Modbux

by ploxc

<img src="./resources/icon.png" alt="Logo" width="60" />

A Modbus Client & Server Simulation Tool

Modbux is the tool I desperately needed four years ago. It handles both Modbus TCP and RTU, lets you assign datatypes to registers, scan for addresses and data, simulate servers for testing, and actually understand what you're reading, all in one app.

Download

Download the latest release

Available for Windows and macOS

Read the documentation

Features

Client Mode:

• Read/write with 12 data types (int16/32/64, uint16/32/64, float, double, bitmap, timestamps, UTF-8 strings)
• Bitmap detail panel: expandable 16-bit view with per-bit toggles, color coding, invert, and inline comments
• Advanced mode: see all numeric data type interpretations simultaneously
• Read configuration mode: efficiently read only configured registers with automatic grouping, group index column, and inline error rows
• Scan Unit IDs and register ranges
• Big-endian / Little-endian support (swap registers)
• Scaling factors and linear interpolation
• Modbus TCP (with hostname/IP) and RTU support with automatic COM port discovery
• Configuration save/load (JSON)

Server Mode:

• Modbus TCP and RTU server modes (serial port, USB converters, socat virtual pairs)
• Simulate up to 10 Modbus devices (TCP mode)
• 256 Unit IDs per server (0-255)
• 12 data types: numeric registers, bitmap, UTF-8 strings, Unix timestamps, and IEC 870-5 datetime
• Static or random value generation with configurable intervals
• Time-based generators for Unix/datetime that output the current system time
• DateTimePicker with UTC toggle for setting exact timestamps
• Redesigned booleans: individual address rows with toggle circles, inline comments, and hover-to-delete
• Configuration save/load (JSON)
• Instant auto-start

Split Mode:

• Run client and server simultaneously in separate windows
• Server opens in a second window, main window becomes the client
• Connect to your own server via 127.0.0.1 for local testing
• Great for learning Modbus hands-on or developing both sides of a communication simultaneously

State Persistence:

• Everything saves automatically between sessions

UI

Why This Exists

Four years ago, I was thrown into a CHP plant commissioning with zero Modbus experience. The client didn't know the addresses. I didn't know the protocol. I had a scanner that found... something... and a hex reader that showed me values I couldn't interpret.

Is that temperature in register 100 or 101? Does the documentation start at 0 or 1? Why does TIA Portal call it 40000 when the manual says holding register 0? Is it big-endian or little-endian? Input registers or holding registers? Which function code do I even need?

Oh, and it was Modbus RTU, so add "are my termination resistors right?" and "is my wiring actually correct?" to the list. Because if those aren't perfect, you're reading absolutely nothing and troubleshooting blind.

This went on for four years.

Gradually, I figured it out. But I watched colleague after colleague hit the same walls. We'd eventually succeed, sure, with blood, sweat, and way too many site visits. The tools we had could read a float or an int if you were lucky, but they never gave you the full picture. Endianness? Swap the registers yourself and hope. Documentation starting at the wrong offset? Good luck finding where the data actually lives.

Modbus TCP helped, at least the physical layer wasn't fighting you anymore. I built a TypeScript implementation for my home solar panels using modbus-serial (thanks yaacov!), and things started clicking. The protocol itself actually made sense once you understood it.

Then came another commissioning. More problems. More frustration. And I snapped.

"I've had enough of this."

I dove into the protocol properly, really understood it, and realized: I already have the pieces. I know how this works now. I can build the tool I've been wishing existed this entire time.

So I did.

I built something that shows you every possible data interpretation at once. That scans not just for devices, but for where the actual data is (because I once spent hours discovering third-party documentation had the wrong register range). That lets you test PLC implementations against simulated devices before you're on-site hoping everything works.

I use it every single day now. The idea of writing Modbus code and just hoping it works when I connect to the real device? That's not how I work anymore.

If you've ever fought with Modbus, you'll understand why this needed to exist.

Built with Electron, React, and Material-UI. Open source because this industry needs better tools.

Why "Modbux"?

It's a typo I always made, modbux instead of modbus. But it fits: it's your user experience (UX) working with Modbus. The typo became the brand.

Pro Tip: Let AI Do the Boring Stuff

Saved configs are JSON. Show Claude or ChatGPT a few example registers, paste your device documentation, ask it to complete the mapping. Load it back into Modbux, done. Welcome to 2026.

Installation

Windows

Download the .exe file from releases.

⚠️ SmartScreen warning: Click "More info" → "Run anyway"

macOS

Download the .dmg file from releases.

⚠️ First time opening: Right-click the app and select "Open" (or go to System Preferences → Security & Privacy → "Open Anyway")

Build It Yourself

Modbux is an Electron application built with React, TypeScript, and Material-UI.

Prerequisites

• Node.js (LTS version recommended)
• Yarn package manager

Setup

Clone and install dependencies:

git clone https://github.com/ploxc/modbux.git
cd modbux
yarn

Development

Run the app in development mode:

yarn dev

Testing

Run unit tests:

yarn test

Run unit tests in watch mode:

yarn test:watch

Run E2E tests:

yarn test:e2e

Build

Create a distributable package for your platform:

# For Windows
yarn build:win

# For macOS
yarn build:mac

# For Linux
yarn build:linux

Linux notes:

The default Modbus port (502) requires elevated access. To allow binding without sudo:

sudo sysctl net.ipv4.ip_unprivileged_port_start=502

To persist across reboots:

echo 'net.ipv4.ip_unprivileged_port_start=502' | sudo tee /etc/sysctl.d/50-unprivileged-ports.conf
sudo sysctl --system

For serial port access (RTU mode), add your user to the dialout group:

sudo usermod -aG dialout $USER

Log out and back in for it to take effect.

Contributing

Found a bug? Have a feature request? Please open an issue! Modbux was born from real-world frustrations, and your feedback helps make it better for everyone in the industry.

Feel free to contribute—whether it's reporting issues, suggesting features, or submitting pull requests. See CONTRIBUTING.md for guidelines. Let's build the Modbus tool we all wish we'd had from day one.

License

This project is licensed under the MIT License - see the LICENSE file for details.

Third-Party Software

This software uses open-source packages with permissive licenses (MIT, ISC, Apache-2.0, BSD). All third-party licenses are included in the distributed application.