Reference
The article explains the Modbus-RTU protocol, detailing its data frame format and communication parameters, essential for managing chlorine sensors.
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 | Residual Chlorine Value | 0x03/0x04 | 16-bit unsigned integer, 100 times the actual value |
| 1010H, 1011H | 44113, 44114 | Residual Residual chlorine slope | 0x03/0x04/0x10 | Floating-point number, actual value |
| 1012H, 1013H | 44115, 44116 | Residual Chlorine Deviation Value | 0x03/0x04/0x10 | Floating-point number, 100 times the 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 Current Chlorine Residual Value
Query Frame:
| Address Code | Function Code | Registry Address | Registry Length | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x03 | 0x00 0x00 | 0x00 0x01 | 0x84 | 0x0A |
Response Frame:
| Address Code | Function Code | Number of Valid Bytes | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x03 | 0x02 | 0x03 0x16 | 0x39 | 0x7A |
Chlorine Residual Calculation: 0316H=790=>Chlorine Residual=7.90mg/L
3.2, Set Deviation Value for Current Chlorine Residual Value for Numerical Correction
Query Frame: If the current device outputs Chlorine Residual of 7.90, and the value needs to be corrected to 8.00, the difference is 8.00-7.90=0.10, 0.1*100=10=>41200000（Floating-point number）， register content is written as 0x4120，0x0000)
| Address Code | Function Code | Registry Address | Registry Length | Byte Length | Registry Content | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x10 0x12 | 0x00 0x02 | 0x04 | 0x41 0x20 0x00 0x00 | 0xAB | 0x4C |
Response Frame:
| Address Code | Function Code | Registry Address | Registry Length | Checksum Low Byte | Checksum High Byte |
|---|---|---|---|---|---|
| 0x01 | 0x10 | 0x10 0x12 | 0x00 0x02 | 0xE5 | 0x0D |
3.3 Setting the Slope for Current Chlorine Residual Value for Numerical Correction
Query Frame: If the current device outputs an Chlorine Residual of 8.00, and you want to correct the value to 9.00, the slope is （9-0.1）/（ 8.00-0.10）≈1.1267=>3F9037B5（Floating-point number），and the register content is written as 0x3F90，0x37B5.
| 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 | 0x3F 0x90 0x37 0xB5 | 0xE4 | 0xDD |
Response Frame:
| Address Code | Function Code | Registry Address | Registry 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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