7 Applied Instructions
7.8 External FX I/O Device
398
FXCPU Structured Programming Manual
(Basic & Applied Instruction)
Handling in the event of occurrence of watchdog timer error
1. Cause of occurrence of watchdog timer error
Watchdog timer error may occur in the following cases.
1) When many special extension equipments are connected.
In a configuration of a large number of connected units of special extension equipments (positioning, cam
switch, link, analog, etc.), the initializing time of the buffer memory executed in PLC RUN mode is long,
and the operation time is extended, and a watchdog timer error may occur.
2) When FROM/TO instructions are driven simultaneously.
When many FROM/TO instructions are executed, or when multiple buffer memories are transferred, the
operation time is extended, and a watchdog timer error may occur.
2. Countermeasure
1) Method of using RBFM, WBFM instruction
→ As for BFM divided reading [RBFM], refer to section 7.25.1.
→ As for BFM divided writing [WBFM], refer to section 7.25.2.
2) Method of changing the time of watchdog timer
The detection time of the watchdog timer can be changed by rewriting the content of D8000 (watchdog
timer time).
By entering the following program, the subsequent sequence programs are monitored by the new
watchdog timer time.
3) Change of execution timing of FROM/TO instruction
Please shorten the operation time by deviating the execution of FROM/TO instruction.
Handling of special extension unit/block
As for the connection method of special extension unit/block, the number of units allowed to be connected,
and handling of input and output numbers, please refer to the manual of the PLC main body or the manual of
each special extension unit/block.
M8002
Initial pulse
Watchdog timer time 300ms
0
Watchdog timer refresh
When WDT instruction is not programmed,
the value of D8000 is valid during END process.
MOV
EN
s
ENO
d
WDT
EN ENO
D8000K300
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