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Posted 5 Nov 2006

Modbus TCP class

, 23 May 2014
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A Modbus TCP class.


This article explains a Modbus TCP common driver class. This class implements a modbus TCP master driver.

It supports the following commands:

  • Read coils
  • Read discrete inputs
  • Write single coil
  • Write multiple cooils
  • Read holding register
  • Read input register
  • Write single register
  • Write multiple register


I don't want to explain how the modbus protocol works in detail, because there is plenty of information around.

Using the code

All commands are sent in synchronous or asynchronous mode. If a value is accessed in synchronous mode, the program will stop and wait for the slave to respond. If the slave does not answer within a specified time, a timeout exception is called. The class uses multi-threading for both synchronous and asynchronous access. The class opens two communication channels for each slave. This is necessary because the synchronous thread has to wait for a command to finish. This would block the asynchronous connection. The class directory contains a help file (ModbusClassTCP\doc) with detailed information for every function.

A sample application is included in the package that shows the basic features. It uses asynchronous mode for all function calls. The sample has some additional code to make the result handling more comfortable.

Points of Interest

Remember that a lot of Modbus clients use a watchdog telegram to make sure the master is still active. If you don’t call this watchdog within a specified time, the slave will close the connection. This feature is used to avoid unused open ports when the connection is interrupted.


Version 3.3 (6/14/2013)
- Fixed: ID high and low byte swapped
- Added: Modbus paramater UNIT added to function calls
Version 3.2
First release


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Stephan Stricker
Systems Engineer
Germany Germany
No Biography provided

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QuestionModBus TCP class for C++ or C Pin
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