Cutler-Hammer D320 PLC, User Manual

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152
D320 PLC User's Manual
Instruction
Mnemonic Clear Watchdog Time Range
■
Bit
Word
WAT WAT: Clear watchdog time
Double words
Ladder
WAT: Clears the watchdog timer while executing the program.
Description
1. This instruction clears the watchdog timer within the CPU module to prevent the program
from stopping even if the scan time exceeds the maximum watchdog time. The default
watchdog time is 3 seconds.
2. Under normal operation, the PLC executes the following process:
Read external inputs.
Process the control program.
Update the external outputs.
One execution of this process is termed a scan. When the time it takes to process a single scan
(the scan time) is excessively long, abnormal results may occur caused by the delay in reading
inputs and updating outputs. For this reason, a watchdog time is set by the PLC which, when
exceeded, indicates that an error has occurred. When this happens, the PLC stops the program
to prevent abnormal operation.
3. Under certain circumstances, extremely lengthy scan times may be allowable. The WAT
instruction allows the user to reset the watchdog timer to prevent the PLC from automatically
going into the error condition and stop mode when the watchdog time is exceeded.
4. This operation will occur on every scan for which the input condition is true (On). To perform
the operation only on a change of input condition, use the rising/falling edge contact.
Example
Program Expression Operation Results
In certain applications, the user program may contain
loops which cause lengthy scan times. In the example,
turning on M0.0 prevents the PLC from stopping when
the watchdog time (maximum of 3 sec) is exceeded. For
normal PLC control applications, this instruction should
not be used.
WAT
R
WAT
M0.0