I get no response from the Belimo unit after wiring. What should I check?

Verify that the RS485 A and B lines are not swapped and that a GND reference wire is connected. Confirm the slave address and baud rate (38400) match between the device and the gateway. Also ensure that the bus is properly terminated if the unit is at the end of the chain.

CO2 readings seem inaccurate or stuck at zero. What could cause this?

Some P-22RT models require a warm-up period of several minutes after power-on before CO2 readings stabilize. Also verify that the CO2 sensor variant is present in your specific model, as not all P-22RT sub-variants include a CO2 sensor.

Can I change the temperature display from Celsius to Fahrenheit remotely?

Yes. Write to register 120 to switch the temperature unit. The change applies to both local display and reported Modbus values.

How do I calibrate the sensor offsets through ModbusCloud?

Use the writable registers at addresses 110 (temperature offset), 111 (humidity offset), and 112 (CO2 offset). Enter the correction value to compensate for mounting position effects. The offset is applied to all subsequent readings.

Free Modbus template: Belimo Room Operating Unit Modbus RTU

Name: Belimo Room Operating Unit Modbus RTU
Brand: Belimo
SKU: 7b93d786-9421-473d-b17b-ef356bbff7cc

Modbus RTU template for the Belimo Room Operating Unit (P-22RT series). Reads room temperature, relative humidity, CO2 concentration, and dew point. Allows remote adjustment of temperature setpoints, ventilation stages, and air quality thresholds.

Registers (51)

13 read / 38 write

Name	Address	Function	Type	Unit	Access
Room Temperature in Celcius	0	FC03	uint16	°C	R
Room Temperature in Selected Unit	1	FC03	uint16	°C	R
Relative Humidity	2	FC03	uint16	%	R
CO2 Value of the Room	3	FC03	uint16	ppm	R
Dew Point Temperature in Celcius	4	FC03	uint16	°C	R
Dew Point Temperature in Selected Unit	5	FC03	uint16	°C	R
Air Quality Status	6	FC03	uint16		R
Digital Input	7	FC03	uint16		R
Bus Termination	99	FC03	uint16		R
Serial Number Device – 1st part	100	FC03	uint16		R
Serial Number Device – 2nd part	101	FC03	uint16		R
Serial Number Device – 4th part	102	FC03	uint16		R
Firmware Version	103	FC03	uint16		R
Display Warning Icon	10	FC06	uint16		W
Display Window Icon	11	FC06	uint16		W
Room/Zone Temperature Setpoint in Celcius	21	FC06	uint16	°C	W
Set Ventilation Stage	22	FC06	uint16	%	W
System Operation Mode	30	FC06	uint16		W
Ventilation Stage Control Mode	31	FC06	uint16		W
Heating and Cooling Application Status	32	FC06	uint16		W
Enable Local Adjustment	33	FC06	uint16		W
Room/Zone Temperature Setpoint in Selected Unit	34	FC06	uint16	°C	W
Relative Room/Zone Temperature Setpoint in Celcius	35	FC06	uint16	°C	W
Relative Room/Zone Temperature Setpoint in Selected Unit	36	FC06	uint16	°C	W
Room Temperature Offset	110	FC06	uint16	K	W
Relative Humidity Offset	111	FC06	uint16	%	W
CO2 Value Offset	112	FC06	uint16	ppm	W
CO2 Limit for Good Air Quality	115	FC06	uint16	ppm	W
CO2 Limit for Medium Air Quality	116	FC06	uint16	ppm	W
Air Quality Indication	117	FC06	uint16		W
Window Icon Function	136	FC06	uint16		W
Mode Temperature on Display	137	FC06	uint16		W
Display Ventilation Stages	138	FC06	uint16		W
Select the preferable temperature unit	120	FC06	uint16		W
Unit for temperature shown on display	121	FC06	uint16		W
Select the preferable differential temperature unit	122	FC06	uint16		W
Light/Dark Color Scheme	130	FC06	uint16		W
Display Room/Zone Temperature	131	FC06	uint16		W
Display Relative Humidity	132	FC06	uint16		W
Display CO2 Value	133	FC06	uint16		W
Display Heating/Cooling Icon	134	FC06	uint16		W
Warning Icon Function	135	FC06	uint16		W
Eco Button Mode	139	FC06	uint16		W
Display Boost Button	140	FC06	uint16		W
Mode On/Off Button	141	FC06	uint16		W
Boost Mode Duration	142	FC06	uint16	s	W
Temperature Setpoint Type	145	FC06	uint16		W
Default Room Temperature Setpoint	146	FC06	uint16		W
Adjustment range temperature setpoint	147	FC06	uint16		W
Ventilation Stage Configuration	148	FC06	uint16		W
Ventilation Control Mode	149	FC06	uint16		W

Belimo Room Operating Unit Modbus RTU

Overview

Key readings

Available commands

Use cases

Installation

Things to watch for

Registers (51)

Working with Modbus in your projects?

Free: RS485 and Modbus RTU installation checklist (PDF)

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