Summary

UTY-VMGX is a Modbus-RTU to Fujitsu VRF network converter that allows a Modbus master (BMS/PLC) to monitor and control VRF indoor units. One converter can supervise up to 128 indoor units. This guide covers capabilities, wiring, addressing, DIP settings, commissioning, and troubleshooting. No links.

Applies to

  • Fujitsu VRF systems using the FGL (VRF network)

  • Modbus RTU (RS-485) integrations for building management systems

Core capabilities

  • Protocol bridge: VRF network (FGL) ↔ Modbus-RTU (RS-485).

  • Scale: Up to 128 indoor units per converter; multiple converters can share a VRF network.

  • Control & feedback: Read/write key points (on/off, mode, setpoint, fan, faults) while wired/IR remotes remain usable.

  • Power: 230 V AC supply to the converter.

System topology

  • VRF side: Connect to the VRF communications trunk shared with indoor/outdoor units.

  • Modbus side: Converter is a slave on an RS-485 trunk with a single Modbus master.

  • Grounding: Bond earth as per panel practice; avoid ground loops between systems.

RS-485 wiring (Modbus side)

  • Terminations: Enable termination at both ends only of the trunk.

  • Polarity: Terminals are COM / D- / D+. Keep polarity consistent across all devices.

  • Shielding: If using shielded cable, earth the shield at one end only. Do not bond shield to COM.

  • Length & layout: Trunk up to ~1000 m; avoid long stubs; use repeaters for large device counts.

Modbus communications settings (DIP SET6)

  • Baud rate: 9600 bps (default) or 19200 bps.

  • Parity: Even (default) or Odd. If Stop = 2, parity is None.

  • Stop bits: 1 (default) or 2.

  • Mode: Use Modbus-RTU (ASCII not used).

Match these to the master; document the settings on the panel label.

Addressing

  • VRF network address (SET1): Unique converter address on the VRF network. Avoid collisions with central controllers or other converters.

  • Modbus slave address (SET4/SET5): 1…247 recommended. Keep a register mapping sheet with the physical device label.

Indoor unit registration (mapping)

There are three supported methods:

  1. Use defaults

    • Pre-mapped addresses; suitable for simple systems.

  2. Automatic scan (SET2-4)

    • Turn SET2-4 ON to start scanning; indoor units will briefly stop while addresses are discovered.

    • LED display cycles the count of detected indoor units.

    • Turn SET2-4 OFF to save. Power-cycle to finalise.

  3. PC tool method

    • Use the configuration utility to read/write registered data. Power must be OFF during a write.

Tip: If multiple converters are on the same VRF network, power only the target refrigerant system during each scan to avoid cross-registration.

Test run & verification

  • Normal LED state after init: steady green indicators for power and run.

  • Test run (SET2-3): Turn ON to command a timed test across registered indoor units (auto-ends or turn OFF to stop).

  • BMS check: From the Modbus master, poll a small set (e.g., first 10 indoors) to confirm address mapping, on/off, mode, setpoint and fault status.

Installation cautions

  • Install the converter indoors in a clean, dry, ventilated location.

  • Provide strain relief on all cables; maintain bend radius and separation from mains.

  • Do not mount with cables exiting directly downward into the enclosure where water could track.

Quick commissioning checklist

  •  VRF trunk connected; converter VRF address unique.

  • RS-485 polarity correct; termination only at the two ends of the trunk.

  • Baud / parity / stop match the master.

  • Modbus slave address recorded on the panel.

  • Scan or mapping completed; indoor count matches as-built.

  •  BMS reads/writes confirmed (on/off, mode, setpoint, fan).

  •  Final LED state normal; test run completed (if used).

  •  As-built register map filed with the O&M.

Troubleshooting

  • No response to polls: Verify baud/parity/stop, slave address, terminations, and shield earthing at one end only.

  • Wrong indoor count / addressing errors: Re-run scan with only the intended system powered; confirm total ≤128; correct any duplicate VRF addresses.

  • Intermittent data: Reduce stub lengths, tighten terminals, check supply stability to the converter, and confirm the master’s poll rate isn’t saturating the bus.

  • VRF comms alarms: Check converter VRF address conflicts and wiring polarity on the VRF side; power-cycle after mapping changes.

Modbus fundamentals (for BMS engineers)

  • Modbus-RTU is master/slave; only one talker at a time.

  • Keep poll rates conservative; allow enough interval for the converter to traverse the VRF network and update data.

  • Group reads/writes by contiguous registers to minimise bus traffic.

Key takeaways

  • UTY-VMGX provides a clean bridge between VRF and Modbus-RTU with capacity up to 128 indoors.

  • Correct RS-485 practice, unique addresses, and a verified scan/map are the critical success factors.

  • Validate with a short BMS script before handing over to operations.