Reference
The article provides a detailed explanation of the Modbus-RTU communication protocol, covering basic parameters, data frame format, register addresses, and practical examples for sensor data reading and calibration.
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 | PLC Address | Content | Supported Function Codes | Definition Description |
|---|---|---|---|---|
| 0000H | 40001 | EC value | 0x03/0x04 | μS/cm; 16-bit unsigned integer, actual value |
| 0001H | 40002 | Temperature | 0x03/0x04 | °C; 10 times the actual value |
| 0002H | 40003 | Salinity | 0x03/0x04 | ppm; 16-bit unsigned integer |
| 0003H | 40004 | TDS | 0x03/0x04 | ppm; 16-bit unsigned integer |
| 0050H | 40081 | Temperature deviation value | 0x03/0x04/0x06/0x10 | 16-bit signed integer, 10 times the actual value |
| 0051H | 40082 | EC deviation value | 0x03/0x04 | 16-bit unsigned integer, actual value |
| 0052H, 0053H | 40083, 40084 | EC temperature compensation coefficient | 0x03/0x10 | Floating-point number, big-endian, default resolution 0.02 |
| 0054H, 0055H | 40085, 40086 | Electrode constant | 0x03/0x10 | Floating-point number, big-endian, default resolution 0.1 |
| 0110H, 0111H | 40273, 40274 | EC calibration | 0x10 | Write 00 04 to register 0110H, write the calibration standard solution value to register 0111H, actual value |
| 07D0H | 42001 | Device address | 0x03/0x04/0x06/0x10 | 1~254 (16-bit unsigned integer, factory default 1) |
| 07D1H | 42002 | 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 EC 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 | 0x27 0x10 0x01 0x09 | 0x30 | 0xD4 |
Conductivity calculation: 2710H=10000 => EC=10000μS/cm
Temperature calculation: 0109H=265=>Temperature=26.5℃
3.2, Set deviation value for current conductivity value for numerical correction
Query frame: If the current sensor output conductivity value is 900, to correct the value to 1000, the difference is 1000-900=100=>64H, write 0064H in the register content
| Address code | Function code | Register address | Register content | Checksum low bit | Checksum high bit |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x50 | 0x00 0x64 | 0x88 | 0x30 |
Response Frame:
| Address Code | Function Code | Register Address | Register Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x06 | 0x00 0x50 | 0x00 0x64 | 0x88 | 0x30 |
3.3 Calibrate the sensor with a 1413μS/cm standard solution
Query Frame: Write 00 04 to 0110H and 0111H respectively, 1413 =>0585H.
During calibration, the electrode is placed in a standard solution with a known conductivity value. The electrode is shaken to accelerate the response, then allowed to stand until the conductivity value stabilizes. This conductivity value is then written into the conductivity solution calibration log and calibrated to complete the calibration.
| Address Code | Function Code | Registry Address | Registry Length | Byte Length | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x01 0x10 | 0x00 0x02 | 0x04 | 0x00 0x04 0x05 0x85 | 0x7C | 0x01 |
Response Frame:
| Address Code | Function Code | Registry Address | Registry Length | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x01 0x10 | 0x00 0x02 | 0x41 | 0xf1 |
3.4, Modify 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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