Mitsubishi FX5U vs FX3U: Which PLC Fits Your 2025 Machine Upgrade?

Mitsubishi’s FX series has been the backbone of countless small and mid-sized machines. In 2025, most engineers are deciding whether to stay with the proven FX3U or move to the newer FX5U (iQ-F series) for new builds and upgrades.

This guide walks through FX5U vs FX3U from a practical engineering and lifecycle perspective: scan performance, I/O capacity, networking, motion, and migration. We’ll also point to real Mitsubishi hardware stocked by Industrial Automation Co. so you can move from planning to procurement without hunting for part numbers.

FX5U vs FX3U at a Glance

At a high level, FX3U is a mature, compact PLC platform. FX5U is its successor in the iQ-F family with significantly more built-in functionality and network-ready design.

Real Hardware Examples You Can Buy Today

Industrial Automation Co. stocks multiple FX3U base units and modules that are directly relevant when you’re deciding between staying with FX3U or planning an FX5U-based future system.

FX3U CPU Base Units

• Mitsubishi FX3U-32MR/ES-A base unit – 16 DC inputs / 16 relay outputs, 100–240 VAC.
• Mitsubishi FX3U-48MR-ES-A base unit – ideal for medium I/O count machines.
• Mitsubishi FX3U-80MR/ES-A base unit – 40 DC inputs / 40 relay outputs for larger panels.
• Mitsubishi FX3U-128MR/ES-A base unit – 128 I/O main unit for higher density systems.

FX3U Expansion & Support Modules

• FX3U-4LC Temperature Loop Controller – closed-loop temperature control for ovens, dryers, and heaters.
• FX3U-FLROM-16 Memory Cassette – program backup and extra storage for FX3U.
• FX3U-64MTDSS Transistor Base Unit – 32 DC inputs and 32 transistor outputs for high-speed output applications.
• FX3U-ENET-ADP Ethernet Module – adds Ethernet connectivity to FX3U systems.
• FX3U-4HSX-ADP High-Speed Positioning Module – for stepper/servo pulse control on FX3U.
• Mitsubishi PSU 50 power supply – designed for FX3U and FX5U series PLCs.

While FX5U CPU units themselves aren’t currently listed in our storefront, many shared peripherals (like PSU 50) and FX3U-side hardware are directly usable in mixed or transitional architectures.

Performance & Architecture: Where FX5U Pulls Ahead

CPU Speed, Memory, and Scan Headroom

FX3U was already a big step up from FX2N, with around 64K steps of program memory and basic instruction times around 0.065 µs. That’s more than enough for classic discrete control, but it can get tight when you start stacking PID loops, data logging, and messaging on the same CPU.

FX5U doubles program memory to roughly 128K steps and accelerates both basic and data instructions. In practical terms, that means:

• Shorter scan times with the same logic.
• More comfortable headroom for heavy math, recipe handling, and string/data manipulation.
• Space to add diagnostics, alarms, and maintenance logic that often get cut on older hardware.

Built-In Analog, Ethernet, and RS-485

On FX3U, almost everything beyond digital I/O is an add-on: analog modules, Ethernet, and some serial options. You can absolutely build a robust system, but it costs you panel space, power budget, module addressing, and more wiring.

FX5U changes that baseline. A typical FX5U CPU (e.g. FX5U-32M variants) gives you:

• 2 channels of 12-bit analog input.
• 1 channel of 12-bit analog output.
• 1× Ethernet port built-in.
• 1× RS-485 port built-in.

For a new design in 2025, that’s a big deal. Where an FX3U panel might have required an analog module, an Ethernet module, and a serial module, an FX5U design may be just CPU + I/O extensions. Fewer modules mean:

• Less wiring and fewer failure points.
• Lower overall power consumption.
• Simpler spares strategy (fewer unique part numbers).

High-Speed Counters and Motion

Both platforms can handle high-speed inputs and motion, but FX5U bakes more of it into the CPU itself.

Typical FX5U CPU capabilities include:

• Up to 8 high-speed counter channels at 200 kHz.
• Up to 4 axes of 200 kHz pulse output for positioning.

That makes FX5U a natural fit for:

• Indexing conveyors and feeders.
• Cut-to-length systems.
• Basic multi-axis pick-and-place or gantry setups.

FX3U can hit similar use cases, but it typically needs additional modules like FX3U-4HSX-ADP for more advanced motion. If you’re starting clean, FX5U gives you more “out of the box” capability before you even add a dedicated motion module.

Networking & Data: FX5U Is Built for Connected Machines

In many plants, Ethernet and serial communications are no longer “nice-to-haves.” HMIs, SCADA, historians, and remote support all assume the PLC is on the network.

FX3U Networking

To get Ethernet on FX3U, you’re typically using something like the FX3U-ENET-ADP Ethernet adapter . It works well, but:

• Consumes expansion slots and power.
• Adds one more module to configure and support.
• Can be a constraint on compact control panels with limited DIN rail space.

FX5U Networking

FX5U’s onboard Ethernet and RS-485 give you Modbus TCP/RTU, HMI/SCADA connectivity, and peer-to-peer PLC links without touching the expansion bus. Combined with SD-card logging, that makes it much easier to:

• Push production data to plant historians or MES.
• Debug intermittent issues with trend logs instead of guesswork.
• Support remote troubleshooting and firmware upgrades.

Lifecycle & Migration Strategy

FX3U remains widely deployed and well-understood. If you’re supporting an existing installed base, there’s a strong argument to keep like-for-like replacements on the shelf:

• Base units like FX3U-80MR/ES-A and FX3U-128MR/ES-A can slot into existing panels with minimal wiring changes.
• Modules like FX3U-4LC and FX3U-FLROM-16 keep legacy systems maintainable without a full redesign.

But for new projects or major refurbishments, FX5U’s lifecycle advantages are hard to ignore:

• Newer silicon and longer expected manufacturer support horizon.
• Modern engineering tools (GX Works3) with better diagnostics and structured programming.
• Higher ceiling for future expansion and feature creep.

Practical Migration Pattern

A common, low-risk migration path looks like this:

1. Keep FX3U spares (CPUs and key modules) on hand to cover unplanned downtime now.
2. Design new machines and major control panel rebuilds around FX5U, even if older lines stay on FX3U.
3. Standardize shared peripherals where possible (e.g. PSU 50 power supply used on both FX3U and FX5U).
4. Plan a phased migration of the most critical FX3U lines as time and budget allow.

Decision Logic: FX5U or FX3U for This Machine?

Here’s a simple way to formalize the choice in your project spec.

// Pseudo-logic for platform selection

IF new_machine == true THEN
    IF requires_ethernet OR requires_analog OR requires_motion THEN
        PLATFORM := FX5U
    ELSE IF I_O_count > 200 OR future_expansion_expected THEN
        PLATFORM := FX5U
    ELSE
        PLATFORM := FX3U   // small, simple, cost-driven machines
    END_IF
ELSE // existing machine
    IF drop_in_replacement AND downtime_risk_high THEN
        PLATFORM := FX3U
    ELSE
        CONSIDER_MIGRATION := true   // evaluate FX5U retrofit case-by-case
    END_IF
END_IF
  

Quick Rules of Thumb

• Choose FX3U when you need a drop-in CPU or module for a running system, or you’re building a small, standalone machine with no Ethernet or analog.
• Choose FX5U when you’re designing anything with networking, data logging, multiple drives/servos, or a 10+ year lifecycle expectation.

Example Application Scenarios

1. Simple Standalone Machine – FX3U Wins on Cost

Application: A small packaging helper machine (simple conveyor, sensor, diverter gate), 32–40 I/O, no remote access, and no analog process values.

In this case, a unit like FX3U-32MR/ES-A or FX3U-48MR-ES-A is usually the most cost-effective and simplest choice. You can keep the design compact and reuse the same program templates across multiple machines.

2. Multi-Axis Indexing Line – FX5U Is the Better Backbone

Application: A 3–4 axis indexing conveyor line with encoders for registration, analog feedback from load cells or tension sensors, integrated HMI, and SCADA connectivity.

Here, FX5U’s combination of:

• Built-in high-speed counters (for encoders).
• 4-axis 200 kHz pulse outputs (for stepper/servo drives).
• Built-in analog and Ethernet.

cuts down on hardware count and simplifies commissioning. The engineering team can also leverage GX Works3’s function blocks and structured programming for reusable motion templates.

Where Industrial Automation Co. Fits In

Whether you stay with FX3U or design forward with FX5U, having the right spares on hand is what actually keeps lines running. Industrial Automation Co. carries a wide range of Mitsubishi FX series parts with a 2-year warranty, including:

• FX3U base units like FX3U-80MR/ES-A and FX3U-128MR/ES-A .
• Expansion and specialty modules such as FX3U-4LC , FX3U-FLROM-16 , and FX3U-64MTDSS .
• Shared accessories like the PSU 50 power supply that support FX3U and FX5U architectures.

For a broader brand comparison, you can also point readers to your existing blog: Top PLC Brands Ranked: Features, Costs, and Benefits .

Summary: 2025 Recommendation

In 2025, the cleanest rule is:

• New designs and major upgrades: Standardize on FX5U.
• Legacy support and simple machines: Continue using FX3U where it makes economic sense.

FX3U remains a solid, field-proven choice when you just need discrete control and a fast drop-in spare. FX5U, however, is where you want to be for long-term, networked, and motion-heavy applications.

Ready to plan your platform strategy or build a spare-parts list? Browse our Mitsubishi FX3U catalog, or contact the Industrial Automation Co. team for help cross-referencing your existing PLCs and modules to in-stock parts.