Reference
This article provides an in-depth guide on the Modbus-RTU communication protocol, covering essential parameters, data frame formats, and detailed register address descriptions for sensor communication.
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, 0001H | Blue-green algae value | 0x03/0x04 | cells/ml; floating-point big-endian, actual value |
| 0002H | Temperature | 0x03/0x04 | °C; 16-bit signed integer, 10 times the actual value |
| 0050H | Blue-green algae deviation value | 0x03/0x04/0x06/0x10 | 16-bit signed integer, actual value |
| 0051H | Temperature deviation value | 0x03/0x04/0x06/0x10 | 16-bit signed integer, 10 times the actual value |
| 0060H | Blue-green algae slope | 0x03/0x04/0x06/0x10 | 16-bit signed integer, 100 times the actual value |
| 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 Blue-Green Algae Values and Temperature
Query Frame:
| Address Code | Function Code | Register Address | Register Length | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x03 | 0x00 0x00 | 0x00 0x03 | 0x05 | 0xCB |
Response Frame:
| Address Code | Function Code | Number of Valid Bytes | Register Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x03 | 0x06 | 0x44 0xEF 0x4F 0x91 0x00 0xE8 | 0x25 | 0x4F |
Blue-green algae calculation: Floating-point big-endian 44EF4F91 = 1914 => 1914 cells/ml
Temperature calculation: 00E8H = 232 => Temperature is 232/10 = 23.2℃
3.2, Set the current blue-green algae value deviation for numerical correction
Query frame: If the current sensor output blue-green algae value is 1200, to correct it to 1000, the difference is 1000-1200=-200, -200=FF38. Calculation method: FFFF-00C8+1=FF38, register content is written as 0xFF38
| Address Code | Function Code | Registry Address | Registry Content | Check Digit Low Bit | Check Digit High Bit |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x50 | 0xFF 0x38 | 0xC9 | 0xF9 |
Response Frame:
| Address Code | Function Code | Register Address | Register Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x50 | 0xFF 0x38 | 0xC9 | 0xF9 |
3.3 Setting the Slope of Blue-Green Algae Concentration
If measuring 2000 cells/ml The standard solution, after setting the deviation value, was measured to be 2200.0 cells/ml. Therefore, 2000/2200 = 0.91 was written into register 0x0060. Multiplying this by 100, the written value is 91, which is 0x005B in hexadecimal.
Query Frame:
| Address Code | Function Code | Registry Address | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x60 | 0x00 0x5B | 0xC8 | 0x2F |
Response Frame:
| Address Code | Function Code | Registry Address | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x60 | 0x00 0x5B | 0xC8 | 0x2F |
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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