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Modbus TCP class

, 23 May 2014 CPOL
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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 haven’t explained how the modbus protocol works in detail, because there is plenty of information around.

Using the code

All commands can be sent in synchronous or asynchronous mode. If a value is accessed in synchronous mode, the program will stop and wait for the slave to response. If the slave doesn't 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 asynchonous connection. The class directory contains a help file (ModbusClassTCP\doc) with detailed information for every function.

An example application that shows the basic features is included in the package. It demos all functions in asynchronous mode. The synchronous functions are very similar. The sample has some additional code to make the results and handling more comfortable.

Points of Interest

Remember that a lot of Modbus clients use a watchdog telegram to make sure the master is still alive. If you don’t call this watchdog within a specified time, the slave will no longer set any output.


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
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United States United States
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Comments and Discussions

QuestionModbus TCP Class not working PinmemberJoseph Frippiat23-Jan-12 2:50 
AnswerRe: Modbus TCP Class not working PinmemberStephan Stricker23-Jan-12 2:56 
GeneralRe: Modbus TCP Class not working PinmemberJoseph Frippiat23-Jan-12 3:13 
GeneralRe: Modbus TCP Class not working PinmemberJoseph Frippiat23-Jan-12 23:24 

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