Delta Electronics Programmable Logic Controller DVP-PLC - Manual (Page 226)

6 Application Instructions API 00-49
DVP-PLC Application Manual 6-14
API Mnemonic Function
07 WDT P Watchdog Timer Refresh
Controllers
ES/EX/SS SA/SX/SC EH/SV
OP Descriptions Program Steps
N/A WDT, WDTP: 1 steps
PULSE 16-bit 32-bit
ES EX SS SA SX SC EH SV ES EX SS SA SX SC EH SV ES EX SS SA SX SC EH SV
Explanations:
1. No operand.
2. The watchdog timer in DVP series PLCs is used for monitoring the operation of the PLC system.
3. WDT instruction can be used to reset Watch Dog Timer. If the PLC scan time (from step 0 to END or when
FEND instruction is executed) exceeds 200ms, PLC ERROR LED will flash. The user will have to turn off PLC
and back On again. PLC will determine RUN/STOP status by RUN/STOP switch. If there is no RUN/STOP
switch, PLC will return to STOP status automatically.
4. When to use WDT:
a) When errors occur in the PLC system.
b) When the executing time of the program is too long, resulting in the scan time being larger than the content in
D1000, the user can improve the problem by the following two methods.
„ Using WDT instruction
t1 t2
STEP0 END(FEND)
WDT
„ Using the set value in D1000 (default value: 200ms) to change the time for watchdog.
Program Example:
Assume the scan time of the program is 300ms, divide the program into two parts and place WDT instruction in the
middle of the two parts, making scan time of the first half and second half of the program being less than 200ms.
X0
END
END
WDT
300ms program
150ms program
150ms program
Dividing the program to two parts
so that both parts' scan time are
less than 200ms.
Watchdog timer reset