SEN0603 MODBUS-RTU RS485 Soil Sensor Reference

This article delves into the ModBus-RTU communication protocol used in soil sensors, explaining data frame formats and register addresses to aid in the analysis of soil health indicators such as conductivity and pH.

Communication Protocol Description

1. Basic communication parameters

Interface	Encoding	Data bits	Parity bits	Stop bits	Error checking	Baud rate
RS485	8-bit binary	8	None	1	CRC	2400bit/s, 4800bit/s, 9600 bit/s configurable, default 9600bit/s

2. Data frame format definition

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

Time for initial structure ≥4 bytes
Address code = 1 byte
Function code = 1 byte
Data area = N bytes
Error checking = 16-bit CRC code
Time to end structure ≥4 bytes
Address code: It is the address of the sensor, which is unique in the communication network (factory default 0x01).
Function code: 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 specific communication data. Note that the high byte of 16bits data is first!
CRC code: two-byte check code.

Host query frame structure:

Address code	Function code	Register starting 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

3. Communication protocol examples and explanations

Example: Read the parameter values of the EC and PH two-in-one device (address 0x01)

Inquiry frame (hexadecimal):

Address code	Function code	Register starting address	Register length	Check code low bit	Check code high bit
0x01	0x03	0x00 0x00	0x00 0x04	0x44	0x09

Response frame (hexadecimal):

Address code	Function code	Return the number of valid bytes	Humidity value	Temperature value	EC value	PH value	Low bit of check code	High bit of check code
0x01	0x03	0x08	0x00 0x00	0x00 0x00	0x03 0xE8	0x00 0x38	0x57	0xB6

EC calculation:

EC: 03E8 H (hex) = 1000 => EC = 1000 us/cm

PH calculation:

PH：0038 H（hex）= 56 => PH = 5.6

4. Register address

Register address	PLC or configuration address	Content	Operation	Definition description
0002H	40003(decimal)	Conductivity	Read-only	Real-time conductivity value
0003H	40004(decimal)	PH value	Read-only	Real-time PH value (expanded 10 times)
0007H	40008(decimal)	Salinity	Read-only	Real-time salinity value (for reference only)
0008H	40009(decimal)	Total dissolved solids TDS	Read-only	Real-time TDS value (for reference only)
0022H	40035(decimal)	Conductivity temperature coefficient	Read-write	0-100 corresponds to 0.0%-10.0% Default 0.0%
0023H	40036(decimal)	Salinity coefficient	Read-write	0-100 corresponds to 0.00-1.00, default is 55 (0.55)
0024H	40037 (decimal)	TDS coefficient	Read and write	0-100 corresponds to 0.00-1.00, default is 50 (0.5)
0052H	40083 (decimal)	Conductivity calibration value	Read and write	Integer
0053H	40084 (decimal)	PH calibration value	Read and write	Integer
07D0H	42001 (decimal)	Device address	Read and write	1-254 (factory default 1)
07D1H	42002 (decimal)	Device baud rate	Read and write	0 represents 2400, 1 represents 4800, 2 represents 9600

Principle

Soil electrical conductivity (EC) is the level of salt in the soil (salinity). These parameters are important indicators of soil fertility and health, and ultimately affect plants. When soil has high salt content, the salt makes it difficult for plants to absorb water, even if the moisture content in the soil is high. This condition will eventually "burn" the plant. Plants grow best in their preferred environment within the optimal range of soil moisture, temperature and EC levels. Growers need data on these parameters and understand what actions need to be taken to ensure plant health and higher yields.

Soil pH, also known as "soil reaction". It is an acid-base reaction of the soil solution, which mainly depends on the concentration of hydrogen ions in the soil solution, expressed by the pH value. A solution with a pH value equal to 7 is a neutral solution; a solution with a pH value less than 7 is an acidic reaction; a solution with a pH value greater than 7 is an alkaline reaction. Soil pH can generally be divided into the following levels:

PH value Soil acidity and alkalinity
＜4.5 Extremely acidic
4.5-5.5 Strongly acidic
5.5-6.5 acidic
6.5-7.5 Neutral
7.5-8.5 alkaline
8.5-9.5 Strongly alkaline
＞9.5 Extremely alkaline

Other Supplementary Information

How to install and use

Quick test method
Select a suitable measurement location, avoid stones, and ensure that the steel needle does not touch hard objects. Throw away the topsoil according to the required measurement depth and maintain the original tightness of the soil below. Hold the sensor tightly and insert it vertically into the soil. Do not move left and right when measuring. It is recommended to measure multiple times within a small range of a measuring point and average it.

Buried measurement method
Dig a pit with a diameter >20cm vertically, insert the sensor steel needle horizontally into the pit wall at a predetermined depth, fill the pit tightly, and after a period of stabilization, measurements and recordings can be made for days, months or even longer.

Things to note

The steel needle must be fully inserted into the soil during measurement.
Avoid strong sunlight directly shining on the sensor, which may cause the temperature to be too high. When using in the field, be careful to prevent lightning strikes.
Do not violently bend the steel needle, do not pull the sensor lead wire forcefully, and do not drop or violently impact the sensor.
The sensor protection level is IP68, and the entire sensor can be immersed in water.
Due to the presence of radio frequency electromagnetic radiation in the air, it is not advisable to leave it powered on for a long time in the air.