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Free Modbus PLC Simulator Home

This protocol simulator started one weekend as a test program while developing a SCADA/HMI with modbus RTU and TCP/IP and afterward came in useful testing an embedded gnu-Linux device too. It has worked so well for me, I decided to put a nice GUI onto it and release it for free.
The modbus protocol, also called open-mbus is implemented in many process control and monitoring devices, while most SCADA or HMI systems have a driver. Please download it to try it out.
- Why is it called MOD_RSSIM?
- Because it started out as a Modbus RS-232 Simulator ...but it grew as all applications do, I added TCP/IP support and later, Allen Bradley DF1 slave. Even a simple DF1 master with fixed polling.
Typical uses include:
- Verifying a SCADA/HMI configuration.
- Developing modbus master and slave drivers, for embedded devices and desktop platforms.
- Learning the Modbus protocols.

Quick-start to using the free Modbus Slave

An intuitive, simple user-interface was my goal with the simulator, next I will try guide you through how to get started.
The main program window in Modbus RTU mode
  1. Download the simulator.
  2. Run it, and select TCP/IP or serial protocol.
  3. If you want serial, you can try using com0com (see the links page) to create a local loopback, and skip the serial cable.
  4. Select your serial port from the settings dialog (or if TCP/IP, simply point your HMI/SCADA to the local IP address) and connect.
  5. Next you can click on the "Comms" button in the bottom-right to see the state of the communication link to see if your master is trying to connect and talk to the simulator already.
  6. Now go and look at your SCADA/HMI, if it's not already talking, go into your 'drivers' and make sure they are started (in most cases no license will be needed from modbus protocol in your SCADA simply because it is so popular for verification), and if still not talking, look at your own communication logs.
  7. Unless you are doing something wrong, you should have zeroed raw values visible in the SCADA/HMI. Basically all SCADA analogs and digitals will be zeroed, or in the case of a 4-20mA, will be out-of range. Now for some fun read the User Documentation.

  • The built-in help engine starts when you hit the universal "F1" key. You will need to have a default web-browser installed (no internet needed) to view the help.
  • Screenshot showing Allen-Bradley MASTER  'JOY SSC DF1' protocol emulating a continuous coal miner. Selecting a different protocol opens up new options for setting up each protocol as relevant.

The simulator in the JOY machine mode using Allen-Bradley DF1 protocol

For Real Hardware

Been developing software for 18 years, some of that embedded, ARM, PPC and Blackfin DSP.  I have programmed purely C, heavy C++ and OO, and many other languages. MFC, ATL, STL, COM are my friend, on Win32 and recently gnu-Linux too. I'm not an electronics engineer at all, but I love digital logic.
 Modicon brick CPU
The 'brick'
Pictured above, one of the controllers I once used for demos.

Beer money

Support This Project
This simulator is free, if you do like it and want to encourage it's development, please donate via Sourceforge (or PayPal etc.) by clicking on the button to the left. It costs me about £15 a year for the domain, so random donations are appreciated.
This software program is released under the GPL version 3 license.

This project is open sourced on SourceForge so if you want the latest source code, just follow the link.


May 2010:
Windows 95 support may be dropped - I have released a Windows 7 (and Vista) update with version 8.19, which is a move forward, and means a minimum of Windows 2000 will be needed in future. Old binaries will still be available for download.

Jan 2010:
Job-search over: I found work at the end of January, after the small company I worked for laid a lot of staff off, ironically someone bought them up at the same time as I got new work again.

The author is currently NOT available for hire


Allen-Bradley is a trademark of Rockwell Corporation USA; MODBUS of Modicon USA; JOY is a trademark of Joy Mining Machinery UK; Windows is a trademark of Microsoft Corporation USA.

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