Watchdog Timer Time [D8000]; Low Battery Voltage Detection [M8005 And M8006] - Mitsubishi Electric MELSEC-Q Structured Programming Manual

FXCPU Structured Programming Manual
[Device & Common]
4.2.2

Watchdog timer time [D8000]

The watchdog timer monitors the operation time of the PLC. If the PLC does not finish operation within the
specified time, it turns ON the following LED and turns OFF all outputs.
FX /FX /FX
3S 3G 3GC
FX /FX /FX
1S 1N 1NC
FX /FX /FX /FX
0S 0 0N
The system transfers "200 ms (or 100 ms)" as the initial value when the power is turned ON. When it is
necessary to execute a program which requires the scan time longer than 200 ms (or 100 ms), the watchdog
timer time can be changed using a program.
1. Program example
M8002
Initial pulse
2. When a watchdog timer error occurs
A watchdog timer error may occur in the following cases. Add the above program near the first step and
extend the watchdog timer time, or shift the FROM/TO instruction execution timing.
1) When many special function units/blocks are connected
If many special function units/blocks (for positioning, cam switch, link, analog, etc.) are connected, it
takes more time to initialize buffer memories when the PLC becomes the RUN mode. As a result, the
operation time becomes longer, and a watchdog timer error may occur.
2) When many FROM/TO instructions are driven at the same time
If many FROM/TO instructions are executed or many buffer memories are transferred, the operation time
becomes longer. As a result, a watchdog timer error may occur.
3) When many high speed counters (software counters) are used
If many high speed counters are programmed and they count high frequency at the same time, the
operation time becomes longer. As a result, 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 the WDT instruction.
This reset method is recommended when the operation time of a specific sequence program is long or when
many special function units/blocks are connected.
→ For the WDT instruction, refer to the FX Structured Programming Manual [Basic & Applied
4. Caution on changing the watchdog timer time
The watchdog timer time can be set to 32767 ms maximum. However, the operation error detection timing is
delayed if the watchdog timer time is long.
It is recommended to use the initial value (200 ms or 100 ms) when no problems are expected in operations.
The watchdog timer time becomes "0" when a value less than "0" is set to D8000.
4.2.3

Low battery voltage detection [M8005 and M8006]

These special devices detect low voltage in the lithium battery used for memory backup.
When the PLC detects low battery voltage, the BATT (BAT) LED turns ON. In FX
LED turns ON when the PLC detects low battery voltage if an optional battery is used and the battery mode is
selected by the parameter setting.
Use the following program to notify low battery voltage to the outside.
Program example
M8005
0
Low battery voltage
4.2 Supplement of Special Devices (M8000 and later and D8000 and later)
/FX /FX PLCs: ERROR (ERR) LED
3U 3UC
/FX /FX PLCs: CPU • E (ERROR) LED
2N 2NC
/FX PLCs: CPU • E LED
U 2C
MOV
EN ENO
s d
K300 D8000
WDT
EN ENO
"Low battery voltage"
Y001
M8006 latches low battery voltage.
4 Operations of Special Devices (M8000 and later, D8000 and later)
Watchdog timer time
300ms
If the watchdog timer time (WDT) instruction
is not programmed, the value stored in D8000
becomes valid at the END processing.
Instruction].
/FX PLCs, the ALM
3G 3GC
217
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