Mitsubishi FX3U-128MR/ESS: FX3U Series Modules,

Technical Dossier

Product Details And Specifications

Mitsubishi Electric FX3U Series Technical Notes

The Mitsubishi Electric FX3U series represents the third-generation flagship of the MELSEC-F family of compact programmable logic controllers. Deployed across global heavy industry sectors — including petrochemical refineries, nuclear auxiliary systems, offshore platforms, and continuous-process chemical plants — the FX3U platform established a benchmark for high-density I/O integration within a compact DIN-rail form factor. Its 64K-step program memory, built-in USB programming port, and support for up to 384 I/O points (via extension modules) made it the dominant choice for machine-level automation from its introduction in 2004 through the mid-2020s. The FX3U-128MR/ESS specifically provides 64 relay-output points and 64 DC input points in a single base unit, making it one of the highest-density standalone controllers in the FX3U lineup.

The Evolution of FX3U Architecture

The FX3U succeeded the FX2N series (introduced 1998), inheriting its extension bus architecture while doubling program capacity and adding native USB connectivity. Key architectural milestones:

FX1S / FX1N (2000–2003): Entry-level fixed I/O controllers; no extension bus. Incompatible with FX3U expansion modules.
FX2N (1998–2010): Established the extension module bus standard used by FX3U. FX2N expansion blocks are electrically compatible with FX3U base units via the same 34-pin bus connector.
FX3U (2004–present, mature phase): 64K steps, USB port, high-speed counters to 100 kHz, built-in RTC. Supports FX3U-specific special function modules (positioning, analog, communication).
FX3UC (compact variant): Connector-type terminal blocks; reduced footprint; same instruction set as FX3U.
FX5U (2016–present, successor): Ethernet-native, 128K steps, SD card slot. Not backward-compatible with FX3U extension modules. Mitsubishi officially positions FX5U as the migration target for new FX3U designs.

The FX3U series entered its mature/end-of-active-design phase circa 2020. Mitsubishi Electric continues to supply spare parts and repair services, but new system designs are directed toward the iQ-F (FX5) platform. This makes long-term spare parts availability a critical operational concern for installed FX3U-based systems.

FX3U Full Catalog & Functionalities (SKU List)

Base Units — Relay Output:

FX3U-16MR/ES: 8 DC inputs / 8 relay outputs, 16-point base unit, 100–240 VAC supply.
FX3U-32MR/ES: 16 DC inputs / 16 relay outputs, 32-point base unit, AC supply.
FX3U-48MR/ES: 24 DC inputs / 24 relay outputs, 48-point base unit, AC supply.
FX3U-64MR/ES: 32 DC inputs / 32 relay outputs, 64-point base unit, AC supply.
FX3U-80MR/ES: 40 DC inputs / 40 relay outputs, 80-point base unit, AC supply.
FX3U-128MR/ES: 64 DC inputs / 64 relay outputs, 128-point base unit, AC supply.
FX3U-128MR/ESS: 64 DC inputs / 64 relay outputs, 128-point base unit, AC supply — safety-certified variant with enhanced EMC compliance.

Base Units — Transistor Output (Sink/Source):

FX3U-32MT/ES: 16 DC inputs / 16 transistor sink outputs, 32-point, AC supply.
FX3U-64MT/ES: 32 DC inputs / 32 transistor sink outputs, 64-point, AC supply.
FX3U-128MT/ES: 64 DC inputs / 64 transistor sink outputs, 128-point, AC supply.
FX3U-32MT/ESS: 16 DC inputs / 16 transistor source outputs, 32-point, AC supply.
FX3U-64MT/ESS: 32 DC inputs / 32 transistor source outputs, 64-point, AC supply.
FX3U-128MT/ESS: 64 DC inputs / 64 transistor source outputs, 128-point, AC supply.

Communication & Network Adapter Modules:

FX3U-232ADP-MB: RS-232C Modbus RTU master/slave adapter; mounts left of base unit.
FX3U-485ADP-MB: RS-485 Modbus RTU adapter; supports multi-drop networks up to 32 nodes.
FX3U-USB-BD: USB programming board; installs in base unit option slot; replaces RS-422 cable.
FX3U-ENET: 100BASE-TX Ethernet module; supports SLMP and MC protocol; mounts on right extension bus.

Analog I/O Modules:

FX3U-4AD: 4-channel analog input; 12-bit resolution; ±10 V / 4–20 mA selectable per channel.
FX3U-4DA: 4-channel analog output; 12-bit resolution; ±10 V / 4–20 mA selectable per channel.
FX3U-4AD-PT-ADP: 4-channel PT100 RTD temperature input adapter.

Sourcing Hard-to-Find & Obsolete FX3U Parts

As the FX3U series transitions from active production to long-term support status, procurement of specific base unit variants — particularly high-density models such as the FX3U-128MR/ESS and FX3U-128MT/ESS — becomes increasingly constrained through standard distribution channels. DriveKNMS maintains a dedicated inventory of new-surplus, refurbished, and tested-serviceable FX3U components sourced from decommissioned plant equipment, authorized overstock, and controlled secondary markets.

Quality Control for the FX3U Range

FX3U base units incorporate a multi-layer backplane bus connecting the CPU, I/O circuits, and extension connector. DriveKNMS applies a structured test protocol to all FX3U units prior to dispatch:

Power Supply Verification: Input voltage range test (85–264 VAC for AC-supply units); inrush current measurement; 24 VDC service output load test.
CPU Functional Test: Program write/read cycle via GX Works2; RTC accuracy verification; USB port enumeration check.
I/O Point Scan: 100% point-by-point input continuity and output switching test using automated fixture; relay contact resistance measurement (target <100 mΩ per point) for MR-type units.
Extension Bus Integrity: FX3U-specific 34-pin bus loopback test; communication handshake verification with FX3U-4AD and FX3U-485ADP-MB reference modules.
High-Speed Counter Test: 100 kHz pulse injection on designated HSC inputs (X000–X005); count accuracy verification against reference counter.
Burn-In: 48-hour powered cycle at 40°C ambient prior to final inspection.