Mitsubishi Electric's City Multi VRF (Variable Refrigerant Flow) system dominates the European commercial air conditioning market for medium to large buildings — hotels, offices, retail. Integrating VRF air conditioning into a KNX smart building requires bridging the proprietary M-NET bus that connects Mitsubishi indoor and outdoor units to either KNX group addresses or a standard Modbus register map. The MAC-397IF-E Modbus adapter is the standard solution for multi-unit installations. Understanding its register structure and gateway configuration is essential for KNX integrators working on commercial HVAC projects.

M-NET: Mitsubishi's proprietary bus

M-NET is Mitsubishi Electric's internal communication protocol — a 2-wire RS-485 bus that connects outdoor units, indoor units, central controllers and system controllers. M-NET is entirely proprietary: protocol documentation is not publicly available, and only Mitsubishi-approved equipment (including the MAC-397IF-E) can read and write to M-NET devices. Each indoor unit has an M-NET address (0-50), set by rotary switch or central controller during commissioning. The M-NET system supports up to 50 indoor units per outdoor refrigerant circuit and additional outdoor units via refrigerant piping grouping.

MAC-397IF-E: M-NET to Modbus translation

The MAC-397IF-E (Mitsubishi Electric MAC-397IF-E Lossnay / Air Conditioning Interface) connects to the M-NET RS-485 bus and presents each indoor unit as a set of Modbus holding registers on a separate RS-485 Modbus RTU port. The adapter supports up to 50 indoor units simultaneously. Power: 24V DC or from M-NET bus. Modbus RTU parameters: 9600 baud, 8 data bits, even parity, 1 stop bit. Modbus slave address of the adapter: configurable 1-247.

Modbus register map per indoor unit

Each indoor unit occupies a block of Modbus holding registers. The register offset = (unit M-NET address × 20). Base registers (add unit offset): Register 0: on/off status (read: 0=off, 1=on; write: 0=off, 1=on). Register 1: operating mode (read/write: 0=auto, 1=heat, 2=dry, 3=cool, 4=fan only). Register 2: fan speed (read/write: 0=auto, 1=low, 2=medium, 3=high, 4=extra high — model dependent). Register 3: setpoint temperature ×10 in Celsius (read/write: 160=16.0°C to 300=30.0°C). Register 4: room temperature ×10 (read only: actual room temperature from indoor unit NTC sensor). Register 5: error code (read: 0=no error, non-zero = error code, see Mitsubishi service manual). Register 6: louver position (vertical, 0=auto, 1-5=fixed positions). Register 10: filter dirty flag (read: 1=filter maintenance required). Write to register with value to command. Read register for status feedback.

KNX-Modbus gateway configuration

For KNX integration, the Modbus RTU output of MAC-397IF-E connects to a Modbus TCP/RTU gateway that presents the registers on the KNX group address space. Suitable gateways: HMS Anybus Communicator (Modbus RTU → Modbus TCP, then KNX-Modbus TCP gateway); Weinzierl KNX ENO 634/635; Intesis IBOX-KNX-MBS gateway (native Modbus-to-KNX, 100 data point slots). For each indoor unit, configure the gateway to read: on/off (1-bit DPT 1.001), mode (1-byte DPT 5.001 or 20.102), setpoint (2-byte DPT 9.001), room temperature (DPT 9.001), error flag (DPT 1.001). And write: on/off command, mode command, setpoint command. With 10 indoor units and 5 data points each, 50 data point slots are required on the gateway.

PAC-IF010B-E: direct KNX interface for single units

For projects with only one or two Mitsubishi indoor units (residential split AC integration), the PAC-IF010B-E provides a direct KNX TP bus interface for a single indoor unit. Connect PAC-IF010B-E to M-NET port of the indoor unit (2-wire, polarity-insensitive). Connect PAC-IF010B-E to KNX TP bus. Load ETS6 application (available in KNX online catalogue). Configure group addresses: on/off (DPT 1.001), operating mode (DPT 20.102 HVAC mode: 0=auto, 1=comfort, 2=standby, 3=economy, 4=protection), setpoint temperature (DPT 9.001), actual temperature feedback (DPT 9.001), fan speed (DPT 5.001). The PAC-IF010B-E is significantly more expensive per unit than MAC-397IF-E (which amortises across 50 units), so it only makes sense for 1-2 unit residential installations.

ETS6 integration pattern for hotel rooms

A typical hotel room KNX HVAC setup with Mitsubishi ceiling cassette: KNX room thermostat (Gira 2096) sends temperature setpoint to group address 3/1/1 (DPT 9.001). Modbus gateway reads KNX value from 3/1/1 and writes to MAC-397IF-E register 3 (setpoint × 10). KNX room thermostat sends occupancy mode via group address 3/1/2 (DPT 20.102): comfort → mode = cool or heat (seasonal), standby → setpoint raised by 2°C via gateway logic, economy → unit off or setpoint raised 4°C. "Do Not Disturb" KNX scene → group address 3/1/3 → gateway sets register 0 = on, register 2 = 1 (low fan speed), register 3 = setback setpoint (27°C summer, 18°C winter). "Welcome" scene → comfort setpoint and auto fan.

Fault monitoring and alert integration

MAC-397IF-E register 5 (error code) should be polled every 60 seconds. If non-zero, the gateway sets KNX group address 9/1/x (alarm bit DPT 1.001) to 1. HA automation on this group address: send push notification with error code and unit address. Cross-reference error code with Mitsubishi service manual table (provided with adapter, 200+ error codes). Common codes: E6 (indoor/outdoor communication fault), P1 (intake sensor fault), E1 (system error — requires site engineer). The MAC-397IF-E adapter itself has a fault output LED and an RS-485 diagnostic mode accessible via Mitsubishi USB interface cable. In large commercial projects, HVAC fault monitoring is contractually required — configure all indoor unit error registers into the KNX energy management system via the gateway.