Replacing Mitsubishi FX3U with FX5U: I/O, Wiring and Program Migration Checklist

Review Expansion Modules One by One

Do not assume that every module beside the FX3U can move across with it. Mark each item in the equipment list as retain, replace, or redesign, with the relevant compatibility condition beside it.

For supported FX3 extension hardware, Mitsubishi provides bus conversion modules. FX5-CNV-BUS serves the cable-type connection arrangement; FX5-CNV-BUSC serves the connector-type arrangement, with additional connection requirements depending on the configuration. Their availability does not establish compatibility for an unlisted module. Mitsubishi iQ-F options

Module order also matters. Mitsubishi states that iQ-F modules cannot be added downstream of the bus conversion module; they belong before it. 

For analog equipment, write down the signal range, scaling, filtering, and response expected by the machine. If a replacement channel produces a different raw value for the same physical measurement, that difference needs to be resolved before the value reaches a limit comparison or control calculation.

Check the completed assembly’s power requirements and physical layout before finalizing the order.

Save More Than the Ladder Program

Keep an unchanged copy of the original engineering project. Where access permits, compare it with the running PLC and collect the current parameters and operating data needed for recovery.

Production setpoints deserve particular attention. A project saved during commissioning may not contain the values the machine uses today. Record where those values come from: retained PLC devices, an HMI recipe, a supervisory system, or an initialization routine.

Confirm that the backup opens on the engineering computer intended for the migration. Locate the authorized passwords and required software before the shutdown. Preserve the original files separately from all conversion work.

Move the Project into GX Works3

Mitsubishi documents importing an FX3-series .gxw project through:

Project → Open Other Format File → GX Works2 Format → Open Project

Select the FX5U target as prompted and save the result as a separate project. 

If the available project was created in GX Developer, GX Works2 provides an import route for that format. Establish a usable GX Works2 project before proceeding with the GX Works3 conversion. 

Read the conversion results before editing away warnings. Mitsubishi’s migration handbook warns that instructions and devices may change, and parameter and intelligent-function-module settings require recreation in GX Works3.

Assign each warning or unresolved setting to a specific check. “Project converted” should mean the file is ready for engineering review—not that the machine is ready to run.

Give counters, timers, and communications their own review

Three items deserve an explicit search in the old project:

• C235–C255: FX3U high-speed counters require compatible-counter configuration and program changes for FX5U.
• T192–T199: Routine timers require parameter setup.
• Serial communication: Channel assignments can change; instructions and parameters must match the new arrangement.

These are documented migration conditions, not problems that disappear after a successful rebuild. 

For each function, define a test that reflects its use. An encoder test should check the expected count and direction. A timed sequence should be observed through the relevant machine states. Communication testing should confirm the actual data exchange, including writes where authorized.