Practical answers you can verify.
FX5U and GX Works3 guidance written as engineering references—not artificial community posts. Every answer shows its source and review date.
ANALOG
How many built-in analog channels does an FX5U have?
The FX5U CPU has two built-in 0–10 V analog inputs and one built-in 0–10 V analog output. FX5UC CPUs do not include this built-in analog function.
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ANALOG
How do I scale an FX5U 0–10 V input to 0–10 bar?
For a raw range of 0–4000 and an engineering range of 0–10.0 bar, multiply the raw value by 100 and divide by 4000. The result then represents bar × 10.
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ANALOG
Why is my FX5U analog value unstable?
First separate an electrical-noise problem from a real process fluctuation. Check shield grounding, cable routing, sensor supply, common wiring and the raw value before changing the program.
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GX WORKS3
How do I connect GX Works3 directly to an FX5U by Ethernet?
Connect the PC to the CPU Ethernet port, open Online → Current Connection Destination, select Ethernet direct connection and run Communication Test.
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GX WORKS3
How do I connect GX Works3 to an FX5U through a network switch?
Configure the FX5U Ethernet-port own-node settings, place the PC in the same IP subnet, and select connection via hub using the CPU IP address.
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GX WORKS3
Should I read the PLC program before making an FX5U change?
Yes. Read and save a dated backup of the running PLC before editing, then compare it with the project you intend to change.
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GX WORKS3
Why did my FX5U Ethernet parameter change not take effect?
After writing applicable Ethernet parameters, the CPU may need a reset or power cycle before the new settings become active.
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TROUBLESHOOTING
Where can I check FX5U errors in GX Works3?
Open Module Diagnostics while online. It provides CPU/module status and diagnostic information that should be checked before changing logic.
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HIGH-SPEED I/O
How many built-in high-speed counter channels can an FX5U use?
The FX5 CPU built-in counter supports up to eight channels, but the maximum frequency per channel depends on CPU size and counter configuration.
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HIGH-SPEED I/O
How do I calculate RPM from FX5U pulse counts?
RPM = pulses counted × 60,000 ÷ (encoder pulses per revolution × sample time in milliseconds). Use latched sample data, not a continuously changing accumulator.
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COMMUNICATION
Which channel numbers are used for FX5U serial communication?
For FX5U, the built-in RS-485 port is CH1, a communication board is CH2, and communication adapters are assigned CH3 and CH4 in order from the CPU side.
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COMMUNICATION
How many Modbus slaves can an FX5U master connect?
Mitsubishi states up to 32 stations over RS-485 and one station over RS-232 for the FX5 Modbus master function.
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COMMUNICATION
Why is my FX5U RS-485 device not communicating?
Check the selected serial channel, polarity, common/reference connection, station number and the complete serial format before changing the PLC program.
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LADDER LOGIC
How do I create a motor start/stop seal-in circuit in FX5U ladder?
Place the healthy stop and overload conditions in series, then place the start contact in parallel with the output holding contact before the motor output coil.
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LADDER LOGIC
How do I latch and reset an alarm in FX5U ladder?
SET the alarm memory when the fault occurs, and RST it only when the reset command is present and the original fault is clear.
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PROGRAMMING
Why did an FX5U value change after power loss?
Check whether the device range is configured as latched/retentive and whether the project parameters currently in the CPU match the offline project.
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PROGRAMMING
What should be initialized on the first PLC scan?
Initialize only values that must return to a known startup state. Do not overwrite retained production data, calibration or recipes unless that is the documented requirement.
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GX WORKS3
How do I avoid losing FX5U device comments?
Treat comments and labels as project data: include them in the applicable read/write operation and keep the dated GX Works3 project backup with the machine documentation.
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GX WORKS3
Should I compile an FX5U project before writing it?
Yes. Rebuild or compile the edited project, resolve relevant errors and warnings, then compare and write only the intended data.
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GX WORKS3
Should I use USB or Ethernet to connect GX Works3 to an FX5U?
Use the interface supported by the exact CPU and site conditions. Ethernet is convenient for networked access; USB is useful for a direct local connection on supported FX5 models.
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SAFETY
What should I check before an FX5U online program change?
Confirm authorization, the correct PLC, a current backup, machine safe state, the exact change scope and a recovery plan before transferring anything.
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