Modbus software: test, monitoring and
master emulation

Olga Weis

Modbus is an openly published communication protocol built on a master-slave architecture. It is implemented in a wide range of industries for establishing communication among various electronic devices. Modbus protocol can be used for sending data over serial lines RS-485, RS-422, RS-232, and TCP/IP (Modbus TCP) protocols. Modbus over UDP is also possible although not widely used.

Modbus software

What is Modbus and how it works


Modbus protocol is a messaging structure of a master-slave communication between intelligent electronic devices. As a rule signals are transferred from devices back to a main controller or a resource gathering the data. A master device can direct a request individually to a slave device or broadcast it to all slaves. When individual signal is received, the slave will respond, whereas when the signal is broadcast, the response is usually not anticipated. The first byte the master sends is the slave address. This way each slave device knows whether to ignore the message or to listen to it after this first byte.

There are two variations of Modbus protocol used in serial communication:

  • Modbus ASCII - uses ASCII characters for protocol communication. The longitudinal redundancy check checksum is used and the messages are framed by a leading colon (":") and a trailing newline (CR/LF).
  • Modbus RTU - this protocol shows data in a compact binary view. RTU checks info for errors by following the commands with a cyclic redundancy check checksum. Bytes are sent consecutively without spaces between them with a 3.5 character space between messages for a delimiter. This is how the software knows that a new message has started. RTU messages are framed by idle periods.

Types of Modbus Software and their usage


As mentioned before Modbus protocol is very widely implemented, especially the serial versions of it (RTU and ASCII ones). Modbus RTU is actually the most common usage of Modbus protocol. Are you developing software that communicates with electronic devices through Modbus RTU? In this case you will surely appreciate a high-quality Modbus RTU software for monitoring and analysis of data sent via this protocol. Same with Modbus ASCII variation.

Modbus monitoring tool

Modbus monitoring software


Serial Port Monitor developed by Eltima Software helps monitor and analyze messages sent through both variations of Modbus protocol. Its friendly interface shows comprehensive views of data and offers flexible filters for precise analysis. Serial Port Monitor offers a convenient way to track down the possible problems during development of an application or a driver, test and optimize serial devices, etc.

This professional Modbus test software allows you to:

  • Connect to a serial port that is already in use by a different program and start monitoring it right there. All data received by the port is shown in real time for immediate tracking of any possible issues.
  • Monitor several serial ports at the same time. You can analyze the activity of several serial ports in one session synchronously. Data flowing in both directions is conveniently recorded in a single log on a first-in first-out basis.
  • See the captured data in a table, terminal, line, and dump views at the same time. You can use various filters for precise monitoring.
  • Emulate the data transfer to a serial device to see exactly how it will react to specific commands.

Master and Slave Modbus Simulators.


  • QModMaster is a Qt-based app simulating a Modbus master. This software is free of charge and offers a comprehensive graphical interface and a bus monitor for traffic analysis.
  • SimplyModbusMaster -can simulate a Modbus RTU or a Modbus ASCII Master. It displays all information in a single window.
  • Modbus RTU Slave and Modbus ASCII Slave Software - works as a Modbus RTU Slave or a Modbus ASCII Slave and monitors master requests. One window is used for entering and displaying the settings, and the second window - for the data.
  • Modbus Poll features a multiple document interface and simulates a Modbus master. A good helper app for those who are developing the Modbus slave devices. This Modbus software can monitor several Modbus slaves and/or data areas simultaneously. Developers have the possibility to create and send their own test strings in the "test center" and get the result from the slave in hexadecimal format.

Serial Port Monitor

Requirements: Windows XP/2003/2008/Vista/7/8/10/Server 2012 , 9.16MB size
Version 7.0.312 () Release notes