Modbus Master and Slave Simulators

(updated July 2023)

I needed to learn about Modbus recently, although I’m in the heart of Canada’s SCADA-rich Oil and Gas country, Modbus has somehow eluded me up to now.

Modbus is a communication protocol for industrial devices developed in 1979 by Modicon, now Schneider Electric. It was designed for communicating with programmable logic controllers (PLCs), but has since become popular for industrial sensors and instruments in general., a U.S. non-profit trade association, now controls the Modbus protocol and provides free access to protocol specifications and technical resources

The protocol was originally described in terms of a (typically) single Master and multiple Slaves on an RS-485 bus. The Master polls a slave for register values it wants, and a Slave only responds when addressed by a Master. The protocol was recently re-defined using Client/Server terminology. A Slave is now called a Server because it provides data, and a Master is now called a Client because it requests data.

When developing a device which will communicate using Modbus, it can be helpful to use a Client or Server simulator for testing or comparison. You can use one of the many available complete simulators, modify an existing open-source project to meet your needs, or roll your own bespoke solution using an open-source Modbus library.

Client (Master) Simulators

A Modbus Client (Master) simulator is used to query data from devices and can be a valuable test when developing a Modbus Server (Slave) device. A large number of Master simulators are readily available, including free of charge, open source, and commercial software.

Free Client (Master) Simulators

Modpoll from proconX is a free proprietary Windows command-line program supporting Modbus ASCII, RTU and TCP. Modpoll is a demonstration utility for proconX’s commercial driver libraries, and could be considered a defacto standard from the number of references to it on the web.

ModbusScope is a free open-source cross-platform app for capturing and graphing Modbus data. In the past, I would capture Modbus data using Modpoll and hammer the output into CSV with sed, or use ModScan to capture CSV directly, and plot using using either DatPlot or spreadsheet software. However, thanks to a recomendation from Ben, ModbusScope is now my go-to plotting app. ModbusScope is a Qt app and coded in C++, and it uses muparser (“fast math parser library”) and QCustomplot (“easy to use plotting widget for Qt”), and icons from Lucide.

CAS Modbus Scanner is a free proprietary application recommended by Steven. It can retrieve coils, inputs, holding registers, and input registers (displaying values in a variety of formats), and also discover Modbus devices on a network (testing every address, function, length, and offset to check for exceptions or responses). I used CAS Modbus Scanner to assist in verification testing for the Modbus interface of a new industrial controller.

QModMaster is a free open-source Qt-based Modbus master based on libmodbus (see Libraries below). QModMaster is licensed using the LGPL and includes a bus monitor for examining traffic on the bus. 

Modbus Tester from Schneider Electric is a free proprietary Windows GUI program for reading Modbus registers. It supports Modbus RTU and TCP.

RMMS (Radzio! Modbus Master Simulator) is a free proprietary Windows utility (GUI) and claims to replace commercial ModScan and Modbus Poll utilities. It supports Modbus RTU and TCP, and multiple Modbus slave devices. 

Commercial Client (Master) Simulators

Simply Modbus Master (RTU and ASCII ). The Free mode allows six request messages before the application must be re-started. C$60. A slave simulator and TCP client are also available. The website has a nice intro to Modbus and Modbus Enron.

Modbus Poll from modbus tools was designed to help developers of Modbus slave devices and others to test and simulate the Modbus protocol. Using a multiple document interface, several Modbus slaves and/or data areas can be monitored at the same time. US$129 per developer. The modbus tools website also has a good intro to Modbus.

ModScan from WinTECH Software was developed to verify correct protocol operation in new or existing systems. Extensions provide third-party data acquisition using Control Automation routines or the MS Jet Database engine.  A debug mode displays raw serial data to and from a connected device, and ModScan32 can execute test scripts with stimulus messages and expected responses for production testing. Single user license US$64.95. WinTech also provide a number of other Modbus-related utilities. 

Server (Slave) Simulators

A Server (Slave) simulator provides a source of data for a Modbus Master, and can be useful as a known source of data when setting up a test workflow for a device being developed. A Slave simulator can also be used to create a model of a new device being developed, and act as both the development specification and a source of expected behavior for validating the device being developed.

Free Server (Slave) Simulators

ModRSsim2 was forked from MOD_RSSIM and seems to be the more active of the two now with a number of updates including compiling on Visual Studio 2010. ModRSsim2 supports RS-232 and TCP/IP connections, the full range of Modbus addresses for all four Modbus types (0xxxxx, 1xxxx, 3xxxx, & 4xxxx addresses), as well as diagnostics with complete traffic byte capture and logging capability. ModRSsim2 supports CSV loading and a scripting environment for testing as well as HTML custom displays. It is free and open-source, and licensed under the GPL.

pyModSlave is a free and open-source Qt-based Python-code ModBus RTU and TCP slave from the developer of QModMaster. A Windows executable is provided and pyModSlave includes a bus monitor for examining all traffic on the bus. pyModSlave is licensed under the LGPL

MOD_RSSIM is a Windows-based Modbus PLC Simulator (and basis of ModRSsim2 above). It is free and open-source, and started as a test program for a SCADA/HMI with Modbus RTU and TCP/IP. Typical uses are to verify device configuration, support development of Modbus master and slave drivers for embedded and desktop platforms, and as an educational tool to learn Modbus protocols. 

UnSlave Modbus Slave Simulator . UnSlave simulates any number of Modbus slaves. UnSlave is provided free from Unserver, possibly as a source of test data for Unserver’s Modbus REST API Server, which provides data from Modbus networks and devices to higher-level clients – and is monetized. The Unserver website includes a nice Complete Modbus Guide.

Commercial Server (Slave) Simulators

WinModbus. Modbus Slave Simulator for Windows. GBP62.50. Lifetime support. 14-day functional demo for evaluation. Polished website.

SimServe by SCADAmatic can simulate Modbus ASCII, RTU, or TCP/IP. It provides a user interface for setting up a network topology of multiple devices simultaneously. The developer James brought it to my attention, and was kind enough to provide a guest key for evaluation. SimServe could be a valuable development aid if you are developing SCADA software and need simulated devices for testing, or if you are developing a device and could benefit from having a digital twin for comparison (assuming SimServe is capable of simulatingyour device).


A number of Modbus libraries are available to leverage application development.

FreeMODBUS is a free open-source implementation of the Modbus protocol with separate ASCII/RTU and TCP ports for a variety of embedded systems. I can recommend FreeMODBUS based on first-hand experience replacing a DIY protocol stack in an embedded industrial controller with an 8-bit MPU. FreeMODBUS is licensed using the BSD 3-clause license. 

libmodbus is a free open-source library for Linux, Mac OS X, FreeBSD, QNX and Win32. The library is written in C, supports RTU (serial) and TCP (Ethernet) communications, and is licensed using the BSD 3-clause license. 

Other Resources

Peter Chipkin has a nice list of various Modbus-related tools.

com0com is a free open-source kernel-mode virtual serial port driver for Windows. An unlimited number of virtual COM port pairs can be created, and any pair can be used to connect one COM port based application to another. The module is signed with a test certificate, and requires configuring Windows to load test-signed boot modules.

35 Replies to “Modbus Master and Slave Simulators”

    1. Hi Bart, thanks for your question. If you have a particular commercial device in mind, please reply with a link so I can understand the device better.

      I haven’t come across any slave simulators that simulate a specific device, but on the other hand I haven’t really looked for any. However, the slave simulators I’ve listed can generally be configured to expose whatever registers you wish, such as the registers your power/energy meter provides, although the register data returned will be nonsensical compared to the actual device.

      If you want to simulate the _behavior_ of the device, then I would probably start with pyModSlave and extend the simulation function so it behaves like an actual real power/energy meter. I hope you write back and clarify your use case, it sounds intriguing!

      Cheers, Dale.

        1. Thanks for the links Bart. I found manuals with descriptions of the Modbus interfaces – there are a LOT of registers! Also the registers are organized in two or more blocks, with one set of registers providing measured or calculated data (and at least for the SDM630 seemed could be written to, although I’m not sure why a user would want to), and one set of registers controlling the device configuration. I also noted the SDM630 manual advised that user interface software is available that might be be more convenient to use than the Modbus interface (especially if the device is configured to require a password to change settings).

          You could configure one of the slave simulators listed in the post to expose registers in the same ranges as these devices, it may not be possible to configure the _exact_ same registers, but probably not too difficult to modify a simulator such as pyModSlave if this was essential. It would take more work if you want the data in the registers to approximate a real device configured in a specific way, and a LOT more work if you want the data to respond like a real device (e.g. to have the calculated power change appropriately if the sytem voltage is changed, given the same measured current).

          I hope this is of help. Certainly anything is possible given enough time and motivation! 🙂

          If anyone is interested, here are the manuals that Google found (no guarantee though they are the correct versions):

          Eastron SDM630

          ABB B23 112-100

          1. “there are a LOT of registers”
            Especially the triple phase ones and the models that also come with bidirectional measurements.

            “and one set of registers controlling the device configuration”
            Some provide an interface with buttons on the device itself to config everything (address, baudrate etc), while there are also some that allow to change settings through modbus.

            “I also noted the SDM630 manual advised that user interface software is available that might be be more convenient to use than the Modbus interface (especially if the device is configured to require a password to change settings).”
            Windows only as far as I can see, so not convenient when the device intended to access and process the data is (for example) a Raspberry Pi.

            “You could configure one of the slave simulators listed in the post to expose registers in the same ranges as these devices, but it may not be possible to configure them for the _exact_ same registers – but probably not too difficult to modify e.g. pyModSlave if required.”
            Well, I was hoping someone already had done something like this, which I could work with.

            “It would be more work if you want the data read from the registers to approximate data from a real device configured a specific way, and a lot more work if you want the data to respond to changes in configuration (e.g. the calculated power changes appropriately if the sytem voltage is changed, given the same measured current).”
            Changing values like in a real world situation would be great :). In the case of kWh-counters as simple as incremental, voltage varying around 230V and power also varying (up to couple thousand watts or so).

            Are there any Python-based slave simulators that might be capable of doing this?

          2. Sorry Bart, if there is such a simulator I’m not aware of it.

            pyModSlave has the basics, and could make a good starting point. You might consider contacting the developer to discuss the posibility and potential development stratgies.

            Another option might be to develop your simulator using a simulation environment such as GNU Octave. There is a Modbus toolkit for interacting with devices as a Modbus Master, which could possibly be used as a guide for creating a Slave toolkit.

            Good luck whatever path you take!

  1. If you would like to generate modbus traffic, I need a Server and a client.

    Ideally the client would simulate values continuously that can be read.
    Is there any opensource solution for this ?

    1. Hi Modbus, please clarify what you mean by “simulate values continuously that can be read” so I understand correctly.

      A Modbus “Server” (aka Slave) provides data (in “registers”) for reading by a Modbus “Client” (aka Master). Do you want the server to simply provide changing register values so that the client gets different data values when it reads from the server? If so, PyModSlave can do this, but the data will be random (IIUC).

      If you want to simulate a specific process (e.g. a temperature change from 0C to 100C over 5 minutes with some given resolution), you could replace the random simulation code in pyModSlave with your desired simulation.

      If you also need a Client (aka Master) simulator, you might consider QModMaster. It is developed by the same developer as pyModSlave, and in relatively active development (fwiw, I have used PyModMaster for simple register reading).

      If you need to graphically plot the server data, you might consider ModbusScope. I added ModbusScope to the list recently after receiving a comment recommendation, and can say based on a (brief) evaluation that it works very nicely for the purpose.

      Good luck!

    1. Thanks Ben! ModbusScope look very cool and I will be testing it tomorrow! For the benefit of others, ModbusScope is a cross platform Qt app, coded in C++, and uses muparser (“fast math parser library”) and QCustomplot (“easy to use plotting widget for Qt”) libraries, and Lucid icons.

      I was thinking it would be a nice educational exercise to work through building ModbusScope on both Windows and Linux. Is it hard to build? I didn’t see a makefile (so that’s how much I know! 😉 ).

      1. It’s QT so should be pretty straightforward, but I must admit I am mainly an avid user. The dependencies are bundled with it. You could always take a peek at the .github folder with the workflows that show the commands used in CI.

  2. I know this post is a few years old, but I wanted to toss out another commercial product that functions as a Modbus Simulator. It’s called SimServe by SCADAmatic. It can simulate Modbus ASCII, RTU, or TCP/IP. It provides a user interface that allows the user to setup a network topology of multiple devices simultaneously.

    1. Fyi, the SimServe URL is for anyone who hasn’t discovered that Jame’s name links there (I hadn’t Doh!).

      Fyi I took a quick look at SimServe and installed it with a guest key (thanks James!). It looks like an interesting product, although I haven’t had time to do any thorough testing yet.

      I hope to start working on a client soon for configuring and monitoring the Kayden CLASSIC flow transmitter and switch. I will need to setup a small network for testing, and will include a SimServe server on the network. Watch out for a post on my progress.

  3. Having used Modbus Poll and ModScan32 for many years (I particularly liked the register naming feature in the former) I initiated a development project in-house to make a master simulator that would allow us to work with the kind of instrument specifications we typically have to exchange with out customers (mixing different register and data types, applying scaling, units, descriptions, grouping etc.). I also wanted the ability to trend and log values and get help during debugging by getting the traffic decoded / described in text. So now there is another shareware option available:

  4. Little late to the party here, but I wanted to share an open source multipurpose Modbus GUI tool I have been working on called ModbusMechanic. It’s Java based so it runs on any platform.

    I originally wrote it to quickly read and simulate float data from instruments but now it does quite a few other things including act as a TCP->RTU gateway and scan the RTU bus for active nodes. Couple of my co-workers found it useful, and I’m always looking for feedback to improve it.

    I hope the community finds it useful!

  5. There’s also “mbpoll” which looks like a rewrite of the “modpoll” command-line utility (it uses exactly the same command-line syntax), but using libmodbus instead of the FieldTalk driver, and adding a few useful options such as quiet mode.

  6. I have used Modbus Tools (old but works) and ModbusMonitor to simulator total complex projects in Windows 10 with good luck.

  7. Hi Allison, the commercial ModScan software supports a “DANIEL/ENROM/OMNI” transmission mode in addition to “Standard”, but I don’t recall seeing DANIEL support in the other simulators.

    If you want an open simulator, you may want to review the capabilities of the modbus library used by qModMaster ( used by qModMaster to see if DANIEL support is baked-in. If it is, forcing qModMaster to use DANIEL mode may not be difficult. Good luck!

  8. Thanks for the summary. I’m looking for something to test a Daniels dialect link which uses function code 65 for FP values. Do you know of one?

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.