Watchdog Timer [D8000]; Battery Voltage Low Detection [M8005 And M8006] - Mitsubishi Electric FX3G SERIES Programming Manual
Programmable controllers
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FX
/FX
/FX
Series Programmable Controllers
3G
3U
3UC
Programming Manual - Basic & Applied Instruction Edition
37.2.2
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.
37.2.3
Battery voltage low detection [M8005 and M8006]
This special device detects low voltage in the lithium battery for memory backup.
In FX
/FX
PLCs, the BATT (BAT) LED turns ON when the PLC detects low battery voltage.
3U
3UC
In FX
PLCs, the ALM LED indicator turns ON when the PLC detects low battery voltage if the optional battery is
3G
installed and the battery mode is selected by the parameter setting.
Use the following program to notify low battery voltage to the outside.
M8005
0
Battery
voltage low
FNC 12
K300
D8000
MOV
The watchdog timer
FNC 07
When WDT (FNC 07) instruction is not programmed, the
WDT
value stored in D8000 is not valid until END processing.
"Battery voltage low"
Y001
M8006 latches the battery voltage low.
37 Operation of Special Devices (M8000 -, D8000 -)
37.2 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.
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