How to Simulate Modbus Devices for Testing: 10 Free Tools Compared

You are developing a SCADA system, configuring a PLC, or building an HMI — but the field devices are not available yet. The energy meters, VFDs, temperature sensors, and I/O modules are still on order, or they are installed at a remote site you cannot access.

A Modbus simulator solves this. It runs on your computer and pretends to be a real Modbus device. Your SCADA master or PLC connects to the simulator, reads holding registers, writes coils, and receives responses — exactly as it would from a real device.

This guide covers 10 free Modbus simulators (slave and master), explains when to use each one, and walks you through setting up a complete Modbus TCP test environment on your laptop in under 5 minutes.

1. What Is a Modbus Simulator

A Modbus simulator is software that emulates a Modbus device on your computer. There are two types:

• Slave simulator (server) — pretends to be a field device (meter, sensor, VFD). It listens for requests from a master and responds with register values you configure.
• Master simulator (client) — pretends to be a PLC or SCADA master. It sends read/write requests to a real or simulated slave device.

For most testing, you need a slave simulator — because your SCADA or PLC is the master, and you need something to talk to.

2. Slave Simulator vs Master Simulator

Feature	Slave Simulator	Master Simulator
Role	Pretends to be a field device	Pretends to be a PLC/SCADA master
Listens on	Port 502 (TCP) or COM port (RTU)	Connects to port 502 or COM port
Use case	Test your SCADA/PLC master	Test your field device or slave firmware
You configure	Register values, coil states	Poll rate, register addresses, function codes
Examples	ModRSsim2, OpenModSim, diagslave	modpoll, Modbus Poll, QModMaster

3. Quick Comparison Table

Slave Simulators (Server)

Tool	License	GUI	TCP	RTU	OS	Key Feature
ModRSsim2	Free (open source)	Yes	✅	✅	Windows	Multiple slave IDs, animation, logging
OpenModSim	Free (open source)	Yes	✅	✅	Windows, Linux	JavaScript scripting, error injection
diagslave	Free (freeware)	No (CLI)	✅	✅	Windows, Linux, Raspberry Pi	Lightweight, cross-platform
Modbus Slave	Free trial (30 days)	Yes	✅	✅	Windows	100 slave windows, OLE automation
ModbusSlaveSimulation	Free (MIT)	Yes	✅	✅	Windows, Linux, macOS	Multiple instances, pre-set values

Master Simulators (Client)

Tool	License	GUI	TCP	RTU	OS	Key Feature
modpoll	Free (freeware)	No (CLI)	✅	✅	Windows, Linux, Raspberry Pi	Scripting, float/32-bit support
Modbus Poll	Free trial (30 days)	Yes	✅	✅	Windows	Professional, multiple polls
QModMaster	Free (open source)	Yes	✅	✅	Windows, Linux	Open source, easy to use
pymodbus	Free (open source)	No (Python)	✅	✅	Any (Python)	Custom simulators, scripting
Node-RED	Free (open source)	Yes (web)	✅	✅	Any (Node.js)	Visual flow programming

4. ModRSsim2 — The Classic Free Slave Simulator

ModRSsim2 is the most widely used free Modbus slave simulator. It supports Modbus TCP and RTU, multiple slave addresses, register animation, and traffic logging.

Feature	Details
License	Free (open source)
Protocols	Modbus TCP, Modbus RTU, Modbus ASCII
Slave addresses	Multiple (1–247) simultaneously
Register types	Coils, Discrete Inputs, Input Registers, Holding Registers
Data simulation	Manual entry, random, increment
Logging	Built-in traffic log with frame decode
OS	Windows
Download	sourceforge.net/projects/modrssim2

Setup: Download → run the .exe → select protocol (TCP or RTU) → set slave address → configure register values → click Start. Your SCADA master can now connect on port 502.

5. OpenModSim — Modern Open-Source Slave Simulator

OpenModSim is a newer, actively maintained open-source slave simulator with JavaScript scripting, error injection, and a clean modern interface.

Feature	Details
License	Free (GPL, open source)
Protocols	Modbus TCP, Modbus RTU
Slave addresses	Configurable
Data simulation	Random, increment, decrement with configurable limits and step
Error injection	Simulate exception codes to test master error handling
Scripting	ECMAScript/JavaScript for advanced scenarios
OS	Windows 7+, Linux (Debian, Ubuntu, Fedora, Rocky)
Download	github.com/sanny32/OpenModSim

Best for: Engineers who need to test how their master handles errors — you can inject specific Modbus exception codes (Illegal Function, Illegal Data Address, etc.) to verify error handling.

6. diagslave — Command-Line Slave Simulator

diagslave is a lightweight command-line Modbus slave simulator. No GUI, no installation — just download and run.

Feature	Details
License	Free (freeware)
Protocols	Modbus TCP, RTU, ASCII, UDP, RTU over TCP
OS	Windows, Linux (x86, ARM), Raspberry Pi, QNX
Download	modbusdriver.com/diagslave.html

Example — start a TCP slave on port 502, address 1:

bash

diagslave -m tcp -a 1 -p 502

Example — start an RTU slave on COM7, 19200 baud, even parity:

bash

diagslave -m rtu -a 1 -b 19200 -p even COM7

Best for: Quick testing, scripted test environments, Raspberry Pi, Linux headless servers.

7. Modbus Slave (ModbusTools) — Professional Slave Simulator

Modbus Slave by ModbusTools is a professional-grade simulator with a 30-day free trial. It supports up to 100 slave windows simultaneously and includes OLE automation for Excel integration.

Feature	Details
License	Free trial (30 days), $129 for permanent license
Protocols	Modbus TCP, RTU, ASCII
Slave windows	Up to 100 simultaneously
OLE automation	Interface with Excel, Python, VBA
OS	Windows
Download	modbustools.com/modbus_slave.html

Best for: Professional SCADA integration testing where you need to simulate many devices at once.

8. ModbusSlaveSimulation — Lightweight GUI Simulator

A standalone Windows app supporting Modbus RTU, TCP, UDP, and ASCII over RTU. Multiple instances can run simultaneously.

Feature	Details
License	Free (MIT, open source)
Protocols	Modbus TCP, RTU, UDP, ASCII over RTU
Pre-set values	Set register values before starting the connection
Multiple instances	Run several simulators with different ports/addresses
OS	Windows, Linux (Mono), macOS (Mono)
Download	github.com/GitHubDragonFly/ModbusSlaveSimulation

9. modpoll — Command-Line Master Simulator

modpoll is the master-side companion to diagslave. It sends Modbus requests from the command line — perfect for quick testing and scripting.

Feature	Details
License	Free (freeware)
Protocols	Modbus TCP, RTU, ASCII, UDP, RTU over TCP
Data types	16-bit, 32-bit integer, float, hex, module 10000
OS	Windows, Linux, Raspberry Pi
Download	modbusdriver.com/modpoll.html

Example — read 10 holding registers starting at 500 from a TCP slave:

bash

modpoll -m tcp -r 500 -c 10 192.168.1.100

Example — read 5 floating-point values starting at register 100:

bash

modpoll -m tcp -t 4:float -r 100 -c 5 192.168.1.100

Example — write value 1234 to register 100:

bash

modpoll -m tcp -r 100 192.168.1.100 1234

10. Modbus Poll (ModbusTools) — Professional Master Simulator

Modbus Poll is the most popular Modbus master simulator for professional use. 30-day free trial. Supports multiple simultaneous poll windows, data logging, and all function codes.

Feature	Details
License	Free trial (30 days), $129 permanent
Protocols	Modbus TCP, RTU, ASCII
OS	Windows
Download	modbustools.com/modbus_poll.html

11. QModMaster — Open-Source Master with GUI

QModMaster is a free, open-source Modbus master with a graphical interface. Built with Qt, it runs on Windows and Linux.

Feature	Details
License	Free (LGPL, open source)
Protocols	Modbus TCP, RTU
OS	Windows, Linux
Download	github.com/ed-chemnitz/qmodmaster

12. pymodbus — Python Library for Custom Simulators

pymodbus is a Python library that lets you build custom Modbus master and slave simulators with full control over behavior, register values, and timing.

Feature	Details
License	Free (BSD, open source)
Protocols	Modbus TCP, RTU, ASCII, TLS
Language	Python 3
Install	pip install pymodbus
Download	github.com/pymodbus-dev/pymodbus

Example — create a Modbus TCP slave with 100 holding registers:

python

from pymodbus.server import StartTcpServer
from pymodbus.datastore import ModbusSlaveContext, ModbusServerContext
from pymodbus.datastore import ModbusSequentialDataBlock

store = ModbusSlaveContext(
    hr=ModbusSequentialDataBlock(0, [0]*100),  # 100 holding registers
    ir=ModbusSequentialDataBlock(0, [0]*100),  # 100 input registers
)
context = ModbusServerContext(slaves=store, single=True)
StartTcpServer(context=context, address=("0.0.0.0", 502))

Best for: Automated testing, CI/CD pipelines, custom device emulation, dynamic value generation.

13. Node-RED with Modbus Nodes

Node-RED is a visual flow programming tool that supports Modbus through community nodes. You can build both master and slave simulators using drag-and-drop flows.

Feature	Details
License	Free (Apache 2.0)
Modbus nodes	node-red-contrib-modbus
Install	npm install node-red-contrib-modbus
OS	Any (Node.js)
Download	nodered.org

Best for: IIoT integration, visual prototyping, connecting Modbus to MQTT/HTTP/databases.

14. How to Set Up a Modbus TCP Test Environment in 5 Minutes

What You Need

• One computer (laptop or desktop)
• A slave simulator (ModRSsim2 or diagslave)
• A master application (your SCADA, PLC simulator, or modpoll)

Step 1. Start the Slave Simulator

Using ModRSsim2:

1. Download and run ModRSsim2
2. Select MODBUS TCP/IP protocol
3. Set Station ID = 1
4. The simulator listens on port 502 automatically
5. Set some values in Holding Registers (e.g., register 0 = 2301, register 1 = 1456)

Using diagslave:

bash

diagslave -m tcp -a 1 -p 502

Step 2. Connect Your Master

Using modpoll:

bash

modpoll -m tcp -a 1 -r 1 -c 10 127.0.0.1

This reads 10 holding registers starting at register 1 from the local slave simulator.

Using your SCADA software:

Configure a Modbus TCP connection to 127.0.0.1, port 502, Unit ID 1.

Step 3. Verify

You should see register values in your master application matching what you set in the slave simulator. If you change values in the simulator, your SCADA updates automatically on the next poll.

15. How to Set Up a Modbus RTU Test Environment

For Modbus RTU, you need virtual COM ports because you do not have a physical RS-485 bus.

Step 1. Install Virtual COM Port Software

• Windows: com0com — creates pairs of virtual COM ports (e.g., COM3 ↔ COM4)
• Linux: socat — creates virtual serial pairs

Linux example:

bash

socat -d -d pty,raw,echo=0 pty,raw,echo=0
# Output: /dev/pts/3 and /dev/pts/4 — use these as your COM ports

Step 2. Start the Slave on One Port

bash

diagslave -m rtu -a 1 -b 9600 -p even COM3

Step 3. Connect the Master on the Other Port

bash

modpoll -m rtu -a 1 -b 9600 -p even -r 1 -c 10 COM4

16. Testing Scenarios Every Engineer Should Run

Test	What to Verify	How
Basic read	Master reads holding registers correctly	Read 10 registers, verify values match simulator
Basic write	Master writes to a register	Write a value, verify it appears in the simulator
Wrong slave address	Master handles no response	Set master to address 5, simulator to address 1
Wrong register address	Slave returns exception code 02	Request a register that does not exist
Wrong function code	Slave returns exception code 01	Send FC 04 to a slave that only supports FC 03
Multiple slaves	Master polls multiple addresses	Run multiple simulator instances with different IDs
Timeout behavior	Master handles slave not responding	Stop the simulator, observe master timeout behavior
Large block read	Verify max register count	Read 125 registers in one request
Float values	Verify 32-bit float decoding	Set two consecutive registers to a known float value
CRC errors (RTU)	Master handles corrupted frames	Disconnect/reconnect the serial link during transfer

17. Common Simulation Problems and Fixes

Problem	Cause	Fix
Connection refused on port 502	Simulator not running, or wrong port	Start the simulator first. Verify port 502.
“Port 502 already in use”	Another application is using port 502	Close the other application or use a different port (e.g., 1502).
No response from simulator	Wrong slave address	Verify master and simulator use the same Unit ID.
All registers read as 0	Default values not set	Set register values in the simulator before connecting.
Exception code 02	Register address out of range	Check the register range configured in the simulator.
Float values show garbage	Wrong byte order	Try swapping byte order (AB CD vs CD AB) in your master.
RTU communication fails	Baud rate or parity mismatch	Verify both sides use identical serial settings.
Firewall blocking port 502	Windows Firewall	Allow the simulator application through the firewall.

Summary

A Modbus simulator lets you test your SCADA, PLC, or HMI without physical field devices. The most common setup is a slave simulator on your laptop that your master connects to over TCP port 502.

The key recommendations:

• Quick GUI testing: Use ModRSsim2 (Windows) or OpenModSim (Windows/Linux)
• Command-line / scripting: Use diagslave (slave) + modpoll (master)
• Custom simulation with Python: Use pymodbus
• Error injection testing: Use OpenModSim (inject exception codes)
• Professional multi-device simulation: Use Modbus Slave by ModbusTools (30-day trial)
• RTU testing without hardware: Use com0com (Windows) or socat (Linux) for virtual COM ports

💡 Tip: Use the free Modbus Frame Decoder Tool to decode any Modbus frame captured during simulation — including MBAP header, function code, register addresses, and CRC validation.

For Modbus protocol fundamentals, see: Modbus Protocol Guide

For Wireshark analysis during testing, see: Wireshark for Modbus TCP