node-red-contrib-modbus-modpackqt 4.0.1

Modbus commissioning, testing & analysis tools for Node-RED. TCP/RTU master and slave, register store and debugging. Free-to-use proprietary software: modification and redistribution restricted. By ModPackQT.

npm install node-red-contrib-modbus-modpackqt

node-red-contrib-modbus-modpackqt

Modbus commissioning, testing & analysis tools for Node-RED. By ModPackQT.

Free to use under proprietary terms. Modification and redistribution are restricted; normal Node-RED configuration and your own flows remain permitted. See LICENSE. Open the ModPackQT web console for register decoding, device simulation and AI assistance.

License transition: versions before 4.0.0 retain their applicable MIT rights. Version 4.0.0 introduces new terms, not a change to earlier copies. Read the license before upgrading. This is a legal restriction, not technical copy protection.

Release correction: 4.0.1 contains only the established nodes; Master Pro is excluded from registration and the published files. Version 4.0.0 prematurely included Master Pro and is deprecated. Master Pro remains in local development pending explicit release approval.

What you get

• Modbus master — read (FC1–FC4) and write (FC5/FC6/FC15/FC16) over TCP or RTU (serial)
• Embedded Modbus TCP slave server — push values from any flow, let PLCs / SCADA / HMIs read them
• Cloud profile pickers — paste your ModPackQT Account Key into the runtime config and a My Connections dropdown loads your saved devices, auto-filling host / port / unit. The slave server node has a matching My Slaves picker. No more retyping IPs across nodes.
• Outputs raw register values — pair with node-red-contrib-bytes-modpackqt to decode int / float / string / bitmask
• Zero external dependencies — Modbus runs inside the Node-RED process

Install

Recommended (one shot): install both palettes — Modbus + decoders — together.

Palette manager → Manage palette → Install:

• node-red-contrib-modbus-modpackqt
• node-red-contrib-bytes-modpackqt

npm:

cd ~/.node-red
npm install node-red-contrib-modbus-modpackqt node-red-contrib-bytes-modpackqt
# then restart Node-RED

5-minute walkthrough

1. Read a temperature float from a PLC

[inject every 5s] → [modbus master read FC3 addr=100 qty=2] → [decode-float32 BE] → [debug]

msg.payload along the wire:

• After Modbus read: [16828, 0] (raw registers)
• After decode-float32: 23.5 (clean float — done)

2. Send a setpoint back

[inject 23.5] → [encode-float32 BE] → [modbus master write FC16 addr=200]

The encoder converts 23.5 → [16828, 0], the write node sends it to the PLC.

Optional dynamic Start Address (free)

Enable Dynamic address → Use msg.address when present in Master Write. One node can then target different coils or registers on the same configured device:

// FC5: turn coil 0 on
msg.address = 0;
msg.payload = true;
return msg;

Send another message with { address: 5, payload: false } to turn coil 5 off. For FC6 use a numeric payload; for FC15/FC16 use the existing array/Quantity settings. Addresses are zero-based integers from 0 to 65535 (decimal integer strings are also accepted), not 40001-style register reference labels.

The option is off by default, preserving existing flows. With it enabled, a missing msg.address falls back to the configured Start Address. Invalid supplied addresses and write ranges extending past 65535 are rejected at runtime before any request is sent and reported to Catch nodes. Successful output includes msg.addressWritten, msg.valuesWritten and msg.success.

Only the address is dynamic. Device, Unit ID, Function Code and Quantity stay configured in the node. Each message is still one Modbus request; this option adds no timer and does not change incoming trigger intervals or queue policy.

Use one shared runtime per physical RTU port; this build does not introduce cross-runtime serial ownership or dynamic per-message unit IDs. Node-level read-only controls do not secure the physical device or the separate local probe HTTP API. Keep network access protected; a gateway key is not Pro licensing or local Modbus authentication.

3. Watch what's happening (debug visibility)

Drop a modbus traffic node anywhere on the canvas, point it at the same runtime config, wire to a Debug node. You'll see one message per Modbus op:

{
  "ts": "2026-05-09T14:23:01.234Z",
  "direction": "read",
  "kind": "master",
  "target": "192.168.1.10:502",
  "unitId": 1,
  "fc": 3,
  "address": 100,
  "quantity": 2,
  "values": [16828, 0],
  "durationMs": 12,
  "ok": true
}

Filter by direction, function code, or target if you only want a slice.

4. Be a Modbus slave (let SCADA read your values)

In the runtime config check Enable embedded Modbus TCP slave server, set port 1502. Then:

[any source] → [encode-int32 BE] → [modbus slave write holding addr=0]

External masters connecting to your-host:1502, unit 1, FC 3, address 0, qty 2 will read the latest value back.

Cookbook (combined with the bytes palette)

Decode a status bitmask

[master read FC3 addr=50 qty=1] → [decode-bitmask bits=8] → [debug]
// payload = [true, true, false, true, false, false, false, false]

Read a device serial number string

[master read FC3 addr=10 qty=8] → [decode-string BE encoding=utf8 trim=true] → [debug]
// payload = "SN-2025-A0042"

Bridge MQTT → Modbus

[mqtt in topic=setpoint] → [encode-float32 BE] → [master write FC16 addr=200]

Bridge Modbus → MQTT

[poll every 5s] → [master read FC3 addr=100 qty=2] → [decode-float32 BE] → [mqtt out topic=temp]

Mirror a remote PLC into local slave server

[poll 1s] → [master read FC3 addr=0 qty=10] → [slave write holding addr=0]
// any local SCADA can now read from your Node-RED slave instead of hammering the PLC

Alert on Modbus errors

[modbus traffic filter=any] → [switch ok==false] → [email out]

Available nodes

Live commissioning with probe nodes

The two probe nodes (modpackqt-master-probe, modpackqt-slave-probe) are tools for the kind of work you do once per device — figuring out a register map, decoding bytes correctly, simulating a device for SCADA development. They have no flow inputs or outputs; instead, each probe registers itself with a small local HTTP runtime, and the modpackqt.com web console attaches live.

Pattern: one probe per device

┌─────────────────────────┐  ┌─────────────────────────┐  ┌─────────────────────────┐
│  master-probe           │  │  master-probe           │  │  slave-probe            │
│  Inverter A             │  │  Energy Meter           │  │  Fake Inverter          │
│  192.168.1.10:502 #1    │  │  192.168.1.20:502 #5    │  │  Listening :1502 #1     │
│  [ Open in Console ]    │  │  [ Open in Console ]    │  │  [ Open in Console ]    │
└─────────────────────────┘  └─────────────────────────┘  └─────────────────────────┘

Click Open in Console on any probe → the web console opens with all probes from this Node-RED instance in the sidebar, the clicked one pre-selected. Switch between devices with one click.

Hidden runtime

The first probe deployed starts a small HTTP server on 127.0.0.1:8502:

The server auto-stops when the last probe is removed.

Network access

Default bind is loopback-only (127.0.0.1) — only browsers on the same machine can reach it. To allow remote browsers on your LAN:

MODPACKQT_PROBE_HOST=0.0.0.0 MODPACKQT_PROBE_PORT=8502 node-red

Port 8502 falls back to 8503–8506 automatically if it's already in use.

Why this palette?

These nodes are built for commissioning, testing, and analysis — the kind of work where you need to quickly probe a device, verify register layouts, decode bytes correctly, simulate a slave for SCADA development, or watch traffic to debug a flaky link. Pair them with the ModPackQT web console for AI-assisted register decoding, profile management, and a full visual tester.

Importing the example flow

This package ships with a complete demo flow under examples/basic-flow.json showing every node combined with the bytes palette (master read → decode-float32, encode-float32 → master write, status bitmask, traffic monitor, slave loop).

Node-RED → Menu → Import → Examples → node-red-contrib-modbus-modpackqt → basic-flow.

Make sure node-red-contrib-bytes-modpackqt is also installed before importing — the example uses both.

Troubleshooting

Reporting bugs & getting updates

• Bugs / feature requests: use our contact page.
• Security issues: report privately via the security page.
• Updates are never automatic. Node-RED's palette manager will show "update available" when we publish a new version — you choose when to upgrade. Pin a major version (^4.0.0) for stability. Users retaining an earlier MIT release should keep their existing major version and review the new terms before upgrading.
• Changelog: the CHANGELOG.md file is shipped inside this package. We follow semver — patch releases for bug fixes only.

Links

• ModPackQT homepage
• Node-RED + ModPackQT docs
• Bytes palette (decoders/encoders)
• Modbus tutorials

License & disclaimer

Free-to-use proprietary — © ModPackQT. See LICENSE for permitted use and restrictions. Earlier MIT copies retain their rights; their notice is preserved in LICENSE-MIT-HISTORICAL. Third-party licenses remain applicable. Provided "as is" without warranty of any kind. You are responsible for validating this software in your environment before any use with real equipment.