Mitsubishi Electric MELSEC FX3U Programming Manual page 473

FX /FX /FX Series Programmable Controllers
3G 3U 3UC
Programming Manual - Basic & Applied Instruction Edition
Acceptance of interrupts while FROM/TO instruction is executed (M8028)
1. While M8028 is OFF
While a FROM/TO instruction is being executed, interrupts are automatically disabled. Input interrupts and
timer interrupts are not executed.
Interrupts generated during the execution of FROM/TO instructions are immediately executed after the
FROM/TO instruction completes.
FROM/TO instructions can be used in interrupt programs.
2. While M8028 is ON
When an interrupt is generated during the execution of a FROM/TO instruction, the FROM/TO operation is
momentarily paused while the interrupt program executes. FROM/TO instructions cannot be used in interrupt
programs.
Action against watchdog timer error
1. Cause of watchdog timer error
A watchdog timer error may occur in the following cases:
1) When many special extension equipment is connected
When many special extension equipment (such as positioning units, cam switches, link units and analog
units) are connected, considerable time may be required to initialize buffer memories when the PLC
mode is set to RUN, the operation time may be long, and a watchdog timer error may occur.
2) When many FROM/TO instructions are driven at the same time
When many FROM/TO instructions are driven at the same time or when many buffer memories are
transferred, the operation time may be long, and a watchdog timer error may occur.
2. Countermeasures
1) Using RBFM (FNC278) or WBFM (FNC279) instruction [Ver.2.20 or later]
2) Changing the watchdog timer time
By overwriting the contents of D8000 (watchdog timer time), the watchdog timer detection time can be
changed.
When the program shown below is input, the sequence program after the input will be monitored with the
new watchdog timer time.
M8002
0
Initial pulse
3) Changing FROM/TO instruction execution timing
Shift FROM/TO instruction execution timing to make the operation time shorter.
Handling of special extension units/blocks
For the special extension unit/block connection method, number of connectable special extension units/
blocks and handling of I/O numbers, refer to the manuals of the PLC and each special extension unit/block.
→ For divided BFM read [RBFM (FNC278) instruction], refer to Section 31.1.
→ For divided BFM write [WBFM (FNC279) instruction], refer to Section 31.2.
FNC 12
K300 D8000
MOV
Watchdog timer refresh
FNC 07
If WDT (FNC 07) instruction is not programmed, the value of
WDT
D8000 is valid during END processing.
15 External FX I/O Device – FNC 70 to FNC 79
15.9 FNC 78 – FROM / Read From A Special Function Block
Watchdog timer time: 300 ms
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