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Water Quality ORP Sensor

Reference

This article offers an in-depth look at the Modbus-RTU communication protocol, detailing data frame formats, register addresses, function codes, and practical examples to effectively manage and configure communication networks, including setting ORP values, temperature adjustments, and device configurations.

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 ORP Value 0x03/0x04 mV; 16-bit signed integer, actual value
0001H Temperature Value 0x03/0x04 °C; 16-bit signed integer, 10 times the actual value
0050H ORP Deviation Value 0x03/0x04/0x06/0x10 16-bit signed integer, actual value
0051H, 0052H ORP Slope 0x03/0x04/0x10 Floating-point big-endian
0053H Temperature Deviation Value 0x03/0x04/0x06/0x10 16-bit signed integer, 10 times the actual value
0120H, 0121H Electrode calibration 0x10 Calibration is performed by writing parameters to registers 0x0120 and 0x0121 using function code 0x10, 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, and 7 represents 1200.

3. Communication Protocol Examples and Explanation

3.1 Reading Current ORP Value 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 0x01 0x35 0x01 0x01 0x2B 0x91

ORP Calculation: 0135H=309=>ORP=309mV
Temperature Calculation: 0101H=257=>Temperature=25.7℃

3.2, Set Deviation Value for Current ORP Value for Numerical Correction

Query Frame: If the current device outputs ORP of 309, and the value needs to be corrected to 293, the difference is 293-309=-16, -16=>Calculation method: FFFF-0010+1=0xFFF0, register content is written as FFF0H)

Address Code Function Code Register Address Register Content Check Digit Low Bit Check Digit High Bit
0x01 0x06 0x00 0x50 0xFF 0xF0 0xC8 0x6F

Response Frame:

Address Code Function Code Registry Address Registry Content Checksum Low Byte Checksum High Byte
0x01 0x06 0x00 0x50 0xFF 0xF0 0xC8 0x6F

3.3 Setting the Slope for Current ORP Value for Numerical Correction

Query Frame: If the current device outputs an ORP of 309, and you want to correct the value to 293, the slope is 293/309≈0.7735, 0.7735=>0x3F460419 (floating-point big-endian), and the register content is written as 3F460419.

Address Code Function Code Registry Address Registry Length Byte Length Registry Content Checksum Low Byte Checksum High Byte
0x01 0x10 0x00 0x51 0x00 0x02 0x04 0x3F 0x46 0x04 0x19 0x19 0x94

Acknowledgement Frame:

Address Code Function Code Registry Address Registry Length Checksum Low Byte Checksum High Byte
0x01 0x10 0x00 0x51 0x00 0x02 0x10 0x19

3.4 Register Calibration of Electrodes

If electrode calibration is required, parameters can be written to registers 0x0120 and 0x0121 using function code 0x10. This device uses two-point calibration, requiring two known ORP standard solutions. For the first calibration point, write 0x0001 to register 0x0120 and the standard ORP value for the first point to register 0x0121; for the second calibration point, write 0x0002 to register 0x0120 and the standard ORP value for the second point to register 0x0121. Calibration complete.

Query Frame: Select 86mV ORP standard solution to calibrate the first point. 86=0056H

Address Code Function Code Register Address Register Length Byte Length Register Content Checksum Low Byte Checksum High Byte
0x01 0x10 0x01 0x20 0x00 0x02 0x04 0x00 0x01 0x00 0x56 0x2D 0xD9

Response Frame:

Address Code Function Code Register Address Register Length Checksum Low Byte Checksum High Byte
0x01 0x10 0x01 0x20 0x00 0x02 0x41 0xFE

Query frame: Select 256mV of ORP standard solution to calibrate the second point. 256 = 0100H

Address Code Function Code Registry Address Registry Length Byte Length Registry Content Checksum Low Byte Checksum High Byte
0x01 0x10 0x01 0x20 0x00 0x02 0x04 0x00 0x02 0x01 0x00 0x5C 0x77

Response Frame:

Address Code Function Code Registry Address Registry Length Checksum Low Byte Checksum High Byte
0x01 0x10 0x01 0x20 0x00 0x02 0x41 0xFE

3.5, 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

3.6, 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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