General Assist Technical Reference
UTY-VMGX Modbus RTU Network Converter
VRF integration, RS-485 wiring, addressing, registration and commissioning
For Fujitsu / AIRSTAGE VRF systems integrated with a BMS or PLC via Modbus RTU
UTY-VMGX is a Modbus-RTU to VRF network converter that allows a Modbus master such as a BMS or PLC to monitor and control VRF indoor units. One converter can supervise up to 128 indoor units.
Qualified personnel only: Installation includes 230 V AC power, VRF communications and RS-485 wiring. Isolate power before wiring or changing settings and confirm the applicable installation documentation before energising equipment.
1. Core capabilities
| Function | Details |
|---|---|
| Protocol bridge | VRF network (FGL) to Modbus-RTU over RS-485. |
| Capacity | Up to 128 indoor units per converter. Multiple converters can share a VRF network. |
| Control and feedback | Read/write key points including on/off, mode, setpoint, fan and fault status. Wired and IR remote controllers remain usable. |
| Power supply | 230 V AC supply to the converter. |
System topology
- VRF side: Connect to the VRF communications trunk shared with the indoor and outdoor units.
- Modbus side: The converter operates as a slave on an RS-485 trunk with a single Modbus master.
- Grounding: Bond earth in accordance with normal panel practice and avoid ground loops between systems.
2. RS-485 wiring - Modbus side
| Item | Requirement |
|---|---|
| Termination | Enable termination at both ends only of the RS-485 trunk. |
| Polarity | Terminals are COM / D- / D+. Maintain consistent polarity across all devices. |
| Shielding | Earth the shield at one end only. Do not bond the shield to COM. |
| Length and layout | Trunk length up to approximately 1000 m. Avoid long stubs and use repeaters where required for larger networks. |
RS-485 success factors: Correct D+/D- polarity, termination at the two physical ends only, conservative stub lengths and a shield earthed at one end are critical for reliable operation.
3. Modbus communication settings
Communication parameters are configured using DIP SET6. The converter settings must match the BMS or PLC master.
| Parameter | Available setting | Default / note |
|---|---|---|
| Baud rate | 9600 or 19200 bps | 9600 bps default |
| Parity | Even or Odd | Even default. When Stop = 2, parity is None. |
| Stop bits | 1 or 2 | 1 default |
| Protocol mode | Modbus-RTU | ASCII is not used. |
Addressing
| Address | Setting | Requirement |
|---|---|---|
| VRF network address | SET1 | Use a unique converter address and avoid conflicts with central controllers or other converters. |
| Modbus slave address | SET4 / SET5 | Use a unique address. 1-247 is recommended. Record the address on the panel and in the register map. |
4. Indoor unit registration and mapping
Indoor units can be registered using one of three methods.
Method 1 - Default mapping
Use the pre-mapped address structure where appropriate for simple systems.
Method 2 - Automatic scan
- Turn SET2-4 ON to start the automatic scan. Indoor units will briefly stop while addresses are discovered.
- The LED display cycles the detected indoor-unit count.
- Confirm the count matches the as-built system.
- Turn SET2-4 OFF to save the registration.
- Power-cycle the converter to finalise the mapping.
Method 3 - PC configuration tool
Use the configuration utility to read or write registered data. Power must be OFF during a write.
Multiple converter sites: When several converters share a VRF network, power only the target refrigerant system during each scan to prevent cross-registration.
5. Test run and verification
- Normal LED state: After initialisation, confirm steady green indicators for power and run.
- Test run: Use SET2-3 to command a timed test across registered indoor units. The test auto-ends or can be stopped by turning the switch OFF.
- BMS validation: Poll a small sample, such as the first 10 indoor units, and confirm address mapping, on/off, operating mode, setpoint and fault status.
Installation cautions
- Install the converter indoors in a clean, dry and ventilated location.
- Provide cable strain relief and maintain bend radius and separation from mains wiring.
- Do not mount the enclosure so cables exit directly downward where water could track into the enclosure.
Quick commissioning checklist
- VRF trunk connected and converter VRF address is unique.
- RS-485 polarity correct and termination installed only at the two ends of the trunk.
- Baud, parity and stop-bit settings match the Modbus master.
- Modbus slave address recorded on the panel.
- Scan or mapping completed and indoor count matches the as-built system.
- BMS reads and writes confirmed for on/off, mode, setpoint and fan.
- Final LED state normal and test run completed where used.
- As-built register map filed with the O&M documentation.
6. Troubleshooting
| Symptom | Checks |
|---|---|
| No response to Modbus polls | Verify baud, parity, stop bits, slave address, end-of-line termination and shield earthing at one end only. |
| Wrong indoor count or addressing errors | Re-run the scan with only the intended system powered. Confirm the total does not exceed 128 and correct any duplicate VRF addresses. |
| Intermittent data | Reduce stub lengths, tighten terminals, verify converter supply stability and confirm the master's poll rate is not saturating the bus. |
| VRF communication alarms | Check converter VRF address conflicts and VRF-side wiring. Power-cycle the converter after mapping changes. |
7. Modbus fundamentals for BMS engineers
- Modbus-RTU is a master/slave protocol with only one master controlling the bus.
- Keep poll rates conservative so the converter has sufficient time to traverse the VRF network and refresh data.
- Group reads and writes into contiguous register ranges where possible to minimise bus traffic.
8. Key points
- UTY-VMGX provides a bridge between the VRF network and Modbus-RTU for up to 128 indoor units.
- Correct RS-485 practice, unique addresses and verified indoor registration are the main success factors.
- Validate the integration with a short BMS test before handover.
General Assist Technical Reference