A cross-platform Modbus simulator — Slave and Master, in one desktop toolkit.
Built with Rust · Tauri 2 · Vue 3
English · 中文
Testing a Modbus integration usually means wiring up a real PLC or borrowing a master station. This project puts both ends on your desktop:
- 🛰️ Slave & Master in one repo — simulate a field device, or drive one, with no external hardware.
- 🔌 Five transports, one core — TCP, TCP+TLS, RTU, ASCII and RTU-over-TCP, covering function codes FC01–FC06 / FC15 / FC16.
- 📈 Built-in data simulation — drive registers with fixed / random / sine / sawtooth / triangle / counter / CSV-playback sources; 20,000+ registers with virtual scrolling.
- 🖥️ Native desktop app — small Rust + Tauri binaries for Windows, macOS and Linux, with in-app auto-update.
- 🌏 Bilingual UI — full English / 简体中文, switchable at runtime.
- Screenshots
- Features
- Download
- Supported Function Codes
- Transport Modes
- Build from Source
- Quick Start
- Architecture
- Contributing
- Changelog
- macOS First Launch
- Anonymous Usage Analytics
- License
Slave · 20,000 registers with live random mutation
ModbusSlave runs a real Modbus TCP server on 0.0.0.0:502 with two slave devices. The register table virtual-scrolls through 20,000+ holding registers; Random Mutation changes values in place (orange flashes) and the value panel on the right decodes the selected register as Signed / Unsigned / Hex / Binary at once.
Master · communication log with decoded TX/RX frames
The bottom log panel records every request/response pair — direction, function code and a readable detail (R 0 x60, 60 regs) side by side, filterable by direction / FC / text and exportable to CSV.
- Multi-transport — TCP, TCP+TLS, RTU (serial), ASCII (serial), RTU-over-TCP
- Modbus TCP over TLS — TLS 1.2+ encryption, PEM and PKCS#12 certificate formats, optional mTLS (mutual authentication with client certificates)
- Multiple slave devices — create connections on any port, add multiple slave devices per connection
- Four register types — Coils (FC01), Discrete Inputs (FC02), Holding Registers (FC03), Input Registers (FC04)
- Full protocol support — Read (FC01–04), Write Single (FC05/06), Write Multiple (FC15/16), with Modbus exception codes
- Register table — address search/filter, inline value editing, Ctrl/Shift multi-select, virtual scrolling (20,000+ registers), multi-format display (Auto / U16 / I16 / Hex / Bin / Float32 with 4 byte orders)
- Default initialization — new slaves pre-fill addresses 0–20,000 across all four register types; batch-add supports up to 50,000 entries per operation
- Value panel — Signed/Unsigned/Hex/Binary (16-bit), Long/Float (32-bit), Double (64-bit), all byte orders (AB CD / CD AB / BA DC / DC BA)
- Dynamic data sources — simulate changing register values: Fixed, Random, Sine, Sawtooth, Triangle, Counter, CSV playback
- Communication log — real-time TX/RX logging with search, direction/function-code filtering, and CSV export
- Project files — save/load complete configurations as
.modbusprojfiles for quick scenario switching - Serial port support — auto-detect system serial ports, configurable baud rate, data bits, stop bits, parity
- Multi-transport — TCP, TCP+TLS, RTU (serial), ASCII (serial), RTU-over-TCP
- Modbus TCP over TLS — TLS 1.2+ encryption, PEM and PKCS#12 certificate formats, accept-invalid-certs mode for self-signed certificate testing
- Scan groups — periodic polling with custom intervals per register group, per-group slave ID override
- Device discovery — slave ID scan (1–247), register address scan, auto-add discovered devices to scan groups
- Multi-format data view — Unsigned, Signed, Hex, Binary, Float32 (AB CD / CD AB), virtual scrolling
- Write operations — write single/multiple coils and registers (FC05/06/15/16)
- Communication log — TX/RX logging with search/filter (direction, function code, text) and CSV export
- Auto-reconnect — configurable reconnection with exponential backoff (1s → 2s → 4s → … → 30s max)
- Project files — save/load connection and scan group configurations
- Connection-on-scan — auto-prompt to scan devices after a successful connection
- In-app auto-update from GitHub Releases (signed bundles, 6 h check throttle, "later" snoozes 24 h)
- Unified error system — structured
ModbusErrorwith categorized error types (connection/protocol/application), serialized to JSON for frontend parsing - Shared Vue components — common composables, types and i18n shared between both Tauri apps via the
shared-frontendnpm workspace
Pre-built installers for every platform are on the Releases page.
| Platform | Installer |
|---|---|
| Windows | .msi / .exe (NSIS) |
| macOS | .dmg (Apple Silicon & Intel) |
| Linux | .AppImage / .deb / .rpm |
Both apps auto-update from GitHub Releases since v0.16.0. macOS users need one extra step on first launch.
Note: As of v0.16, the native egui edition (assets with the
-egui-suffix) is discontinued. Please migrate to the Tauri installers listed above.
Users in mainland China may have unstable access to GitHub Releases. Recommended mirror for direct installer downloads:
Starting from v0.16.0, the in-app updater automatically falls back through multiple proxies — no manual action needed. However, the very first upgrade from a pre-0.16 build has no in-app updater at all; download and install the new version once via the mirror above, after which updates route through proxies automatically.
| Code | Function | Slave (Server) | Master (Client) |
|---|---|---|---|
| FC01 | Read Coils | Read | Read/Poll |
| FC02 | Read Discrete Inputs | Read | Read/Poll |
| FC03 | Read Holding Registers | Read | Read/Poll |
| FC04 | Read Input Registers | Read | Read/Poll |
| FC05 | Write Single Coil | Write | Write |
| FC06 | Write Single Register | Write | Write |
| FC15 | Write Multiple Coils | Write | Write |
| FC16 | Write Multiple Registers | Write | Write |
| Mode | Transport | Framing | Use Case |
|---|---|---|---|
| TCP | TCP/IP socket | MBAP header | Standard Modbus TCP |
| TCP+TLS | TLS over TCP | MBAP header | Secure Modbus TCP (TLS 1.2+) |
| RTU | Serial port | Slave ID + CRC-16 | RS-485/RS-232 devices |
| ASCII | Serial port | : + hex + LRC + CRLF |
Legacy serial devices |
| RTU-over-TCP | TCP/IP socket | Slave ID + CRC-16 | Industrial gateways |
# install frontend dependencies (npm workspaces — run once at the repo root)
npm install
# run the Slave
cd crates/modbussim-app && cargo tauri dev
# run the Master
cd crates/modbusmaster-app && cargo tauri devcd crates/modbussim-app && cargo tauri build
cd crates/modbusmaster-app && cargo tauri buildcargo test --workspaceA full round-trip in four steps — drive the simulated Slave from the Master, no hardware required. (Screenshots show the Chinese UI; flip to English any time with the 中 / EN toggle.)
Open ModbusSlave and click 新建连接 (New Connection): pick TCP, a port (e.g. 502) and random initialization — the server starts with a device pre-filled across all four register types (coils, discrete inputs, holding and input registers). Turn on 随机变化 (Random Mutation) to make values move; click any row to decode it in the value panel.
Open ModbusMaster and click 新建连接 (New Connection). The defaults already target the local Slave: address 127.0.0.1, port 502, slave ID 1. Tick 启用 TLS (Enable TLS) for a secure link. Click 创建 (Create), then 连接 (Connect).
Add scan groups (e.g. holding registers 0–59 and coils 0–31) and start polling. The connection tree shows each group with its function code and range; the table refreshes on every poll with the values served by the Slave.
Expand 通信日志 (Communication Log) at the bottom: every TX/RX pair is decoded — direction, function code and a readable detail. Back on the Slave, mutated values surface in the Master's table on the next poll. Write back from the Master (FC05/06/15/16) and confirm the change lands on the Slave.
ModbusSim/
├── crates/
│ ├── modbussim-core/ # Core library: protocol, transport, registers, logging
│ │ └── src/
│ │ ├── slave.rs # Slave connection (TCP/RTU/ASCII/RtuOverTcp dispatch)
│ │ ├── master.rs # Master connection with multi-transport support
│ │ ├── frame.rs # RTU/ASCII frame encode/decode
│ │ ├── pdu.rs # Modbus PDU request/response parsing
│ │ ├── transport.rs # Transport enum, serial config, TLS config, port enumeration
│ │ ├── mbap.rs # MBAP frame encoding/decoding (for TLS mode)
│ │ ├── tls_slave.rs # TLS-enabled Modbus TCP slave server
│ │ ├── tls_master.rs # TLS-enabled Modbus TCP master client
│ │ ├── register.rs # Register types, encoding/decoding
│ │ ├── data_source.rs # Dynamic data sources for register simulation
│ │ ├── reconnect.rs # Reconnect policy with exponential backoff
│ │ ├── error.rs # Unified ModbusError enum
│ │ ├── project.rs # .modbusproj file save/load/migrate
│ │ └── log_collector.rs # Thread-safe log ring buffer
│ ├── modbussim-app/ # Slave Tauri application
│ └── modbusmaster-app/ # Master Tauri application
├── frontend/ # Slave Vue 3 frontend
├── master-frontend/ # Master Vue 3 frontend
└── shared-frontend/ # Shared Vue components, composables, i18n
| Layer | Stack |
|---|---|
| Backend | Rust, Tokio (async runtime), tokio-modbus, tokio-serial, native-tls (macOS Security.framework / Linux OpenSSL / Windows SChannel), serialport |
| Frontend | Vue 3, TypeScript, Vite, @tanstack/vue-virtual |
| Desktop | Tauri 2 |
Issues and pull requests are welcome. For a code change, please make sure cargo test --workspace passes (plus the frontend type checks) before opening a PR.
See CHANGELOG.md or the Releases page.
Starting from v0.16.0, both apps check GitHub Releases on startup and prompt to install new versions. Users on pre-0.16 builds need to upgrade manually once.
The bundles are not Apple-notarized (no paid Developer Program). On first launch macOS shows "ModbusSlave / ModbusMaster cannot be opened — Apple could not verify…" with only Done and Move to Trash buttons. This is the standard macOS 15 (Sequoia) block for ad-hoc-signed apps — the app is not damaged.
How to allow it (pick one)
1. GUI path
- Double-click the
.app, see the block dialog, click Done. - Open System Settings → Privacy & Security, scroll to the bottom.
- You'll see "ModbusSlave was blocked…" — click Open Anyway and enter your password.
- The next dialog has an Open button; click it. Subsequent launches go straight through.
2. One-line Terminal
xattr -dr com.apple.quarantine "/Applications/ModbusSlave.app"
xattr -dr com.apple.quarantine "/Applications/ModbusMaster.app"Strips the quarantine flag so macOS stops blocking.
ModbusSim sends an anonymous app_started event on launch via Aptabase, so the author can see install counts, active usage, and version/OS distribution. It collects no personal data — only app version, OS, locale, and an approximate country derived from your IP (the IP itself is never stored). You can turn it off anytime via the ⓘ "About" popover in the toolbar.