Watch Dog Timer (Wdl) - Mitsubishi Electric MELSEC-A Series User Manual

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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.
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