MITSUBISHI FX3U-485-BD – RS-485 Serial Communication Board | FX3U Platform
Engineering Background
The FX3U-485-BD is a dedicated RS-485 half-duplex serial communication expansion board designed exclusively for the Mitsubishi MELSEC FX3U series programmable logic controller. As industrial automation systems increasingly demand open, multi-node serial networking, this board provides a factory-integrated solution that eliminates the need for external converters or gateway devices. It mounts directly onto the FX3U main unit’s left-side expansion connector, occupying no DIN rail space and drawing power directly from the CPU module.
Product Positioning
Targeted at OEM machine builders, system integrators, and plant automation engineers, the FX3U-485-BD enables the FX3U CPU to act as a MODBUS RTU master or slave, participate in Mitsubishi N:N peer-to-peer networks, and communicate with variable frequency drives, HMIs, barcode readers, and third-party field devices over a robust, noise-immune RS-485 bus. It is the preferred choice when a single additional serial port is required without upgrading to a larger CPU or adding a separate communication module.
Complete Technical Specifications
| Parameter | Specification |
|---|---|
| Model Number | FX3U-485-BD |
| Compatible CPU | MELSEC FX3U series (all versions) |
| Interface Standard | RS-485 (EIA/TIA-485), half-duplex, 2-wire |
| Communication Protocol | MODBUS RTU (master/slave), N:N Network, Inverter Communication, Non-protocol (free port) |
| Baud Rate | 300 / 600 / 1200 / 2400 / 4800 / 9600 / 19200 / 38400 / 57600 / 115200 bps |
| Data Bits | 7 or 8 bits |
| Parity | None / Even / Odd |
| Stop Bits | 1 or 2 |
| Max. Network Nodes | Up to 16 stations (N:N); up to 32 nodes (MODBUS RTU) |
| Max. Cable Length | 500 m (at 19200 bps); 50 m (at 115200 bps) |
| Connector Type | Screw terminal block (SDA, SDB, SG) |
| Termination Resistor | Built-in switchable 110 Ω termination |
| Power Supply | Supplied from FX3U CPU (no external power required) |
| Current Consumption | 20 mA DC (from FX3U internal bus) |
| Isolation | Non-isolated (signal ground shared with CPU) |
| Operating Temperature | 0 °C to +55 °C |
| Storage Temperature | −20 °C to +70 °C |
| Humidity | 5 % to 95 % RH (non-condensing) |
| Vibration Resistance | 10–57 Hz: 0.075 mm amplitude; 57–150 Hz: 9.8 m/s² |
| Shock Resistance | 147 m/s², 3 times each in 3 axes |
| Dimensions (W × H × D) | Approx. 40 × 90 × 20 mm |
| Weight | Approx. 50 g |
| Certifications | CE, UL (cULus), RoHS compliant |
| Country of Origin | Japan |
Proven Application Environments
- MODBUS RTU Master/Slave Networks – Poll sensors, drives, and meters; integrate with SCADA and HMI systems using standard MODBUS function codes (01, 02, 03, 04, 05, 06, 0F, 10).
- Mitsubishi N:N Peer-to-Peer Linking – Share data registers and bit devices across up to 8 FX3U CPUs without a master PLC, ideal for distributed machine cells.
- Variable Frequency Drive Control – Direct inverter communication using Mitsubishi inverter protocol (FR-A700, FR-E700, FR-D700 series) for speed reference, run/stop, and fault monitoring.
- HMI / GOT Integration – Connect Mitsubishi GOT1000/GOT2000 series HMIs or third-party panels via RS-485 without occupying the main RS-422 port.
- Barcode / RFID Reader Integration – Non-protocol (free port) mode enables ASCII/binary data acquisition from serial peripherals.
- Building Automation & Energy Monitoring – Interface with MODBUS-compatible power meters, temperature controllers, and BACnet gateways.
Engineering Advantages
- Zero Footprint Expansion – Plugs directly into the FX3U left-side BD connector; no additional DIN rail space, no external wiring to a separate module.
- Simultaneous Port Operation – The FX3U CPU’s built-in RS-422 port remains fully available; the FX3U-485-BD adds a second independent serial channel (CH2).
- Built-in Termination Switch – On-board 110 Ω switchable termination resistor eliminates the need for external resistors at bus endpoints.
- GX Works2 / GX Developer Native Support – Configured entirely through PLC parameters (Special Function Block settings); no separate configuration software required.
- Protocol Flexibility – Supports MODBUS RTU, Mitsubishi proprietary protocols, and free-port (non-protocol) mode within the same hardware platform.
- Proven Industrial Reliability – Manufactured to Mitsubishi Electric FA standards with full CE and UL certification for global deployment.
System Architecture & Signal Flow
The FX3U-485-BD connects to the FX3U CPU via the dedicated BD expansion connector on the left side of the main unit. Internally, the board interfaces with the CPU’s serial communication ASIC, appearing as Channel 2 (CH2) in GX Works2 PLC parameters. The RS-485 transceiver drives the SDA (+) and SDB (−) differential pair to the screw terminal block. The SG (signal ground) terminal should be connected to the cable shield or remote device ground to minimize common-mode noise. The built-in termination resistor switch (SW1) should be set to ON at both physical ends of the RS-485 bus and OFF at all intermediate nodes.
Wiring summary: SDA (+) → RS-485 A line | SDB (−) → RS-485 B line | SG → Shield/GND. Maximum recommended stub length per node: < 1 m.
Sourcing & Quality Verification
- All units supplied by Konmask.com are sourced from authorized Mitsubishi Electric FA distributors or verified secondary-market channels with full traceability documentation.
- Each unit undergoes incoming inspection: label verification (model, date code, lot number), physical integrity check, and functional power-on test where applicable.
- Genuine units feature Mitsubishi Electric holographic authenticity labels and factory-sealed ESD packaging. Counterfeit indicators (blurred markings, mismatched date codes, non-standard connector finish) are screened at intake.
- Certificate of conformity and test records available upon request for project documentation.
Procurement Process
- Step 1 – Confirm Compatibility: Verify your CPU is FX3U series (not FX3G, FX3S, or FX5U — those require different BD boards). Confirm no other BD board is already installed (only one BD board permitted per FX3U CPU).
- Step 2 – Place Order: Add to cart and complete checkout. HKD pricing displayed; multi-currency checkout available.
- Step 3 – Order Confirmation: Automated confirmation email with order number sent within minutes. For B2B/bulk orders (>5 units), contact support@konmask.com for a formal quotation.
- Step 4 – Shipping: Orders dispatched within 1–3 business days. Express (DHL/FedEx) and standard (China Post/ePacket) options available at checkout.
- Step 5 – After-Sales: 12-month warranty against manufacturing defects. DOA (Dead on Arrival) replacement within 7 days of receipt with photo evidence.
Technical FAQ
Q: Can I install the FX3U-485-BD alongside an FX3U-232-BD or FX3U-USB-BD?
A: No. The FX3U CPU has a single BD expansion slot. Only one BD board can be installed at a time. If two serial ports are required in addition to the built-in RS-422, use an FX3U-485ADP-MB special adapter instead.
Q: Does the FX3U-485-BD support MODBUS TCP?
A: No. MODBUS TCP requires an Ethernet interface (e.g., FX3U-ENET or FX3U-ENET-ADP). The FX3U-485-BD supports MODBUS RTU over RS-485 only.
Q: What is the maximum number of MODBUS slaves I can poll?
A: The RS-485 standard supports up to 32 unit loads on a single segment. Mitsubishi’s MODBUS RTU implementation on the FX3U supports addressing up to 247 slave IDs, but practical bus loading limits the physical node count to 32 (or more with repeaters).
Q: Is the FX3U-485-BD compatible with FX5U or iQ-F series PLCs?
A: No. The FX3U-485-BD is exclusively compatible with the FX3U series. FX5U and iQ-F CPUs use a different expansion connector and require FX5-485ADP or equivalent adapters.
Q: How do I configure the baud rate and protocol in GX Works2?
A: Navigate to PLC Parameters → Serial Communication Setting → CH2. Select the protocol (MODBUS RTU, N:N, Inverter, or Non-protocol), set baud rate, data bits, parity, and stop bits. Download parameters to the CPU. No ladder logic is required for MODBUS RTU master operation — use the ADPRW instruction.
Q: Can I use the FX3U-485-BD for long-distance communication beyond 500 m?
A: The 500 m limit applies at 19200 bps with proper shielded twisted-pair cable (e.g., Belden 9841 or equivalent). For longer distances, reduce baud rate, use RS-485 repeaters, or consider fiber-optic media converters.
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