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FX3G/FX3U/FX3UC Series Programmable Controllers
Programming Manual - Basic & Applied Instruction Edition
8 Program Flow – FNC 00 to FNC 09
8.8 FNC 07 – WDT / Watchdog Timer Refresh
1
Introduction
2
Overview
3
Instruction
List
4
Devices
in Detail
5
Specified the
Device &
Constant
6
Before
Programming
7
Basic
Instruction
8
FNC00-FNC09
Program Flow
9
FNC10-FNC19
Move & Compare
10
FNC20-FNC29
Arith. & Logic
Operation
8.8 FNC 07 – WDT / Watchdog Timer Refresh
Outline
This instruction refreshes the watchdog timer in a sequence program.
1. Instruction format
2. Set data
3. Applicable devices
Explanation of function and operation
When the operation cycle (time until END or FEND instruction is executed after the step 0) of a PLC exceeds 200 ms,
a watchdog timer error (indicating abnormal operation) occurs; The CPU error LED lights, and the PLC stops.
When the operation cycle is long, insert WDT instruction in the middle of the program to avoid the watchdog timer
error.
Related device
Cautions
1. When a watchdog timer error occurs
A watchdog timer error may occur in the following cases. To avoid the error, input a program shown below near the
head step to extend the watchdog timer time, or shift FROM/TO instruction execution timing.
1) Caution when many special extension devices are connected
In such configuration that many special extension devices (such as positioning units, cam switches, analog units
and link units) are connected, the buffer memory initialization time may become longer, thus the operation time
may become longer, and a watchdog timer error may occur.
2) Caution when many FROM/TO instructions are driven at one time
When many FROM/TO instructions are executed or when many buffer memories are transferred, the scan time
may become longer, and a watchdog timer error may occur.
3) Caution when there are many high speed counters (software counters)
When many high speed counters are provided and high frequency are counted at one time, the operation time
may become longer, and a watchdog timer error may occur.
Operand type Description Data type
– There is no set data. –
Oper-
and
Type
Bit Devices Word Devices Others
System User Digit Specification System User
Special
Unit
Index
Con-
stant
Real
Number
Charac-
ter String
Pointer
XYMTCSD.b KnX KnY KnM KnS T C D R U\G VZModifyKH E ""P
– There are no applicable devices.
Device Name Description
D8000 Watchdog timer time Up to 32767 ms can be set in units of ms (initial value: 200 ms).
P
FNC 07
WDT
WDT
Continuous
Operation
Pulse (Single)
Operation
Mnemonic Operation Condition
Independent Inst.
1 step
WDTP
FNC 07
WDTP
Command
input
One scan is executed.
Command
contact
Command
input
Each scan is
executed.
FNC 07
WDT
Command
contact