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UDMI / Docs / Specs / Modbus

Modbus Specification

UDMI supports reading Modbus points by specifying them via a modbus:// URL schema.

URI Schema

modbus://<host>[:port]/<unitid>/<function>/<address>[/<quantity>][?interpretation]

• host: The host name or IP address of the Modbus endpoint.
  • If the host maps to a named network, as indicated in the networks field in the gateway model, then the parameters there define the effective parameters to use.
• port: (Optional) The TCP port.
• unitid: The Slave ID (Unit Identifier).
• function: The Function Code (see Function Codes).
• address: The starting register address.
• quantity: (Optional) The number of registers to fetch.
• interpretation: (Optional) Query string properties that are necessary to properly interpret data fetched from a register.

Function Codes

• 1: Read Coils
• 2: Read Discrete Inputs
• 3: Read Holding Registers
• 4: Read Input Registers
• 5: Write Single Coil
• 6: Write Single Register
• 15: Write Multiple Coils
• 16: Write Multiple Registers

Interpretation Parameters

Parameters passed in the query string define how to interpret the fetched register data:

• border: i.e., MSB (Big-Endian) or LSB (Little-Endian).
• type: i.e., INT16, UINT32, BOOLEAN, ASCII.
• worder: i.e., HWF (High-Word First) or LWF (Low-Word First) (for 32-bit values).
• scale: i.e., 1.0, 0.01, 100.0 (scale factor).

Network Parameters

The host maps to a named network in the device’s model_localnet.json (under the networks field). Each named network can define the following parameters for communication:

• baud: The baud rate.
• protocol: i.e., RTU, TCP.
• parity: For serial RTU.
• data bits: For serial RTU.
• stop bits: For serial RTU.

Examples

The metadata values in the examples below map to the following complete Modbus URIs:

• Network RTU Point: modbus://modbus_rtu_1/1/3/40001/1?type=INT16&border=MSB
• fan_status: modbus://modbus_rtu_1/2/1/101?type=BOOLEAN
• filter_differential_pressure: modbus://modbus_rtu_1/2/4/30005?type=UINT32&worder=LWF&scale=0.01

Network Configuration Example

The serial-bus RTU network specification as part of a gateway metadata.json file:

{
  "localnet": {
    "networks": {
      "modbus_rtu_1": {
        "family": "modbus",
        "adjunct": {
          "protocol": "RTU",
          "baud": 9600,
          "parity": "none",
          "data_bits": 8,
          "stop_bits": 1,
          "device": "COM1"
        }
      }
    }
  }
}

Proxy Device Example

For a proxied Modbus device, the gateway block in the metadata.json specifies the target device ID (Unit ID), while the pointset block defines the individual points and their register mappings.

{
  "gateway": {
    "target": {
      "family": "modbus",
      "addr": "2",
      "network_id": "modbus_rtu_1"
    }
  },
  "pointset": {
    "points": {
      "fan_status": {
        "ref": "1/101?type=BOOLEAN"
      },
      "filter_differential_pressure": {
        "units": "Pascals",
        "ref": "4/30005?type=UINT32&worder=LWF&scale=0.01"
      }
    }
  }
}

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