Mitsubishi FX3U-128MR/ESS PLC Module – MELSEC FX3U Series

Mitsubishi FX3U-128MR/ESS — 128-Point Relay Output CPU with Native Ethernet in the MELSEC FX3U Platform

The FX3U-128MR/ESS occupies a well-defined position in Mitsubishi Electric’s compact PLC hierarchy: it is the highest-density relay-output variant of the FX3U base unit, and the only configuration in that output class to integrate a 10BASE-T/100BASE-TX Ethernet port directly on the CPU board. For procurement engineers specifying a controller that must handle 64 discrete inputs, switch 64 relay-contact outputs, and communicate over standard Ethernet infrastructure without auxiliary adapter modules, this unit eliminates three separate line items from the BOM — the base CPU, the Ethernet adapter (FX3U-ENET-ADP), and the associated DIN-rail space budget.

The relay contact architecture is the defining hardware characteristic. Each of the 64 output points is driven by an independent electromechanical relay rated at 2 A resistive at 240 VAC or 30 VDC, with a common bus rated at 8 A. This means the output stage can switch inductive loads — motor starters, solenoid valves, contactor coils — without external snubber networks or transient suppression diodes that transistor-output variants require. The mechanical contact also provides galvanic isolation between the PLC logic rail and the field load circuit, a property that simplifies compliance with IEC 60204-1 machinery safety wiring requirements.

The CPU executes basic instructions at 0.065 µs per step across a 64,000-step program memory (expandable with a memory cassette). The scan cycle follows the standard FX3U sequence: input refresh, program execution, output refresh, and peripheral service. At a 64,000-step program with typical mixed logic, the resulting scan time is well within the 10 ms range required for most discrete manufacturing applications. Six built-in high-speed counter channels operate at up to 100 kHz independently of the main scan cycle, enabling encoder position feedback without consuming scan-cycle bandwidth.

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Technical Parameters

Hardware Logical Analysis

The FX3U-128MR/ESS is built on Mitsubishi Electric’s third-generation FX compact CPU architecture, which separates the program execution engine from the I/O refresh bus using an internal dual-port RAM structure. This architecture allows the CPU to continue executing the current scan while the I/O refresh controller independently latches input states and drives output coils — a design that reduces effective I/O latency without requiring a dedicated co-processor.

Relay Output Driver Stage: Each relay output channel is driven by a dedicated relay driver IC that interfaces between the 5 VDC logic rail and the relay coil. The relay coil is suppressed by a flyback diode integrated on the PCB, preventing inductive kickback from propagating onto the logic supply. The relay contacts themselves are rated for 10 million mechanical operations under no-load conditions, and 200,000 operations at the 2 A rated resistive load — a figure that translates to approximately 5.5 hours of continuous 1 Hz switching at full load before the statistical end-of-life threshold is reached. For typical industrial duty cycles (1–10 operations per minute), this represents decades of service life.

EMC Design: The FX3U platform meets EN 61000-6-2 (industrial immunity) and EN 61000-6-4 (industrial emissions). The CPU board uses a multi-layer PCB with dedicated ground planes to contain high-frequency switching noise from the relay driver stage. Input circuits are optocoupler-isolated with a minimum isolation voltage of 500 VAC between the field wiring and the logic rail, providing protection against common-mode transients on the input side. The Ethernet PHY is magnetically isolated via integrated transformer in the RJ-45 connector assembly, meeting IEEE 802.3 isolation requirements.

High-Speed Counter Architecture: The six HSC channels are implemented in dedicated hardware counters clocked independently of the main CPU scan. Channels C235–C240 support single-phase counting; channels C241–C255 support two-phase (A/B quadrature) encoder input. The hardware counter registers are read by the CPU during the input refresh phase, ensuring that encoder position data is always current at the start of each scan — a critical requirement for closed-loop positioning applications where scan-cycle jitter would otherwise introduce position error.

Ethernet Stack Implementation: The built-in Ethernet port implements the MC Protocol (MELSEC Communication Protocol) in both 3E frame (binary) and 1E frame (ASCII) formats, allowing direct register read/write access from SCADA systems, HMI panels, and data historians without a gateway device. CC-Link IE Field Basic support enables the FX3U-128MR/ESS to act as a master controller for distributed I/O slaves over standard 100BASE-TX infrastructure, eliminating the need for proprietary CC-Link coaxial cabling in new installations.

System Integration Benefits

• Zero-adapter Ethernet connectivity: The /ESS suffix denotes factory-integrated Ethernet on the CPU board. No FX3U-ENET-ADP is required, freeing one expansion slot for analog or special function modules and reducing panel wiring complexity.
• Galvanic isolation on all I/O: Optocoupler isolation on inputs (500 VAC) and relay contact isolation on outputs provide two independent isolation barriers between field wiring and CPU logic, simplifying compliance with IEC 60204-1 and reducing ground-loop interference in multi-panel installations.
• Deterministic scan cycle: The dual-port RAM I/O architecture ensures that input data captured at the start of each scan remains consistent throughout program execution, eliminating mid-scan input state changes that can cause non-deterministic behavior in ladder logic with multiple coil references.
• Sub-millisecond interrupt response: Six dedicated interrupt input channels (I000–I050) trigger interrupt service routines with a response latency of less than 200 µs, enabling time-critical event handling — such as emergency stop processing or registration mark detection — independent of the main scan cycle length.
• Scalable I/O architecture: The right-side expansion bus supports up to 8 extension modules, scaling total I/O from 128 to 256 points. Mixed digital/analog expansion is supported; FX3U-4AD and FX3U-4DA modules integrate directly without additional power supplies.
• Backward-compatible migration path: FX3U ladder programs are structurally compatible with FX2N instruction sets. GX Works2 includes a project conversion wizard that migrates FX2N-128MR programs to FX3U format, preserving device addresses and rung structure with documented exceptions for unsupported instructions.
• Distributed I/O via CC-Link IE Field Basic: As a CC-Link IE Field Basic master, the FX3U-128MR/ESS can manage up to 16 slave stations over standard Ethernet cabling, each contributing up to 64 I/O points. This enables distributed panel architectures without dedicated fieldbus cabling infrastructure.
• Transparent remote diagnostics: MC Protocol access over Ethernet allows SCADA systems and maintenance tools to read device memory (D registers, M coils, timer/counter values) in real time without interrupting PLC execution, supporting predictive maintenance data collection and remote fault diagnosis without on-site access.

Quality Assurance & Global Logistics

Every FX3U-128MR/ESS unit supplied by siemensplc.com is sourced as genuine Mitsubishi Electric product, shipped in original factory packaging with intact holographic authenticity labels and factory test documentation. Units are never sourced from gray-market brokers or unauthorized secondary channels. Provenance documentation — including distributor invoices and date codes — is available on request for supply chain audit purposes.

Pre-shipment Inspection Protocol: Each unit undergoes a structured functional verification before dispatch: power-on self-test (POST) completion, I/O point continuity check across all 128 points, Ethernet link establishment and MC Protocol handshake confirmation, and firmware version recording. Results are logged against the unit serial number.

Warranty Coverage: 12-month warranty from the date of shipment, covering manufacturing defects and component failures under normal operating conditions as defined in Mitsubishi Electric’s FX3U hardware manual. Warranty claims are processed with a target response time of 48 hours from receipt of the defective unit.

Logistics from Xiamen, China: Our warehouse is located in Xiamen, Fujian Province — a major port city with direct air freight connections to international hubs including Hong Kong, Singapore, Dubai, Frankfurt, and Los Angeles. Standard international shipments are dispatched within 1–3 business days of order confirmation. Express air freight (DHL, FedEx, UPS) delivers to most destinations in Europe, North America, and Southeast Asia within 3–7 business days. Full export documentation is provided: commercial invoice, packing list, certificate of origin, and HS Code 8537.10 classification for customs clearance. For orders requiring EXW, FOB Xiamen, or CIF terms, contact our logistics team for tailored documentation.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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