ModbusScop Slave is a free Modbus slave / server simulator with a modern, dockable Dear ImGui interface. It serves requests over Modbus TCP, Modbus UDP, and Modbus RTU (serial) — and can also serve RTU-framed requests over TCP/UDP for terminal servers that bridge a network link to a serial RTU bus. Answer as one or many simulated devices at once, so engineers can exercise a Modbus master, HMI, SCADA, or gateway without any real field hardware.
Free to use and redistribute under the permissive BSD 2-Clause License.
Developed by Carlos Nardi.
ModbusScop Slave is organized as a tree: Channels → Devices → Maps.
- A Channel is one server endpoint. It is either a Network channel — a TCP listener (serving several masters at once, up to a configurable limit) or a UDP datagram server — or a Serial channel (a Modbus RTU bus on a COM port). Network channels also carry a per-channel framing choice: standard Modbus TCP (MBAP) or Modbus RTU (slave id + PDU + CRC, no MBAP). Each channel runs on its own I/O thread; start and stop them independently.
- A Device is one simulated unit (Modbus/slave ID) inside a channel. A single channel can host many devices with different unit IDs, and requests are routed to the matching device. A device can optionally reply to any ID (catch-all) for single-device setups.
- Each device exposes its data as maps in one of two models:
- RTU model — the four classic Modbus tables (Coils, Discrete Inputs, Holding Registers, Input Registers) as separate, overlap-checked blocks.
- PLC model — one continuous 65,536-word memory with automatic bit/word aliasing, so the same memory is reachable by both bit and register function codes.
- TCP, UDP, and Serial — serve masters over Modbus TCP, Modbus UDP, or Modbus RTU on a serial port, chosen per channel in the New Channel dialog (Network / Serial, and for Network TCP / UDP).
- Modbus RTU over the network — a per-channel framing selector lets a TCP or UDP channel accept and answer RTU-style frames (slave id + PDU + CRC16, no MBAP header), the way terminal servers present serial RTU devices over Ethernet.
- Multi-device, multi-master simulation — many channels, each with many devices, each answering on its own unit ID; TCP channels serve multiple masters concurrently.
- RTU and PLC data models — overlap-checked RTU blocks (FC 01–04) or a full 64K PLC memory with bit/word aliasing and selectable bit order (LSB / MSB).
- Editable map windows — a Master-style grid with per-cell datatype (16 / 32 / 64-bit signed, unsigned, hex, binary, float, double), selectable byte order, per-cell aliases, Base-1 register numbering, configurable rows-per-column, and a per-bit editor. Cells flash when a master writes them (FC 05 / 06 / 15 / 16); the animation is configurable under View → Animation Colors.
- Per-cell simulation — drive individual cells with Increment, Random, or Sine (words) and Toggle / Random (bits), datatype-aware and paced by a per-cell period.
- Request behavior / fault injection — per-map response delay, forced exception code, and no-reply (silent drop); disabled regions answer Illegal Data Address (Exc 02).
- Communication Monitor — a timestamped, filterable log of every request and response frame (by device and by master), with an enable toggle, optional log-to-file with size-based rotation, and a line counter.
- Status Messages — a scrolling, timestamped log of channel start/stop, master connect/disconnect, workspace and map I/O events.
- Dashboard — live per-channel traffic counters (Rx / Tx / exceptions / bytes), connected-master count, and uptime, with a totals row.
- Workspaces — save and reload your entire setup: channels (including transport and framing), devices, RTU blocks and PLC memory, values, per-cell formats, aliases, simulations, and test behaviors. Individual maps can also be exported and imported on their own.
- Themes — dark / light / classic with a customizable accent color; layout and preferences are remembered between runs.
- Go to the Releases page and download the latest
ModbusScopSlavearchive for Windows. - Unzip it anywhere and run
ModbusScopSlave.exe— no installation required.
Requirements: Windows 10/11 (64-bit).
Rendering: ModbusScop Slave uses the GPU by default; you can switch to CPU (software) rendering under View → Rendering (handy over Remote Desktop or in VMs).
- + Network or + Serial — create a channel.
- Network — choose TCP or UDP, the Framing (Modbus TCP
(MBAP) or Modbus RTU over the network), the bind IP (e.g.
127.0.0.1or0.0.0.0), the port (default 502), and — for TCP — the maximum number of masters. - Serial — pick the COM port and set baud rate, data bits, parity, and stop bits.
- Network — choose TCP or UDP, the Framing (Modbus TCP
(MBAP) or Modbus RTU over the network), the bind IP (e.g.
- + Device — add one or more devices to the channel, each with its own unit ID, as RTU or PLC.
- + Block (RTU) or open Memory (PLC) — define the data, then edit values, formats, aliases, and simulations directly in the map window.
- Press Start on the channel and point your Modbus master at the IP/port (or COM port). Watch the traffic in the Communication Monitor and the counters in the Dashboard.
Use File → Save Workspace (Ctrl+S) to keep the whole configuration and reload it later with Open Workspace (Ctrl+O).
ModbusScop Slave is built with these open-source components, each under its own license:
| Library | Used for | License |
|---|---|---|
| Dear ImGui (docking) | user interface | MIT |
| GLFW 3 | window / OpenGL context | Zlib/libpng |
| OpenGL 3 | rendering | — |
| Asio (standalone) | Modbus TCP / UDP / RTU server | Boost Software License 1.0 |
| stb_image | logo / splash decoding | MIT / public domain |
ModbusScop Slave is released under the BSD 2-Clause License. It is provided "as is", without warranty of any kind; the author is not responsible for any damage or loss caused by its use.
BSD 2-Clause License
Copyright (c) 2026, Carlos Nardi
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