Modbus Action

The Modbus action writes register and coil values to Modbus TCP and Modbus RTU devices.

Properties

Property Name	Optional	Description
endpoint	false	Host and port of the Modbus server (e.g. 192.168.1.50:502) or Unix device path.
mode	true	Protocol framing mode: "tcp" (default) or "rtu_over_tcp" / "rtu".
unitId	true	Target slave unit ID (0..=255). Default: 1. Alias: slaveId.
function	true	Target function code: "holding" (FC06/FC16, default) or "coil" (FC05/FC15).
registerAddress	false	0-based wire address of the target register or coil.
field	false	The field name in the output record whose value should be written.
dataType	true	Value type: bool, uint16, int16, uint32, int32, uint64, int64, float32, float64. Default: uint16.
byteOrder	true	Word/byte layout for multi-register fields: ABCD (default), CDAB, BADC, or DCBA.
timeout	true	Socket write timeout (e.g. 1000ms). Default: 1000ms.

Other common sink properties are supported. Refer to sink common properties for more information.

Example

Write a setpoint to holding register 100:

{
  "id": "rule_chiller_actuation",
  "sql": "SELECT target_setpoint FROM telemetry_stream WHERE target_setpoint IS NOT NULL",
  "actions": [
    {
      "modbus": {
        "endpoint": "192.168.1.50:502",
        "unitId": 1,
        "function": "holding",
        "registerAddress": 100,
        "field": "target_setpoint",
        "dataType": "float32",
        "byteOrder": "CDAB"
      }
    }
  ]
}

Dead-Letter Queue (DLQ) Integration

When a write command fails with Modbus exception codes:

• 0x01: Illegal Function
• 0x02: Illegal Data Address
• 0x03: Illegal Data Value

The engine classifies the failure as a permanent delivery error. It sends the output record to the dead-letter queue under DlqStage::SinkDelivery. For configuration options, refer to Dead-Letter Queues.