Scope

Applies to AirGradient ONE Modbus (8PSLM) and AirGradient ONE Display Edition with Modbus (8PSLM-DE). Note that Modbus is not supported on the Standard model.

Applies to firmware version 9.2.6 onwards.

General

Protocol:
Modbus RTU / RS-485
Number of addresses: 1 .. 247
Factory default: 1
Transmission format: 8 bits, none / odd / even parity, 1 or 2 stop bits
Factory default; 8-E-1
Baudrate: 1200 .. 256000
Factory default: 19200

Timings

Scan Rate

Depending on settings the monitor will perform a measurement cycle every few minutes and provide the newly measured values on the Modbus interface once the cycle has successfully completed. When scanning at a rate of e.g. 10s then the maximum resulting latency is 10s.

Response Timeout

The monitors typically respond within 50 .. 60ms, hence a response timeout setting of 100ms is recommended. For baudrates < 9600 better results might be achieved with a higher setting.

Delay between Polls

A setting of > 50ms between slave polls is recommended.

Pinout

The connector pinout is

Voltage

The 12V .. 32V range is supported.

Function 04 Read Registers

These are the registers that the Master can read.

Register	Parameter	Data Type	Range	Unit	Comment
`3x0000`	current CO2	signed 16 bit	-1 .. 10,000	ppm	raw value -1 indicates an error condition
`3x0001`	current PM2.5	signed 16 bit	-1 .. 1,000	ppm	raw value -1 indicates an error condition OR the sensor is not ready yet
`3x0002`	current PM10	signed 16 bit	-1 .. 1,000	ppm	raw value -1 indicates an error condition OR the sensor is not ready yet
`3x0003`	current temperature	signed 16 bit	-`10001` -400 .. 900	tenth of °C, eg. 126 for 12.6°C	raw value -`10001` indicates an error condition
`3x0004`	current relative humidity	signed 16 bit	-1 .. 100	%	raw value -1 indicates an error condition
`3x0005`	current TVOC	signed 16 bit	-1 .. 30,000 OR -1 .. 500	ppb (SGP 30) OR SGP 40 index, see https://sensirion.com/products/catalog/SGP40/	temperature and humidity compensated -1 indicates an error condition, in case of SGP 30 ppb’s are delivered, in case of the SGP 40 an index


`3x0010`	sensorId byte #1	16 bit	0x00 .. 0xff		MSB
`3x0011`	sensorId byte #2	16 bit	0x00 .. 0xff
`3x0012`	sensorId byte #3	16 bit	0x00 .. 0xff
`3x0013`	sensorId byte #4	16 bit	0x00 .. 0xff
`3x0014`	sensorId byte #5	16 bit	0x00 .. 0xff
`3x0015`	sensorId byte #6	16 bit	0x00 .. 0xff		LSB
`3x0020`	friendly name char #1	8 bit char	ASCII		a user friendly name, up to 24 chars
..
`3x0038`	friendly name char #24

Function 05 Write Single Coil

These are the single bit registers the Master can write.

Register	Parameter	Data Type	Range	Unit	Comment

Register	Parameter	Data Type	Range	Comment
`0x0000`	calibrate CO2 to 400ppm	bit 0	1 / 0	the calibaration is performed upon next measurement cycle, normally within a few minutes
`0x0000`	led test mode	bit 1	1 / 0	not implemented yet
`0x0000`

Function 16 Write Multiple Registers

These are the registers the Master can write.

Register	Parameter	Data Type	Range	Comment
`0x0000`	friendly name char #1	8 bit char	ASCII	a user friendly name, up to 24 chars
..
`0x0018`	friendly name char #24

Modbus Function Specification