Advanced Function Instruction
7-76
FUN 91 P
RSWDT
RESET WATCHDOG TIMER
FUN 91 P
RSWDT
This instruction has no operand.
z When execution control "EN" = 1 or "EN↑" ( P instruction), the WDT timer will be reset (i.e. WDT will start
timing again from 0).
z The functions of WDT have already been described in FUN90 (WDT instruction).
The operation principles of watch dog timer are as follows:
The watchdog timer is normally implemented by a hardware one-shot timer (it can not be software,
otherwise if CPU fail, the timer becomes ineffective, and safeguards are quite impossible). "One-shot"
means that after triggered the timer once, the timing value will immediately be reset to 0 and timing will
restart. If WDT has begun timing, and never triggered it again, then the WDT timing value will continue
accumulating until it reach the preset value of N, at that time WDT will be activated, and PLC will be shut
down. If trigger the WDT once every time before the WDT time N has been reached, then WDT will never
be activated. PLC can use this feature to ensure the safety of the system. Each time when PLC enters into
system housekeeping after finished the program scanning and I/O refresh, it will usually trigger WDT once,
so if the system functions normally and scan time does not exceed WDT time then WDT is never activated.
However, if CPU is damaged and unable to trigger WDT, or the scan time is too long, then there will not be
enough time to trigger WDT within the period N, WDT will be activated and will shut off PLC.
z In some applications, when you set the WDT time (FUN90) to desire, the scan time of your program in certain
situations may temporarily exceed the preset time of WDT. This situation can be anticipated and allowed for,
and you naturally do not wish PLC to shut down for this reason. You can use this instruction to trigger WDT
once and avoid the activation of WDT. This is the main purpose of this instruction.
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