node-red-contrib-modbus-modpackqt 1.1.7

Node-RED nodes for ModPackQT — read/write Modbus TCP registers (master & slave) via the ModPackQT Gateway app. Supports FC1/FC2/FC3/FC4 reads and FC5/FC6/FC15/FC16 writes.

npm install node-red-contrib-modbus-modpackqt

node-red-contrib-modbus-modpackqt

Node-RED Modbus nodes powered by ModPackQT — drag-and-drop nodes to read and write Modbus TCP registers (master & slave) from your Node-RED flows. No hand-written HTTP calls. No protocol handling. Just connect and go.

What is ModPackQT?

ModPackQT is an industrial Modbus Master/Slave platform for engineers and technicians. It provides:

• Modbus TCP and RTU communication via a local gateway app
• Built-in slave simulator — simulate a PLC register map without hardware
• Real-time data visualization, profile management, and AI-assisted setup
• A REST API that this Node-RED palette talks to

👉 Download ModPackQT · Modbus Tutorial · Resources Hub

Requires the ModPackQT Gateway app to be running locally. The gateway is what the nodes talk to — not the cloud server directly.

Installation

Option 1 — Node-RED Palette Manager (recommended)

1. Open Node-RED → Menu (☰) → Manage palette → Install
2. Search for modbus-modpackqt
3. Click Install
4. Restart Node-RED — the ModPackQT palette group appears in the sidebar

Option 2 — npm CLI

cd ~/.node-red
npm install node-red-contrib-modbus-modpackqt
# Restart Node-RED after installation

Prerequisites

1. Sign up at modpackqt.com A free account works for all master nodes. Slave nodes require a paid plan.

2. Download and start the ModPackQT Gateway app Download it from modpackqt.com/download and run it. By default it listens on port 8502 (localhost:8502).

3. Create an API key In the ModPackQT web app go to Settings → API Keys → New Key. Paste it into the modpackqt-config node.

4. For slave nodes — create and start at least one slave in the ModPackQT app first (Slave Simulator → New Slave). Note the Slave ID shown — you'll need it in Node-RED.

Plan limits

Feature	Free	Paid
Master Read (modpackqt-master-read)	✓	✓
Master Write (modpackqt-master-write)	✓	✓
Slave Write (modpackqt-slave-write)	✗	✓
Slave Read (modpackqt-slave-read)	✗	✓

View plans & upgrade →

Available Nodes

Node-RED Modbus Master — Read Registers (FC1/FC2/FC3/FC4)

The modpackqt-master-read node reads registers from any Modbus TCP slave device by routing the request through the ModPackQT Gateway app.

Supported function codes:

• FC1 — Read Coils
• FC2 — Read Discrete Inputs
• FC3 — Read Holding Registers
• FC4 — Read Input Registers

Config options:

Output:

{
  "payload": { "success": true, "values": [100, 200, 300] },
  "topic": "modbus/read/192.168.1.10:502"
}

Node-RED Modbus Master — Write Registers (FC5/FC6/FC15/FC16)

The modpackqt-master-write node writes registers to any Modbus TCP slave device.

Supported function codes:

• FC5 — Write Single Coil
• FC6 — Write Single Holding Register
• FC15 — Write Multiple Coils
• FC16 — Write Multiple Holding Registers

Input: msg.payload — a number (FC5/FC6) or array of numbers (FC15/FC16)

Output: Original msg with msg.success = true

Building a Modbus Slave from Node-RED (Paid plan)

Use modpackqt-slave-write to make Node-RED the data source for a live Modbus slave device. Any external Modbus master (PLC, SCADA, HMI) that connects to the ModPackQT slave port will read whatever values you last pushed from Node-RED.

Typical flow:

[MQTT / Sensor / Timer] → [Function: scale to integer] → [modpackqt-slave-write addr=0] → [External Modbus master reads]

Use cases:

• Publish MQTT sensor data as Modbus TCP registers
• Simulate a PLC register map for HMI/SCADA testing
• Bridge OPC-UA, REST, or database data to Modbus

Slave write input — all of these work:

msg.payload = 234;               // single integer
msg.payload = [234, 1013, 65];   // array — writes 3 registers starting at address 0
msg.payload = "[234, 1013, 65]"; // JSON string also works (inject string type)

Override at runtime:

msg.slaveId      = "42";    // select a different slave per message
msg.address      = 10;      // start at register 10 instead of the configured address
msg.registerType = "coil";  // override register type

Slave read output:

{
  "payload": {
    "values": [234, 1013, 65],
    "registerType": "holding",
    "address": 0,
    "quantity": 3,
    "slaveId": "42"
  },
  "topic": "slave/42/holding/0"
}

Example Flows

Poll Holding Registers Every 5 Seconds (Modbus Master)

[Inject (repeat 5s)] → [modpackqt-master-read FC3 target=192.168.1.10:502 unitId=1 addr=0 qty=10] → [Debug]

Write a Setpoint Register from a Button

[Inject (payload=1234)] → [modpackqt-master-write FC6 target=192.168.1.10:502 unitId=1 addr=100] → [Debug]

Build a Slave — Publish Sensor Data as Modbus (Paid plan)

[MQTT in] → [Function: msg.payload = parseInt(msg.payload * 10)] → [modpackqt-slave-write Holding addr=0] → [Debug]

Any Modbus master on the network now reads live sensor values from the slave.

Import the Demo Flow

1. Open Node-RED → Menu → Import
2. Choose examples/basic-flow.json from the package directory
3. Update the config node with your gateway host/port
4. Click Deploy

Configuration Node

Add one modpackqt-config node per gateway instance. All other nodes reference it.

API Endpoints Used

Troubleshooting

Links

• ModPackQT Homepage
• Download ModPackQT Gateway
• Free Modbus Tutorial
• Modbus Resources Hub
• Node-RED Docs
• Contact / Report an Issue

License

MIT — © ModPackQT