node-red-contrib-modbus-modpackqt

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

100% free, MIT-licensed, no usage limits. Open the ModPackQT web console for register decoding, device simulation and AI assistance.

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]

| Step | Setting | |---|---| | Add a runtime config | TCP, timeout 3000 ms | | Master read | Host 192.168.1.10, port 502, unit 1, FC 3, address 100, qty 2 | | decode-float32 | Endian BE (try LE_SWAP if value looks wrong — many PLCs use CDAB) |

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.

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

| Node | Purpose | |---|---| | modpackqt-config | Shared runtime — master mode (TCP/RTU), serial settings, optional slave server | | modpackqt-master-read | Read FC1/FC2/FC3/FC4 from a remote Modbus device | | modpackqt-master-write | Write FC5/FC6/FC15/FC16 to a remote Modbus device | | modpackqt-slave-read | Read from the embedded slave's register store (verify what masters see) | | modpackqt-slave-write | Push values into the embedded slave's register store | | modpackqt-traffic | Passive monitor — emits one message per Modbus op with full visibility into what's happening on the wire | | modpackqt-master-probe | Live commissioning probe for a single Modbus TCP device. One node per device. Click Open in ModPackQT Console to launch the web tester live-attached. | | modpackqt-slave-probe | Live simulator probe for a single fake slave. One node per port. Web console gives you a real-time register editor. |

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:

| Endpoint | Purpose | |---|---| | GET /api/health | Runtime health + probe count | | GET /api/probes | List all registered probes | | GET /api/probes/:id | Probe details | | POST /api/probes/:id/read | Master probe: read registers { fc, address, quantity } | | POST /api/probes/:id/write | Master probe: write registers { fc, address, values } | | GET /api/probes/:id/store?type=&address=&quantity= | Slave probe: inspect register values | | PUT /api/probes/:id/store | Slave probe: set register values |

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

| Issue | Solution | |---|---| | Decoded float looks like garbage | Try a different word order (BE ↔ LE_SWAP) — common conventions are ABCD and CDAB | | Serial port not configured for RTU mode | Open runtime config → set Serial Port (e.g. /dev/ttyUSB0 or COM3) | | EADDRINUSE on slave port | Another process already uses that port. Pick a different one (e.g. 1502). | | connect ECONNREFUSED | Target Modbus device is unreachable. Check IP / port / firewall. | | Embedded slave is disabled | Open runtime config → check Enable embedded slave server | | Nodes don't appear after install | Fully restart Node-RED. |

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 (^3.0.0) for stability.
• 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

MIT — © ModPackQT. Provided "as is" without warranty of any kind. You are responsible for validating this software in your environment before any use with real equipment.