Reference
This article delves into the Modbus-RTU communication protocol, offering insights into basic parameters, data frame formats, and register addresses, along with practical examples for reading sensor data, setting deviation values, calibrating turbidity, and modifying communication settings.
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 | 1200bit/s，2400bit/s, 4800bit/s, 9600bit/s, 19200bit/s, 38400bit/s, 57600bit/s, 115200bit/s configurable, default 4800bit/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 0x03/0x04 and writes the register function code 0x06/0x10
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.
Register address
| Register Address | Content | Supported Function Codes | Definition Description |
|---|---|---|---|
| 0000H | Turbidity Value | 0x03/0x04 | NTU; 16-bit unsigned integer, 10 times the actual value |
| 0001H | Temperature | 0x03/0x04 | °C; 16-bit signed integer, 10 times the actual value |
| 0050H | Turbidity Deviation Value | 0x03/0x04/0x06/0x10 | 16-bit unsigned integer, 10 times the actual value |
| 0051H | Temperature Deviation Value | 0x03/0x04/0x06/0x10 | 16-bit signed integer, 10 times the actual value |
| 1010H, 1011H | Turbidity Calibration | 0x10 | Uses 2-point calibration. Write 0x0001 to 0x1010 and 0x0000 to 0x1011 to calibrate the zero point; write 0x0002 to 0x1010 and 0x0001 to 0x1011 to calibrate the full-scale point. |
| 07D0H | Device Address | 0x03/0x04/0x06/0x10 | 1~254 (16-bit unsigned integer, factory default 1) |
| 07D1H | Device Baud Rate | 0x03/0x04/0x06/0x10 | 0 represents 2400, 1 represents 4800, 2 represents 9600, 3 represents 19200, 4 represents 38400, 5 represents 57600, 6 represents 115200, 7 represents 1200 |
3. Communication Protocol Examples and Explanation
3.1 Reading Turbidity and Temperature
Query Frame:
| Address Code | Function Code | Registry Address | Registry Length | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x03 | 0x00 0x00 | 0x00 0x02 | 0xC4 | 0x0B |
Response Frame:
| Address Code | Function Code | Number of Valid Bytes | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x03 | 0x04 | 0x0D 0x2E 0x00 0xDB | 0xD8 | 0xCD |
Turbidity Calculation: 0D2EH=3374 => Turbidity=337.4NTU
Temperature Calculation: 00DBH=219=>Temperature=21.9℃
3.2, Set Deviation Value for Current Turbidity Value for Numerical Correction
Query Frame: If the current sensor output turbidity value is 1010, to correct the value to 1000, the difference is 1000-1010=-10, multiplied by 10 to -100=>FF9CH, calculation method: FFFF-0064+1=FF9C, register content is written as 0xFF9C.
| Address Code | Function Code | Registry Address | Registry Content | Check Digit Low Bit | Check Digit High Bit |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x50 | 0xFF 0x9C | 0xC8 | 0x42 |
Response Frame:
| Address Code | Function Code | Registry Address | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x50 | 0xFF 0x9C | 0xC8 | 0x42 |
3.3 Turbidity Calibration
Query Frame: After the sensor stabilizes in a 0 turbidity environment, write 0x0001 to register 0x1010 and 0x0000 to register 0x1011.
| Address Code | Function Code | Registry Address | Registry Length | Byte Length | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x10 0x10 | 0x00 0x02 | 0x04 | 0x00 0x01 0x00 0x00 | 0x6E | 0xA3 |
Response Frame:
| Address Code | Function Code | Registry Address | Registry Length | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x10 0x10 | 0x00 0x02 | 0x44 | 0xCD |
Query Frame: When calibrating the full-scale point, a standard turbidity solution of 1000 NTU should be selected. Write 0x0002 to 0x1010 and 0x0001 to 0x1011.
| Address Code | Function Code | Register Address | Register Length | Byte Length | Register Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x10 0x10 | 0x00 0x02 | 0x04 | 0x00 0x02 0x00 0x01 | 0x5F | 0x63 |
Response Frame:
| Address Code | Function Code | Register Address | Register Length | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x10 0x10 | 0x00 0x02 | 0x44 | 0xCD |
3.4, Change the current address to 0x02
Query Frame:
| Address Code | Function Code | Registry Address | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x07 0xD0 | 0x00 0x02 | 0x08 | 0x86 |
Response Frame:
| Address Code | Function Code | Registry Address | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x07 0xD0 | 0x00 0x02 | 0x08 | 0x86 |
3.5, Modify the current baud rate to 9600
Query Frame:
| Address Code | Function Code | Registry Address | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x07 0xD1 | 0x00 0x02 | 0x59 | 0x46 |
Response Frame:
| Address Code | Function Code | Registry Address | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x07 0xD1 | 0x00 0x02 | 0x59 | 0x46 |
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