Watchdog Timer [D8000] - Mitsubishi FX3U Series Programming Manual

FX /FX Series Programmable Controllers
3U 3UC
Programming Manual - Basic & Applied Instruction Edition

36.2.2 Watchdog timer [D8000]

The watchdog timer monitors the operation (scan) time of the PLC. When the operation is not completed
within the specified time, ERROR (ERR) LED light turns on and all outputs are turned OFF.
When the power is initially turned ON, "200 ms" is transferred from the system to D8000 as the default value.
For executing a program beyond 200 ms, the contents of D8000 must be changed by the user program.
1. Example program
M8002
0
Initial pulse
2. When a watchdog timer error occurs
A watchdog timer error may occur in the following cases. Add the above program to somewhere near the first
step or adjust the number of execution FROM/TO instructions at the same scan.
1) When using many special function units/blocks
When many special function units/blocks (such as positioning, cam switches, link and analog) are used,
buffer memory initial setting time becomes long at turning on the PLC, thus extending the operation time
and allowing the possibility for a watchdog timer error to occur.
2) When executing many FROM/TO instructions at the same time
When many FROM/TO instructions are executed or when many buffer memories are transferred, it
extends the scan time, and a watchdog timer error may occur.
3) When using many high speed counters (software counters)
When many high speed counters are programmed and high frequency is counted at the same time, it
extends the scan time, and a watchdog timer error may occur.
3. How to reset the watchdog timer
The watchdog timer can be reset in the middle of a sequence program using WDT (FNC 07) instruction.
It is recommended to reset the watchdog timer by WDT (FNC 07) instruction when the scan time of a
particular sequence program is extended or when many special function units/blocks are connected.
4. Cautions on changing the watchdog timer
The watchdog timer time can be set to a maximum of 32767 ms. However, CPU error detection is delayed
when the watchdog timer time is extended.
It is recommended to use the default value (200 ms) when no problems are to be expected in operation.
830
FNC 12
K300 D8000
MOV
The watchdog timer is refreshed.
FNC 07
When WDT (FNC 07) instruction is not programmed, the
WDT
value stored in D8000 is not valid until END processing.
36 Operation of Special Devices (M8000 -, D8000 -)
36.2 Supplement of Special Devices (M8000 - and D8000 -)
Setting value of watchdog timer is set to "300 ms."
→ For WDT (FNC 07) instruction, refer to Section 8.8.