Click here to Skip to main content
13,346,027 members (59,577 online)
Click here to Skip to main content
Add your own
alternative version


148 bookmarked
Posted 6 Nov 2006

Modbus TCP class

, 23 May 2014
Rate this:
Please Sign up or sign in to vote.
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

You may also be interested in...

Comments and Discussions

AnswerRe: Disconnects and holding registers value displacement Pin
Stephan Stricker20-Mar-12 3:51
memberStephan Stricker20-Mar-12 3:51 
GeneralMy vote of 5 Pin
Nikolay Ugorliev24-Feb-12 2:27
memberNikolay Ugorliev24-Feb-12 2:27 
QuestionModbus says error: Illegal function! Pin
xeroxvrey6-Feb-12 6:57
memberxeroxvrey6-Feb-12 6:57 
AnswerRe: Modbus says error: Illegal function! Pin
Stephan Stricker6-Feb-12 7:07
memberStephan Stricker6-Feb-12 7:07 
GeneralRe: Modbus says error: Illegal function! Pin
xeroxvrey6-Feb-12 11:27
memberxeroxvrey6-Feb-12 11:27 
QuestionModbus TCP Class not working Pin
Joseph Frippiat23-Jan-12 2:50
memberJoseph Frippiat23-Jan-12 2:50 
AnswerRe: Modbus TCP Class not working Pin
Stephan Stricker23-Jan-12 2:56
memberStephan Stricker23-Jan-12 2:56 
GeneralRe: Modbus TCP Class not working Pin
Joseph Frippiat23-Jan-12 3:13
memberJoseph Frippiat23-Jan-12 3:13 
the function code returned in the answer can not be zero. It must be equal to 0x01 if there is no error or 0x81 if there is an error.
I do not understand why it is zero.
It seems that when the event OnReceive is fired, the tcpAsyClBuffer is not filled.
GeneralRe: Modbus TCP Class not working Pin
Joseph Frippiat23-Jan-12 23:24
memberJoseph Frippiat23-Jan-12 23:24 
QuestionCould you help me Pin
Dantty27-Dec-11 5:42
memberDantty27-Dec-11 5:42 
BugTCP Network Bug Pin
mrraphi16-Nov-11 22:20
membermrraphi16-Nov-11 22:20 
GeneralRe: TCP Network Bug Pin
Member 841032817-Nov-11 3:07
memberMember 841032817-Nov-11 3:07 
GeneralRe: TCP Network Bug Pin
mrraphi17-Nov-11 4:43
membermrraphi17-Nov-11 4:43 
QuestionNo connection could be made because the target machine actively refused it Pin
ChiWaiLam6-Oct-11 2:00
memberChiWaiLam6-Oct-11 2:00 
AnswerRe: No connection could be made because the target machine actively refused it Pin
Member 841032817-Nov-11 3:09
memberMember 841032817-Nov-11 3:09 
QuestionMODBUS TCP Server in Pin
arulchris8-Sep-11 22:50
memberarulchris8-Sep-11 22:50 
GeneralHelp with address mapping Pin
sandeepbaynes17-May-11 20:17
membersandeepbaynes17-May-11 20:17 
Questionnewbie interested in Modbus TCP/IP Pin
autocatch3-May-11 17:27
memberautocatch3-May-11 17:27 
AnswerRe: newbie interested in Modbus TCP/IP Pin
Stephan Stricker4-May-11 9:36
memberStephan Stricker4-May-11 9:36 
GeneralRe: newbie interested in Modbus TCP/IP Pin
autocatch4-May-11 14:30
memberautocatch4-May-11 14:30 
GeneralBig Bug Pin
mrraphi29-Mar-11 4:36
membermrraphi29-Mar-11 4:36 
GeneralRe: Big Bug Pin
Stephan Stricker6-Apr-11 6:05
memberStephan Stricker6-Apr-11 6:05 
GeneralMy vote of 5 Pin
JongchanAhn24-Mar-11 23:26
memberJongchanAhn24-Mar-11 23:26 
GeneralRe: My vote of 5 Pin
Entrase11-Dec-15 23:11
memberEntrase11-Dec-15 23:11 
Generalconnection lost error Pin
ttusharbb23-Jan-11 19:28
memberttusharbb23-Jan-11 19:28 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

Permalink | Advertise | Privacy | Terms of Use | Mobile
Web03 | 2.8.180111.1 | Last Updated 23 May 2014
Article Copyright 2006 by Stephan Stricker
Everything else Copyright © CodeProject, 1999-2018
Layout: fixed | fluid