Mitsubishi FX3U-485-BD Communication Board – FX3U Series

Mitsubishi FX3U-485-BD RS-485 Communication Board: Serial Network Interface for FX3U PLC Systems

The Mitsubishi FX3U-485-BD is a plug-in RS-485 serial communication expansion board engineered for direct integration with the MELSEC FX3U programmable logic controller platform. Within a distributed control architecture, this board occupies the dedicated BD slot on the FX3U main unit and extends the CPU’s native communication capability to a balanced, differential RS-485 physical layer — enabling deterministic, noise-immune data exchange across multi-drop field bus topologies. Unlike external communication adapters that introduce additional wiring complexity and latency, the FX3U-485-BD draws its 5 VDC operating power directly from the FX3U backplane, eliminating auxiliary power supply requirements and reducing panel footprint.

In process control environments where multiple variable-frequency drives, remote I/O stations, or third-party instruments must be coordinated under a single PLC master, the FX3U-485-BD provides the physical and protocol foundation for that integration. Its support for Modbus RTU, Mitsubishi inverter communication protocol, N:N peer-to-peer network, and non-protocol (free-format) serial communication makes it applicable across a broad spectrum of industrial field bus scenarios — from simple point-to-point inverter speed reference transmission to multi-node Modbus RTU polling networks with up to 16 addressable slave devices.

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Technical Parameters

Hardware Logical Analysis

The FX3U-485-BD interfaces with the FX3U CPU module through the dedicated BD expansion connector, which provides a direct 5 V power rail and a serial data path managed by the CPU’s built-in SIO (Serial I/O) controller. This architecture means the communication timing is governed by the FX3U scan cycle and the SIO interrupt handler — not by an external UART chip with its own clock domain — which results in tighter synchronization between PLC ladder logic execution and serial frame transmission.

Differential Signal Integrity: RS-485 uses a balanced differential pair (A/B lines) with a common reference (SG). The FX3U-485-BD’s transceiver maintains a minimum differential output voltage of ±1.5 V across a 54 Ω load, well above the ±200 mV receiver threshold defined by EIA-485. This 7.5× margin provides robust noise immunity in environments with high-frequency switching noise from variable-frequency drives and servo amplifiers operating in the same panel.

Built-in Switchable Termination: Long RS-485 cable runs (typically above 10–15 m at 115,200 bps) require line termination to suppress signal reflections. The FX3U-485-BD incorporates a hardware-selectable 110 Ω termination resistor matched to the characteristic impedance of standard shielded twisted-pair cable. Enabling this resistor at the physical bus endpoints eliminates standing wave reflections that would otherwise corrupt frame parity and cause CRC errors at high baud rates.

2-Wire vs. 4-Wire Configuration: In 2-wire half-duplex mode, the board uses a single differential pair for both transmit and receive, with automatic direction control managed by the SIO controller’s RTS signal. This is the standard configuration for Modbus RTU master-slave polling. In 4-wire full-duplex mode, separate TX and RX pairs allow simultaneous bidirectional data flow, which is required for certain Mitsubishi dedicated protocols and point-to-point full-duplex links. The wiring configuration is selected at the terminal block without any firmware change.

EMC Design: The board’s PCB layout routes the RS-485 differential pair with controlled impedance traces and places decoupling capacitors at the transceiver power pins to suppress high-frequency switching transients. The terminal block includes a functional earth (FE) terminal for cable shield grounding, which, when connected to the panel earth at a single point, forms a Faraday shield that attenuates common-mode interference induced by nearby power cables.

System Integration Benefits

• Zero additional power wiring: The board draws 20 mA from the FX3U’s internal 5 V bus, eliminating the need for a dedicated 24 VDC auxiliary supply and simplifying panel wiring design and BOM.
• Protocol flexibility without hardware change: Switching between Modbus RTU, inverter communication, N:N network, and non-protocol modes is accomplished entirely through GX Works2 parameter settings — no hardware modification or additional module required.
• Deterministic scan-cycle integration: Because the SIO controller is managed by the FX3U CPU’s interrupt system, serial frame reception can trigger immediate ladder logic execution within the same scan cycle, reducing response latency compared to external gateway solutions that add one or more communication cycle delays.
• Multi-drop network scalability: A single FX3U-485-BD supports up to 16 N:N network stations or up to 32 Modbus RTU slave addresses on one physical bus segment, allowing incremental expansion of field devices without additional hardware at the PLC end.
• Inverter integration without gateway: The board natively supports Mitsubishi’s dedicated inverter communication protocol, enabling direct frequency reference, run/stop command, and status readback from FR-A700/E700/D700 series inverters — eliminating the cost and latency of a separate RS-485-to-Ethernet gateway.
• Diagnostic transparency: The FX3U CPU’s special data registers (D8120–D8129 for communication format, D8401–D8409 for error codes) provide real-time visibility into communication status, frame error counts, and timeout events — accessible from GX Works2 device monitor without interrupting production.
• Compact panel footprint: At approximately 40 × 90 × 18 mm and 20 g, the board adds negligible volume to the FX3U main unit. No DIN rail space is consumed, and no additional cable duct routing is required beyond the RS-485 terminal block connections.
• Long cable reach: RS-485 supports bus segment lengths up to 1,200 m at low baud rates (e.g., 9,600 bps), making the FX3U-485-BD suitable for distributed architectures where field devices are located far from the control panel — a common requirement in water treatment, conveyor, and building automation installations.

Quality Assurance & Global Logistics

Every Mitsubishi FX3U-485-BD unit supplied by siemensplc.com is sourced through verified industrial distribution channels with full supply chain traceability. Each unit undergoes a structured incoming inspection protocol: label and date-code verification against Mitsubishi Electric’s published part marking standards, PCB visual inspection for solder joint integrity and component placement conformance, and a functional communication handshake test at 9,600 bps and 115,200 bps to confirm transceiver operation within specification. Units that do not pass all inspection stages are quarantined and not offered for sale.

Our warehouse is located in Xiamen, Fujian Province, China — a major international logistics hub with direct access to DHL Express, FedEx International Priority, UPS Worldwide Express, and SF International services. In-stock orders placed before 15:00 CST are typically dispatched the same business day. Transit times to major destinations: Europe 3–5 business days (DHL Express), North America 3–5 business days (FedEx IP), Southeast Asia 2–3 business days (SF International). Full export documentation — commercial invoice, packing list, and certificate of origin — is provided with every shipment. HS Code: 8537.10.90. A 12-month warranty covers manufacturing defects from the date of shipment.

Contact Information

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WhatsApp: +86 18359268345
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Location: Xiamen, China
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