Modbus - Data Type Conversion

In use cases where the device resource uses an integer data type with a float scale, precision can be lost following transformation.

For example, a Modbus device stores the temperature and humidity in an Int16 data type with a float scale of 0.01. If the temperature is 26.53, the read value is 2653. However, following transformation, the value is 26.

To avoid this scenario, the device resource data type must differ from the value descriptor data type. This is achieved using the optional rawType attribute in the device profile to define the binary data read from the Modbus device, and a valueType to indicate what data type the user wants to receive.

If the rawType attribute exists, the device service parses the binary data according to the defined rawType, then casts the value according to the valueType defined in the properties of the device resources.

The following extract from a device profile defines the rawType as Int16 and the valueType as Float32:

EdgeX 2.0

For EdgeX 2.0 the device profile has many changes. Please see Device Profile section for more details.

Example - Device Profile

deviceResources:
  - name: "humidity"
    description: "The response value is the result of the original value multiplied by 100."
    attributes:
      { primaryTable: "HOLDING_REGISTERS", startingAddress: "1", rawType: "Int16" }
    properties:
      valueType: "Float32"
      readWrite: "R"
      scale: "0.01"
      units: "%RH"

  - name: "temperature"
    description: "The response value is the result of the original value multiplied by 100."
    attributes:
      { primaryTable: "HOLDING_REGISTERS", startingAddress: "2", rawType: "Int16" }
    properties:
      valueType: "Float32"
      readWrite: "R"
      scale: "0.01"
      units: "degrees Celsius"

Read Command

A Read command is executed as follows:

1. The device service executes the Read command to read binary data
2. The binary reading data is parsed as an Int16 data type
3. The integer value is cast to a Float32 value

Write Command

A Write command is executed as follows:

1. The device service cast the requested Float32 value to an integer value
2. The integer value is converted to binary data
3. The device service executes the Write command

When to Transform Data

You generally need to transform data when scaling readings between a 16-bit integer and a float value.

The following limitations apply:

• rawType supports only Int16 and Uint16 data types
• The corresponding valueType must be Float32 or Float64

If an unsupported data type is defined for the rawType attribute, the device service throws an exception similar to the following:

Read command failed. Cmd:temperature err:the raw type Int32 is not supported

Supported Transformations

The supported transformations are as follows:

From rawType	To valueType
Int16	Float32
Int16	Float64
Uint16	Float32
Uint16	Float64