SEN0659 Industrial RS485 CO2 Sensor Reference: API, Protocol & Pinout

The article provides an in-depth explanation of communication protocols for CO2 sensors, emphasizing RS485 and Modbus-RTU, detailing data frames, error checking with CRC, and sensor calibration methods.

Communication Protocol Description

1. Basic communication parameters

Interface	Encoding	Data bit	Parity bit	Stop bit	Error check	Baud rate
RS485	8-bit binary	8	None	1	CRC	2400bit/s, 4800bit/s, 9600 bit/s,19200bit/s,38400bit/s, 115200bit/s can be set, default is 9600bit/s

2. Data frame format definition

Using Modbus-RTU communication protocol, the format is as follows:

Initial structure ≥4 bytes of time

Address code = 1 byte

Function code = 1 byte

Data area = N bytes

Error check = 16-bit CRC code

End structure ≥4 bytes of time

Address code: The address of the sensor, which is unique in the communication network (factory default 0x01).

Function code: The function indication of the command sent by the host. This sensor reads the register function code 0x03 and writes the register function code 0x06

Data area: The data area is the specific communication data. Note that the high byte of 16-bit data is in front!

CRC code: A two-byte check code.

Host inquiry frame structure:

Address code	Function code	Register start address	Register length	Check code low bit	Check code high bit
1byte	1byte	2byte	2byte	1byte	1byte

Slave response frame structure:

Address code	Function code	Number of valid bytes	Data area 1	Data area 2	Nth data area	Check code
1byte	1byte	1byte	2byte	2byte	2byte	2byte

Register address

Register address	PLC or configuration address	Content	Supported function code	Definition description
0000H	40001	CO2 value	0x03	Actual value
006BH		CO2 correction value	0x03/0x06	The modifiable range is ±1000, that is, the value range is 0-1000 or 64535-65535
0066H	40103	Device address	0x03/0x06	1~249 (factory default 1)
0067H	40104	Device baud rate	0x03/0x06	1 represents 2400, 2 represents 4800, 3 represents 9600, 4 represents 19200, 5 represents 38400, 6 represents 115200

3. Communication protocol example and explanation

3.1. Read carbon dioxide value

Inquiry frame: Read value function code 0x03

Address code	Function code	Register start address	Register length	Check code low bit	Check code high bit
0x01	0x03	0x00 0x00	0x00 0x01	0x84	0x0A

Response frame:

Address code	Function code	Return valid bytes	Carbon dioxide value	Check code low bit	Check code high bit
0x01	0x03	0x02	0x02 0xFC	0xB9	0x65

CO2 value:

02FC (hexadecimal) = 764=> CO2 value = 764ppm

3.2, Read the current CO2 correction value (factory default is 0)

Inquiry frame:

Address code	Function code	Register start address	Register length	Check code low position	Check code high position
0x01	0x03	0x00 0x6B	0x00 0x01	0xF5	0xD6

Response frame:

Address code	Function code	Return valid bytes	Correction value	Check code low position	Check code high position
0x01	0x03	0x02	0x00 0x64	0xB9	0xAF

Calibration value:

0064 (hexadecimal) = 100=> Calibration value = 100ppm

3.3. Modify the CO2 calibration value

When the data is different from the reference standard, we can reduce the display error by adjusting the "calibration value". The calibration difference can be modified in the range of ±1000, that is, the value range is 0-1000 or 64535-65535. For example, when the displayed value is 100 less, we can correct it by adding 100. In the command, 100 is hexadecimal 0x64; if you need to reduce it, you can set a negative value, such as -100, which is calculated as FFFF-64+1=FF9C, which is hexadecimal 0xFF 9C

Inquiry frame:

Address code	Function code	Register start address	Modify value	Check code low position	Check code high position
0x01	0x06	0x00 0x6B	0x00 0x64	0xF9	0xFD

Response frame:

Address code	Function code	Register address	Correction value	Check code low position	Check code high position
0x01	0x06	0x00 0x6B	0x00 0x64	0xF9	0xFD

Correction value:

0064 (hexadecimal) = 100=> correction value = 100ppm

3.4, Modify the current address

Inquiry frame: (modify the current address to 0x02)

Address code	Function code	Register address	Modify value	Check code low bit	Check code high bit
0x01	0x06	0x00 0x66	0x00 0x02	0xE8	0x14

Response frame:

Address code	Function code	Register address	Modify value	Check code low bit	Check code high bit
0x02	0x06	0x00 0x66	0x00 0x02	0xE8	0x27

3.5, Modify current baud rate

Inquiry frame: (Modify current baud rate to 38400)

Address code	Function code	Register address	Modify value	Check code low bit	Check code high bit
0x01	0x06	0x00 0x67	0x00 0x05	0xF8	0x16

Response frame:

Address code	Function code	Register address	Modify value	Check code low bit	Check code high bit
0x01	0x06	0x00 0x67	0x00 0x05	0xF8	0x16

3.6, Query current address, baud rate

Inquiry frame:

Address code	Function code	Register address	Data length	Check code low bit	Check code high bit
0xFA	0x03	0x00 0x66	0x00 0x02	0x31	0x9F

Response frame:

Address code	Function code	Return valid bytes	Address	Baud rate	Check code low bit	Check code high bit
0x01	0x03	0x04	0x00 0x01	0x00 0x03	0xEB	0xF2

The real address of the device read is 01, and the baud rate is 0x03, which is 9600.

Principle

This infrared carbon dioxide sensor uses the principle of non-dispersive infrared (NDIR technology) to detect CO2 in the air.