Mitsubishi Electric MELSEC FX3U Programming Manual page 249

FX /FX /FX Series Programmable Controllers
3G 3U 3UC
Programming Manual - Basic & Applied Instruction Edition
Cautions
1. When a watchdog timer error occurs
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.
1) 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.
2) 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
scan time may become longer, and a watchdog timer error may occur.
3) 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.
2. 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.
M8002
0
Initial pulse
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.
Program
whose
operation cycle
is 240 ms
2. When a label (P) of CJ instruction is located in a step number smaller than the step number
of CJ instruction
Put WDT instruction after the label (P).
M8000
Label
Pn
RUN monitor
Command
→ For details on changing watchdog timer time, refer to Subsection 37.2.2.
FNC 12
K300
MOV
Watchdog timer refresh
FNC 07
If WDT (FNC 07) instruction is not programmed, the value of
WDT
D8000 is valid during END processing.
END
FNC 07
WDT
When the command
Program
contact turns ON
FNC00
Pn
CJ
8 Program Flow – FNC 00 to FNC 09
8.8 FNC 07 – WDT / Watchdog Timer Refresh
Watchdog timer time: 300 ms
D8000
Program whose
operation cycle
is 120 ms
FNC07
WDT
Program whose
operation cycle
is 120 ms
END
If an input relay (X) is used as the
command
disabled, so the program execution
cannot be returned from the area
between P and CJ.
As the command contact, use such
device that can be set to OFF in a
program being jumped.
contact, input refresh
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