Belimo · HVAC controller
VAV-Universal VRU-D3-BAC / VRU-M1-BAC / VRU-M1R-BAC Modbus register map
Default configuration
- Protocol
- RS485
- Port
- 502
- Unit ID
- 1
- Addressing
- 0-based
- Byte order
- Big endian
- Word order
- Low word first
- Baud rate
- 38400
- Parity
- None
- Data bits
- 8
- Stop bits
- 2
This device has an RS-485 interface. Reach it through a Modbus RTU-to-TCP gateway, configured with the factory serial settings above.
Register map
| Address | Count | Name | Type | Unit | Scale | Access |
|---|---|---|---|---|---|---|
| 0 | 1 | Setpoint Setpoint for the demanded flow, pressure or damper position according to the selected application and control mode; active when setpoint source (register 118) is bus. Scaled between Min (register 105) and Max (register 106). Range 0...10000. Default 0. | UInt16 | % | 0.01 | RW |
| 1 | 1 | Override control 0 = none (default), 1 = open, 2 = close, 3 = min, 5 = max, 8 = motor stop. Acts on the active setpoint source. | UInt16 | - | 1 | RW |
| 2 | 1 | Command 0 = none (default), 1 = adaptation, 2 = test run, 3 = synchronization. The register returns to 0 after the command has been executed. | UInt16 | - | 1 | RW |
| 3 | 1 | Actuator type 1 = air / water, 2 = VAV / EPIV, 3 = fire, 4 = energy valve, 5 = 6-way EPIV. Reads 65535 when the selected application has no local actuator. | UInt16 | - | 1 | R |
| 4 | 1 | Relative position Related to the adapted mechanical range. Range 0...10000; 65535 when the application has no local actuator. | UInt16 | % | 0.01 | R |
| 5 | 1 | Absolute position Angular position over the entire range of rotation, range 0...9600 (0.01 degrees), i.e. 0...96 degrees. 65535 when the application has no local actuator. | UInt16 | ° | 0.01 | R |
| 6 | 1 | Relative volumetric flow Relative to the nominal volumetric flow in register 110. Range 0...15000. 65535 when the application does not measure flow. | UInt16 | % | 0.01 | R |
| 7 | 1 | Absolute volumetric flow Range 0...65535; 65535 when the application does not measure flow. | UInt16 | m3/h | 1 | R |
| 8 | 1 | Sensor 1 value Depends on sensor type 1 (register 107): voltage 0...65535 mV, resistance 0...65535 ohm, 0/1 for a switch. | UInt16 | mV | 1 | R |
| 10 | 2 | Absolute volumetric flow in the selected unit 32-bit value in the unit chosen by register 117, low word first (the document's worked example: high word at Register No. x+1, low word at x). 65535 in both words when the application does not measure flow. | UInt32 | UnitSel | 0.001 | R |
| 12 | 1 | Analog setpoint Setpoint in percent when setpoint source (register 118) is analog. | UInt16 | % | 0.01 | R |
| 50 | 1 | Relative differential pressure Relative to the nominal differential pressure in register 128. Range -15000...15000. | Int16 | % | 0.01 | R |
| 51 | 1 | Absolute differential pressure Signed integer, range -1000...10000 (x0.1 Pa). | Int16 | Pa | 0.1 | R |
| 53 | 2 | Absolute differential pressure in the selected unit Signed 32-bit value in the unit chosen by register 145, low word first. | Int32 | UnitSel | 0.001 | R |
| 99 | 1 | Bus termination 0 = disabled (default), 1 = enabled: the 120 ohm bus termination is active. Set with the service tools. | UInt16 | - | 1 | R |
| 100 | 1 | Series number, 1st part Parts 1, 2 and 4 of the four-part series number are readable; the example 00839-31324-064-008 is read as 00839 / 31324 / 008. | UInt16 | - | 1 | R |
| 101 | 1 | Series number, 2nd part | UInt16 | - | 1 | R |
| 102 | 1 | Series number, 4th part | UInt16 | - | 1 | R |
| 103 | 1 | Firmware version | UInt16 | - | 1 | R |
| 104 | 1 | Malfunction and service information Bit 4 error in the dP sensor, bit 6 airflow not reached, bit 8 internal activity, bit 9 gear train disengaged (manual override pressed), bit 10 bus watchdog triggered, bit 11 actuator type does not fit the selected application. | Binary | - | 1 | R |
| 105 | 1 | Min Lower limit, range 0...Max. | UInt16 | % | 0.01 | RW |
| 106 | 1 | Max Upper limit, range 2000...10000. | UInt16 | % | 0.01 | RW |
| 107 | 1 | Sensor 1 type 0 = none (default), 1 = active (shows the analog setpoint in mV in hybrid mode), 2 = passive, 4 = switch. | UInt16 | - | 1 | RW |
| 108 | 1 | Bus fail position Position taken when the communication watchdog (register 109) expires. Range 0...10000. | UInt16 | % | 0.01 | RW |
| 109 | 1 | Communication watchdog Timeout in seconds, range 0...3600. | UInt16 | s | 1 | RW |
| 110 | 1 | Nominal volumetric flow Range 0...60000. | UInt16 | m3/h | 1 | R |
| 112 | 2 | Nominal volumetric flow in the selected unit Low word at register 112, high word at 113; range up to 60000000. | UInt32 | UnitSel | 0.001 | R |
| 116 | 1 | Control mode Visualisation of the control mode selected by the damper manufacturer: 0 = position control (Min/Max refer to the adapted mechanical range), 1 = flow control (Min/Max refer to the nominal volumetric flow). Default 1. | UInt16 | - | 1 | R |
| 117 | 1 | Unit selection flow Unit for the 32-bit flow registers: 0 = (not used), 1 = m3/h (default), 2 = l/s, 6 = cfm. | UInt16 | - | 1 | RW |
| 118 | 1 | Setpoint source 0 = analog input on wire 3 (register 12 active), 1 = bus (default, register 0 active). | UInt16 | - | 1 | RW |
| 119 | 1 | Operation mode 0 = negative pressure, 1 = positive pressure (default). Only selectable on the VRU-M1R-BAC. | UInt16 | - | 1 | RW |
| 123 | 1 | Room pressure cascade 0 = disabled (default), 1 = enabled, 2 = enabled fast. Only available when register 124 is flow control or room pressure control; the fast variant only on the VRU-M1R-BAC. | UInt16 | - | 1 | R |
| 124 | 1 | Application selection Selected by the damper manufacturer: 0 = flow control (default), 1 = pressure control, 2 = room pressure control, 3 = flow measurement. | UInt16 | - | 1 | R |
| 125 | 1 | System altitude Altitude above sea level, range 0...3000 m. Default 0. | UInt16 | m | 1 | RW |
| 126 | 1 | Nominal differential pressure in the selected unit Unit chosen by register 145. | UInt16 | UnitSel | 0.1 | R |
| 128 | 1 | Nominal differential pressure Range depends on the actuator: VRU-D3-BAC 0...5000, VRU-M1-BAC 0...6000, VRU-M1R-BAC 0...750 Pa. Used as dp@Vnom in flow control and as the measurement limit in pressure control. | UInt16 | Pa | 0.1 | R |
| 145 | 1 | Unit selection differential pressure 0 = pascal (default), 2 = inches of water. Applies to register 126. | UInt16 | - | 1 | RW |
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Related guides
Notes
Source: Belimo 'Modbus Interface Description - VAV-Universal VRU-D3-BAC / VRU-M1-BAC / VRU-M1R-BAC, Controller for VAV/CAV and pressure applications', edition 2026-01 / V1.04 (belimo_Modbus-Register_VRU_V1_4_en-gb.pdf). www.belimo.com refuses automated downloads (HTTP 403), so the document was read from Belimo's own regional mirror https://www.belimo.ro/mam/general-documents/system_integration/Modbus/belimo_Modbus-Register_VRU_V1_4_en-gb.pdf (same MAM document and edition); the URL above is the canonical vendor location. This is not the VAV-Compact map: the VAV-Universal adds the differential-pressure block (registers 50-53 and 126-145), the application/operation-mode registers (119, 123, 124) and system altitude (125), and it scales the absolute position at 0.01 degrees instead of 1 degree. Factory Modbus settings: 38400 Bd, 1-8-N-2 (8 data bits, no parity, 2 stop bits), address 1; the same four transmission formats and 9600/19200/76800/115200 Bd are selectable, addresses 1...247. Addressing comes from the document's twin columns - 'addressed by 1..n (Register No.) or 0..n-1 (Address)' - so the values here are the Address column, i.e. zero_based. Word order: 32-bit values are 'little endian byte swap / LSW first' (low word at the lower Register No.), hence LowFirst for registers 10, 53 and 112; the signed rows (50, 51, 53) are entered as Int16/Int32 because the document marks them '**) signed integer'. The document prints register 113's address column as '12', a typo for 112: the neighbouring rows and the text ('The register shows the LowWord' / 'HighWord') make 112/113 the pair, and that is what is stored. Deliberately left out: registers 9, 11, 52, 111, 113-115, 120-122, 127 and the '..' gap 129-144, all printed as '–'. The flow and pressure readings are inactive (65535) unless the configured application supports them, which the row descriptions say.