Modbus-Mqtt

This module bridges Modbus and MQTT in both directions. It publishes Modbus read/write results to MQTT and accepts MQTT commands that write to Modbus devices.

Supported Modbus Functions

Modbus-to-MQTT topic structure

modbus-iot/{modbusId}/{functionName}

Example; modbus-iot/12/readInputRegisters

MQTT-to-Modbus write commands

Publish a JSON command to the following topic:

modbus-iot/{modbusId}/command/{functionName}

Supported write functions are writeCoil, writeRegister, writeCoils, and writeRegisters. The payload must contain an integer addr (or address) and value (or values).

# Write one coil
mosquitto_pub -t modbus-iot/1/command/writeCoil -m '{"addr":0,"value":true}'

# Write one holding register
mosquitto_pub -t modbus-iot/1/command/writeRegister -m '{"addr":10,"value":123}'

# Write multiple holding registers
mosquitto_pub -t modbus-iot/1/command/writeRegisters -m '{"addr":20,"values":[10,20,30]}'

Each command publishes a result to modbus-iot/{modbusId}/command/{functionName}/result. Successful commands include addr and value; failures also include err. Modbus write results are additionally published to the existing modbus-iot/{modbusId}/{functionName} topic.

Tests

npm test starts local in-memory MQTT and Modbus TCP devices to verify both directions:

• Modbus holding-register read → MQTT message
• MQTT writeRegister command → Modbus holding-register update

Example

Modbus to MQTT

const ModbusMqtt = require('./index')

const options = {
  mqttOptions: {
    brokerUrl: 'mqtt://broker.mqttdashboard.com'
  },
  modbusOptions: {
    type: 'TCP',
    tcp: {
      ip: '127.0.0.1',
      port: 802
    },
    id: 1
  }
}

const modbusMqtt = new ModbusMqtt(options)

modbusMqtt.on('modbus-connect', () => {
  setInterval(() => {
    modbusMqtt.modbusClient.readInputRegisters(0, 10, (err, response) => {
      if (err) return console.log('Err ', err.message)
      console.log('READ INPUT REGISTERS ', response)
    })
  }, 1000)

  setInterval(() => {
    modbusMqtt.modbusClient.readInputRegisters(20, 10)
  }, 2000)
})

MQTT to Modbus

With the bridge running, use any MQTT client to publish a command. This Node.js example writes holding register 10 with the value 123 and prints the result:

const mqtt = require('mqtt')

const client = mqtt.connect('mqtt://broker.mqttdashboard.com')
const commandTopic = 'modbus-iot/1/command/writeRegister'
const resultTopic = `${commandTopic}/result`

client.on('connect', () => {
  client.subscribe(resultTopic, err => {
    if (err) throw err
    client.publish(commandTopic, JSON.stringify({ addr: 10, value: 123 }))
  })
})

client.on('message', (topic, message) => {
  if (topic !== resultTopic) return

  console.log('Modbus write result:', JSON.parse(message.toString()))
  client.end()
})

Output:

{"addr":10,"value":123}

The bridge also publishes the Modbus write response to modbus-iot/1/writeRegister.