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4, A2CCPU
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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 msec.
This setting can be changed to between 10 and 2000 msec (in intervals
of 10 msec) using a parameter.
(2) Watch
dog
timer
operation
(a) The watch
dog
timer is reset after
the
execution of
the
END instruc-
tion
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) The A2C stops communication with a remote
module.*
However, sequence program operation continues
though
the
RUN key switch is set
for
STOP because swltchinq from RUN to
STOP is to be
done
after
the
execution of
the
END instruction.
2) The A2CCPU can be monitored with a peripheral.
Test operation of
the
A2CCPU or read/write/verify of a sequence
program using
the
peripheral is not possible.
3) Error
code
"25'
is stored in 09008.
(b) END instruction executed after
the
watch
dog
timer
setting expires
1) The A2C
stops
communication with a remote
module.*
Only sequence program operation continues (even when
the
RUN switch is set in
the
STOP position).
2) After
the
execution of
the
END instruction,
the
A2CCPU can
communicate with a peripheral.
3) After
the
execution of
the
END instruction, error
code
"25'
is
stored in 09008.
4) The scan time required until
the
execution of
the
ENO instruction
is stored in 09019 and 09018.
4
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