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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)


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Stephan Stricker
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Comments and Discussions

QuestionVery good and very useful article...but unable to see data PinmemberJaywant.Kulkarni27-Feb-15 1:51 
QuestionReading registers PinmemberMember 97548706-Nov-14 12:15 
AnswerRe: Reading registers PinmemberGarth J Lancaster6-Nov-14 17:21 
AnswerRe: Reading registers PinmemberMember 975487017-Nov-14 8:42 
Questionerror in connection after a period of time PinmemberMember 104508328-Oct-14 21:26 
Suggestion5 Star project, thanks. Pinmemberzeytin7-Sep-14 7:01 
QuestionHow to communicate with many devices at the same time? PinmemberMember 109289987-Aug-14 23:31 
QuestionHow Can I give command to any hardware unit using modbus ??? PinmemberPdeveloper29-Jul-14 2:04 
QuestionBug in CreateReadHeader code PinmemberMember 861764024-Jun-14 12:38 
QuestionAsync timeout ,byte swapping and a Modbus slave project PinmemberMember 861764024-Jun-14 8:29 
QuestionIssues with using this code in C++ project PinmemberMike Grove19-Jun-14 1:19 
QuestionMessage Transaction Id Pinmemberkhalprin27-May-14 5:17 
BugHow to wait the end of ModBus readcoil function PinmemberRhyzko3-Apr-14 6:37 
GeneralRe: How to wait the end of ModBus readcoil function PinprofessionalStephan Stricker23-May-14 4:57 
QuestionModbus Slave application Pinmemberromasimeiz26-Feb-14 4:25 
AnswerRe: Modbus Slave application PinmemberStephan Stricker27-Feb-14 8:59 
GeneralRe: Modbus Slave application Pinmembernasher21927-Jul-14 23:31 
QuestionHow to download??? Pinmemberrquirch10-Jan-14 11:40 
QuestionModbus TCP Class for 2008 PinmemberThe Real John Richards10-Jan-14 1:02 
AnswerRe: Modbus TCP Class for 2008 PinmemberStephan Stricker7-Feb-14 9:46 
QuestionThanks! PinmemberMember 92730099-Jan-14 5:04 
QuestionSimple way to run class from console PinmemberTadejP8122-Nov-13 12:41 
QuestionVS ?? PinmemberMember 103773933-Nov-13 6:09 
AnswerRe: VS ?? PinmemberFlavi114-Nov-13 6:44 
NewsModbus Framework for .NET PinmemberFlavi126-Oct-13 2:29 
GeneralRe: Modbus Framework for .NET PinmemberMember 399685331-Oct-13 7:41 
GeneralRe: Modbus Framework for .NET PinmemberFlavi18-Nov-13 2:51 
QuestionIllegal Data Address Pinmembersoftwaremonkey3-Oct-13 4:43 
AnswerRe: Illegal Data Address Pinmembersoftwaremonkey3-Oct-13 5:16 
GeneralRe: Illegal Data Address Pinmembermehmet_alixx16-Jan-14 8:53 
Questionmodbus message response is not receiving in same order PinmemberUma Mahes17-Sep-13 21:10 
AnswerRe: modbus message response is not receiving in same order PinmemberStephan Stricker18-Sep-13 4:30 
GeneralRe: modbus message response is not receiving in same order PinmemberUma Mahes21-Jan-14 2:48 
SuggestionCan not read/write Address more than 32767. Edited conversion [modified] Pinmemberhadihidayat9-Sep-13 16:52 
GeneralMy vote of 5 Pinmember陳志杰25-Aug-13 18:49 
SuggestionSome improvements necessary Pinmembermuharrem27-Jul-13 14:17 
GeneralRe: Some improvements necessary PinmemberStephan Stricker29-Jul-13 8:10 
GeneralRe: Some improvements necessary Pinmembermuharrem30-Jul-13 5:51 and ModbusTCP class Pinmemberenfrost15-Jul-13 9:57 
AnswerRe: and ModbusTCP class PinmemberStephan Stricker15-Jul-13 12:10 
QuestionMore simple sample, please!) PinmemberMember 101198262-Jul-13 2:57 
NewsModbus TCP Sample ( [modified] Pinmemberhadihidayat2-Jul-13 1:36 
QuestionNot able to read from Modbus TCP simulator PinmemberMayurShah.in28-Jun-13 22:51 
AnswerRe: Not able to read from Modbus TCP simulator PinmemberStephan Stricker29-Jun-13 21:55 
QuestionModbus RTU over TCP PinmemberJosepLluis17-Jun-13 11:18 
AnswerRe: Modbus RTU over TCP PinmemberStephan Stricker17-Jun-13 14:33 
GeneralRe: Modbus RTU over TCP PinmemberJosepLluis18-Jun-13 3:31 
GeneralRe: Modbus RTU over TCP Pinmemberkselvia@eprod.com22-Jun-13 0:21 
QuestionSlave_ID and Slave_Address PinmemberSBerube13-Jun-13 3:04 
AnswerRe: Slave_ID and Slave_Address PinmemberStephan Stricker14-Jun-13 4:51 

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