Advanced Function Instruction
7 -7 5
FUN 90
P
WDT
WATCHDOG TIMER
FUN 90
P
WDT
N : The watchdog time. The range of N is 5~120, unit
in 10mS (i.e. 50ms~1.2 sec)
z
When execution control "EN" = 1 or "EN
↑
" (
P
instruction) transition from 0 to 1, will set the watchdog time
to Nx10ms. If the scan time exceeds this preset time, PLC will shut down and not execute the application
program.
z
The WDT feature is designed mainly as a safety consideration from the system view for the application. For
example, if the CPU of PLC is suddenly damaged, and there is no way to execute the program or refresh I/O,
then after the WDT time expired, the WDT will automatically switch off all the I/Os, so as to ensure safety. In
certain applications, if the scan time is too long, it may cause safety problems or problems of
non-conformance with control requirements. This instruction can used to establish the limitation of the scan
time that you require.
z
Once the WDT time has been set it will always be kept, and there is no need to set it again on each scan.
Therefore, in practice this instruction should use the
P
instruction.
z
Default WDT time is 0.25 sec.
z
For the operation principles of WDT please refer to the RSWDT(FUN 91) instruction.
