan
time is within the setting).
(b)
A
watch dog timer error will occur if processing is not
completed within the predetermined time due to
a
long
sequence program scan time or faulty hardware. In this
case, operation stops.
(3)
Response to watch dog timer errors
After the watch dog timer counts up, the following processing
occurs.
(a) Infinite loop
1) All outputs
(Y)
are turned
off
electrically.
However, se uence program operation continues
though the
&I
key switch is set for
[STOP/
because
switching from
[=I
tom] is to be done after the
execution of the
w1
instruction.
Test operation of the A2CCPU or read/write/verify of
a
sequence program using the peripheral is not possible.
2) The A2CCPU
can
be monitored with
a
peripheral.
3)
Error code "25" is stored in D9008.
(b) END instruction executed after the watch dog timer setting
expires
1) All outputs
(Y)
are turned
off
electrically.
Only sequence program operation continues (even
when the switch is set in the
w]
position).
2) After the exeuction of the instruction, the
3)
After the execution
of
the
rml
instruction, error code
4) The scan time required until the execution of the
A2CCPU can communicate with a peripheral.
"22" is stored in 09008.
instruction is stored in D9019 and D9018.
Scan time
(Fl
counting value)
Fl
error occurs
if
the
(WDT]
counter value
exceeds the setting
END
t~l
reset
(PI
is reset
when
(ENDI
is
executed)
Fig.
4.7
Resatting
Watch
Dog
Timer
4-9
I6
INA)
66232-A
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