Special instructions
1000
Part III FP Instructions
F142_WDT
Watchdog timer update
The scan ‘check watchdog timer’ is automatically set at the start of a scan with the value of the
system register (No. 30). To monitor the transit of a processing block, set the watchdog timer with
this instruction immediately before transition and set again immediately after that.
This instruction also exists as a P instruction (for FP2/2SH, FP3/5, FP10/10SH PLC types), which
is only executed at the rising edge of the EN trigger. Select [Insert P instruction] from the
"Instructions" pane if you require a P instruction. To facilitate reuse, the instruction then appears
under "Recently used" in the pop-up menu. Press <Ctrl>+<Shift>+<v> within the programming
area to open the list of recently used elements.
PLC types
Availability of F142_WDT (see page 1321)
Variable Data type Function
s INT specifies watchdog timer value
Description
The scan ‘check watchdog timer’ is preset with the constant specified by s* if the trigger EN is in
the ON-state. The value specified by s* is 1 to 255 and the preset time becomes 2.5 ms * s* (637.5
ms).
Data types
Example
In this example the function has been programmed in ladder diagram (LD) and structured text
(ST).The same POU header is used for all programming languages.
POU header
All input and output variables used for programming this function have been declared in the POU
header.
Body
When the variable start is set to TRUE, the function is carried out.
LD
ST
When programming with structured text, enter the following:
IF start THEN
(* Watchdog timer value is changed to 123.4ms *)
F142_WDT(1234);
END_IF;
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