GX Works3 Cannot Connect to an FX5U: Ethernet and Adapter Troubleshooting

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Before changing an FX5U’s parameters, establish which connection GX Works3 is trying to use. A laptop connected directly to the CPU needs a different setup from one connected through a machine switch. A USB-to-Ethernet adapter adds another detail: GX Works3 must use that network interface, even though it plugs into a USB socket.

This guide covers the FX5U CPU’s built-in Ethernet port. Connections through an additional Ethernet module, a GOT or a serial adapter require their own settings.

Start by saving the exact communication-error message and a screenshot of the current connection destination. Leave the PLC program and operating mode unchanged while investigating the laptop-side connection.

First, identify where the connection breaks

“The PLC will not connect” can describe several different problems. The following observations help narrow the search without changing the controller.

Observation	Useful next check
Windows does not show the Ethernet adapter	Adapter installation, USB connection and driver status
The adapter appears, but reports the cable disconnected	Cable, connectors, powered Ethernet port and adapter
Windows shows a link, but GX Works3 fails	Selected adapter, connection method and CPU-side interface
Direct connection works, but connection through the switch fails	IP addressing, network path and MELSOFT connection configuration
Another laptop connects through the same cable and port	Differences in adapter selection, software setup and security policy
Communication succeeds, but a read or write is refused	The exact access or operation error, rather than the physical connection

These are diagnostic clues. For example, another laptop connecting successfully makes the first laptop worth investigating; it does not identify its adapter as defective.

Select the adapter that actually carries the cable

A maintenance laptop may have Wi-Fi, a docking-station Ethernet port, a USB network adapter and virtual interfaces installed at the same time. The name “Ethernet” alone may not identify the port connected to the PLC.

Open a Windows command prompt and run:

ipconfig /all

Find the adapter carrying the PLC connection and note its description, IPv4 address and subnet mask. Microsoft documents this command as displaying the full TCP/IP configuration of physical and logical adapters.

A USB-to-Ethernet adapter should appear as a network interface. If Windows does not recognise it correctly, resolve that problem before adjusting GX Works3. Use the adapter manufacturer’s supported driver for the installed Windows version.

For direct connection, explicitly select the relevant adapter in GX Works3 where the selection is available. Mitsubishi documents this option for FX5 CPUs. FX5 Hardware Manual, section 16.6

The USB plug does not make this a USB programming connection. The communication path from the adapter to the FX5U is Ethernet.

If the problem began after changing docks or adapters, revisit this selection first. The cable may now be attached to a different interface from the one previously used.

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Use direct connection only for a direct cable

For a point-to-point connection to the built-in port:

Connect the laptop’s Ethernet interface directly to the FX5U.
Open Online → Current Connection Destination.
Select Ethernet in the direct-connection setting.
Select the physical network adapter carrying the cable.
Run Communication Test.

In the detailed settings, the built-in CPU port uses Ethernet Board on the PC side and CPU Module on the PLC side, with Ethernet Port Direct Connection selected.

Mitsubishi’s FX5U training manual explains that direct connection does not require entering the destination IP address or host name. That makes it useful when the installed address is unknown.

There are still exceptions. The Ethernet manual notes that some PC network settings can prevent direct communication; setting an appropriate PC IP address may then be necessary. It also warns against using direct-connection mode on a LAN or through a hub.

On an operating machine, disconnecting the existing Ethernet cable could interrupt an HMI or another controller. Arrange a suitable maintenance window before changing that connection.

Through a switch, use the actual PLC address

When a switch sits between the laptop and the FX5U, use the connection-via-HUB method and specify the CPU’s actual address.

For this path, review:

The laptop’s address and subnet mask.
The CPU address selected in the connection destination.
The CPU’s existing MELSOFT connection configuration.

Mitsubishi’s Ethernet manual describes MELSOFT connection setup under the CPU’s Ethernet-port parameters. On an existing machine, inspect the approved configuration rather than downloading a new configuration simply to test connectivity.

A practical IPv4 example

Mitsubishi lists the default FX5 CPU address as 192.168.3.250, with subnet mask 255.255.255.0. An installed machine may use something entirely different.

For an isolated test where the CPU is confirmed to retain that default:

Device	Example IPv4 address	Subnet mask
FX5U	192.168.3.250	255.255.255.0
Laptop Ethernet adapter	192.168.3.10	255.255.255.0

The laptop address must be unused. With this subnet mask, the first three octets match while the host addresses differ. Apply the setting to the Ethernet adapter connected to the PLC, not to Wi-Fi. Mitsubishi FAQ 22319

On a plant network, obtain an approved address instead of choosing one from this example. Changing the PLC address to match a laptop can disrupt every device that already communicates with it.

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Use Ping to answer a limited question

Once the CPU address is confirmed, a Ping test can help assess IP reachability. For the default-address example:

ping 192.168.3.250

Ping sends ICMP echo requests. A reply demonstrates an IP response from that address; it does not test a GX Works3 engineering session.

The practical interpretation is:

Ping succeeds, GX Works3 fails: investigate the engineering connection settings, security filtering and target identity.
Ping fails: revisit the address, subnet and network path. A timeout alone is insufficient evidence of a damaged CPU.
GX Works3 connects directly despite an unsuccessful Ping to a guessed address: establish the actual CPU address before drawing conclusions from that Ping result.

Keep the Windows network test and the GX Works3 communication test separate in the service notes. They answer different questions.

Isolate laptop-side problems without rewriting the PLC

Mitsubishi identifies PC security software as a possible cause of blocked Ethernet communication. Use an approved application allowance or an IT-supervised test rather than leaving protection disabled.

For an adapter or docking-station problem, change one item at a time. A known-good cable, followed by a known-good network adapter, produces a useful comparison. Replacing the adapter, changing the IP address and reinstalling software together makes the result harder to interpret.

If GX Works3 runs inside a virtual machine, the guest operating system must have a usable path to the physical Ethernet connection. The host laptop recognising the adapter does not establish that the guest can use it. Have the virtual-network configuration reviewed; a permitted test outside the virtual machine can help isolate that layer.

Before escalating the issue, collect the GX Works3 version, Windows version, adapter model, selected connection method and exact error message. “USB adapter does not work” leaves several distinct causes unresolved.

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Frequently Asked Questions

Do I need a crossover cable to connect directly to an FX5U?

No. Mitsubishi specifies Category 5 or higher STP Ethernet cable and permits a straight cable. A crossover cable can also be used for direct PC connection. Buying a crossover cable is therefore not the first remedy for a failed connection.

Can I use a USB cable instead of Ethernet?

Do not confuse an FX5U with an FX5UJ or FX5S. The shared hardware manual’s direct USB procedure applies to FX5UJ and FX5S CPUs. A USB-to-Ethernet adapter connected to an FX5U still requires Ethernet settings in GX Works3.

Must I know the PLC’s IP address before trying direct connection?

Not normally. Mitsubishi documents direct FX5U connection without entering the destination IP address or host name. Use a genuine point-to-point cable connection, select the correct adapter and run the communication test. PC network-setting exceptions may still need attention.

Why does direct connection work while connection through the switch fails?

The switch path introduces addressing and network configuration requirements that direct connection does not test in the same way. Review the actual CPU address, laptop subnet, connection-via-HUB selection and existing MELSOFT settings. A successful direct test is useful evidence, but it does not validate the switched path.

Should I set the laptop to 192.168.3.250?

No, not when that is the PLC’s address. The devices need distinct addresses. The default PLC address is a starting reference, not a value to assign to both devices or assume on every installed machine.

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Confirm the connection before making changes

After Communication Test succeeds, confirm that the connected CPU is the intended controller. Read diagnostics or authorised project information before attempting a download.

There is no reason to initialise PLC memory merely because the laptop could not connect. If Ethernet parameters genuinely need changing, preserve the existing configuration and plan the interruption: Mitsubishi’s documented parameter-writing procedure includes a CPU reset or power cycle to activate the relevant settings.

For a confirmed hardware replacement requirement, contact Kwoco with the complete FX5U reference and a clear description of the port involved. Distinguish the CPU’s built-in Ethernet port from any installed expansion module so the enquiry concerns the correct component.

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GX Works3 Cannot Connect to an FX5U: Ethernet and Adapter Troubleshooting

Table of Contents

First, identify where the connection breaks