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7 Applied Instructions (Program Flow)
7.8 WDT / Watchdog Timer Refresh
126
FXCPU Structured Programming Manual
[Basic & Applied Instruction]
Cautions
1) Instructions of pulse operation type are not provided in the FX0S, FX0 or FX0N PLC.
To execute pulse operation, make the instruction execution condition pulse type.
2) A watchdog timer error may occur in the following cases. To avoid the error, input a program shown
below near the head step to extend the watchdog timer time, or shift FROM/TO instruction execution
timing.
- Caution when many special extension devices are connected.
In such configuration that many special extension devices (such as positioning units, cam switches,
analog units and link units) are connected, the buffer memory initialization time may become longer,
thus the operation time may become longer, and a watchdog timer error may occur.
- Caution when many FROM/TO instructions are driven at one time.
When many FROM/TO instructions are executed or when many buffer memories are transferred, the
operation time may become longer, and a watchdog timer error may occur.
- Caution when there are many high speed counters (software counters).
When many high speed counters are provided and high frequency are counted at one time, the
operation time may become longer, and a watchdog timer error may occur.
3) The watchdog timer time can be changed.
By overwriting the contents of D8000 (watchdog timer time), the watchdog timer detection time (initial
value: 200 ms) can be changed.
By inputting the program shown below, the sequence program after this insertion is monitored by a new
watchdog timer time.
Program examples
1. When the operation cycle is long and causes an error
For example, by dividing a program whose operation cycle is 240 ms into two portions and inserting WDT
instruction between them, the operation cycle becomes less than 200 ms in both the former half portion and
the latter half portion.
2. When a pointer number or ladder block label of CJ instruction is located in a step number
smaller than the step number of CJ instruction
Put WDT instruction after the pointer number or ladder block label.
If an input relay (X) is used as the
command contact, input refresh is
disabled, so the program
execution cannot be returned from
the area between Pn and CJ.
As the command contact, use
such device that can be set to
OFF in a program being jumped.
M8002
Initial pulse
Watchdog timer time 300ms
0
Watchdog timer refresh
If WDT (FNC 07) instruction is not programmed,
the value of D8000 is valid during END processing.
MOV
EN
s
ENO
d
K300
D8000
WDT
EN ENO
END
Program whose
operation cycle
is 240 ms
END
Program whose
operation cycle
is 120 ms
WDT
EN ENO
Program whose
operation cycle
is 120 ms
M8000
RUN monitor
Command
Program
When the command
contact turns ON
WDT
EN ENO
CJ
EN ENO
p
Pn
Pn: