mV-RS485 Adapter Modbus-RTU Register Information


The following Modbus register information can be used to read/write the Modbus address, read the mV values from a connected sensor, and read/write the calibration values within the mV-RS485 adapter.

 

Read Input Registers

Read Current Address Using Universal Address

The mV-RS485 adapter is programmed such that any sensor will respond to Modbus address 251. This is implemented for the event that the sensor’s current address is unknown.

Function 0x04 (Read input registers)
Registers Data Type Data Size Purpose
0x1000 16-bit integer 1 register Uses the univeral address (251) to read the current mV-RS485 adapter address.

 

Example:

Request Response
Field Name Hex Field Name Hex
Slave Address 0xFB Slave Address 0xFB
Function 0x04 Function 0x04
Start Address Hi 0x10 Byte count 0x02
Start Address Lo 0x00 Register Value Hi 0x00
Quantity of register 0x00 Register Value Lo 0x07
Quantity of register 0x01 CRC Lo 0x20
CRC Lo 0x21 CRC Hi 0xE6
CRC Hi 0x50
  • Using the universal address of 251 (0xFB), the user has read that the current Modbus address of the sensor is 7 (0x0007).

 

Read mV Value

Function 0x04 (Read input registers)
Registers Data Type Data Size Purpose
0x0002, 0x003 32-bit Float, Big-endian 2 registers Requests the mV value recorded by the mV-RS485 adapter.

 

Example:

Request Response
Field Name Hex Field Name Hex
Slave Address 0x01 Slave Address 0x01
Function 0x04 Function 0x04
Start Address Hi 0x00 Byte count 0x04
Start Address Lo 0x02 Register Value Hi 0xC1
Quantity of register 0x00 Register Value Lo 0xC8
Quantity of register 0x02 Register Value Hi 0xBF
CRC Lo 0xD0 Register Value Lo 0x9A
CRC Hi 0x0B CRC Lo 0xB7
CRC Hi 0xDD
  • The user has requested the mV value reading from the mV-RS485 adapter. The sensor responded with 0xc1c8bf9a (-25.09356 mV).

 

Read Point 1 Calibration Values (mV and concentration)

Function 0x04 (Read input registers)
Registers Data Type Data Size Purpose
0x4000, 0x4001, 0x4002, 0x4003 32-bit Float, Big-endian 4 registers Requests the point 1 calibration values (usually 0) set internally on the mV adapter.

 

Example:

Request Response
Field Name Hex Field Name Hex
Slave Address 0x01 Slave Address 0x01
Function 0x04 Function 0x04
Start Address Hi 0x40 Byte count 0x08
Start Address Lo 0x00 Register Value Hi 0x00
Quantity of register 0x00 Register Value Lo 0x00
Quantity of register 0x04 Register Value Hi 0x00
CRC Lo 0xE5 Register Value Lo 0x00
CRC Hi 0x8D Register Value Hi 0x00
Register Value Lo 0x00
Register Value Hi 0x00
Register Value Lo 0x00
CRC Lo 0x31
CRC Hi 0x3D
  • The user has requested the point 1 calibration values from the mV-RS485 adapter. The sensor responded with 0x00000000 (0 mV, 0 value).

 

Read Point 2 Calibration Values (mV and concentration)

Function 0x04 (Read input registers)
Registers Data Type Data Size Purpose
0x4004, 0x4005, 0x4006, 0x4007 32-bit Float, Big-endian 4 registers Requests the point 2 calibration values set internally on the mV adapter.

 

Example:

Request Response
Field Name Hex Field Name Hex
Slave Address 0x01 Slave Address 0x01
Function 0x04 Function 0x04
Start Address Hi 0x40 Byte count 0x08
Start Address Lo 0x04 Register Value Hi 0x3F
Quantity of register 0x00 Register Value Lo 0x80
Quantity of register 0x04 Register Value Hi 0x00
CRC Lo 0xA4 Register Value Lo 0x00
CRC Hi 0x4C Register Value Hi 0x43
Register Value Lo 0x73
Register Value Hi 0xCA
Register Value Lo 0x3D
CRC Lo 0x80
CRC Hi 0x8F
  • The user has requested the point 2 calibration values from the mV-RS485 adapter. The sensor responded with 0x3f8000004373ca3d (1 mV, 243.79 value).

 

 

Write Multiple Registers

Change the Modbus Address

Function 0x10 (Write Multiple registers)
Registers Data Type Data Size Purpose
0x1000 16-bit Integer 1 register Changes the Modbus Address of the mV-RS485 adapter.

 

Example: 

Request Response
Field Name Hex Field Name Hex
Slave Address 0x01 Slave Address 0x01
Function 0x10 Function 0x10
Start Address Hi 0x10 Start Address Hi 0x10
Start Address Lo 0x00 Start Address Lo 0x00
Quantity of register 0x00 Quantity of Outputs Hi 0x00
Quantity of register 0x01 Quantity of Outputs Hi 0x01
Byte count 0x02 CRC Lo 0x05
Outputs Value Hi 0x00 CRC Hi 0x09
Outputs Value Lo 0x05
CRC Lo 0x77
CRC Hi 0x92
  • Using the known address of 1, the user has changed the Modbus address of the sensor with address 1 to 5.

 

Write New Point 2 mV Calibration Value

Function 0x10 (Write Multiple registers)
Registers Data Type Data Size Purpose
0x4004, 0x4005 32-bit Float, Big-endian 2 registers Change the point 2 mV calibration value.

 

Example: 

Request Response
Field Name Hex Field Name Hex
Slave Address 0x01 Slave Address 0x01
Function 0x10 Function 0x10
Start Address Hi 0x40 Start Address Hi 0x40
Start Address Lo 0x04 Start Address Lo 0x04
Quantity of register 0x00 Quantity of Outputs Hi 0x00
Quantity of register 0x02 Quantity of Outputs Hi 0x02
Byte count 0x04 CRC Lo 0x14
Outputs Value Hi 0x3f CRC Hi 0x4D
Outputs Value Lo 0x80
Outputs Value Hi 0x00
Outputs Value Lo 0x00
CRC Lo 0xDB
CRC Hi 0x53
  • The new point 2 mV calibration value has been set to 1.0 (0x3f800000).

 

Write New Point 2 Concentration Calibration Value

Function 0x10 (Write Multiple registers)
Registers Data Type Data Size Purpose
0x4006, 0x4007 32-bit Float, Big-endian 2 registers Change the point 2 concentration calibration value.

 

Example: 

Request Response
Field Name Hex Field Name Hex
Slave Address 0x01 Slave Address 0x01
Function 0x10 Function 0x10
Start Address Hi 0x40 Start Address Hi 0x40
Start Address Lo 0x06 Start Address Lo 0x06
Quantity of register 0x00 Quantity of Outputs Hi 0x00
Quantity of register 0x02 Quantity of Outputs Hi 0x02
Byte count 0x04 CRC Lo 0xB5
Outputs Value Hi 0x43 CRC Hi 0x8D
Outputs Value Lo 0x7C
Outputs Value Hi 0x8F
Outputs Value Lo 0x5C
CRC Lo 0xE7
CRC Hi 0x23
  • The new point 2 concentration calibration value has been set to 252.56 (0x437c8f5c).

 

Note: Point 1 calibration values are typically set at zero; however, the Modbus registers for the point 1 values are 0x4000,0x4001 (mV) and 0x4002, 0x4003 (concentration).