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Modbus TCP class

, 23 May 2014 CPOL
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A Modbus TCP class.

Introduction

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

Background

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.

History

-----------------------------------------
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

License

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

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About the Author

Stephan Stricker
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Comments and Discussions

 
QuestionDisconnects and holding registers value displacement Pinmemberharizhent16-Mar-12 4:31 
AnswerRe: Disconnects and holding registers value displacement PinmemberStephan Stricker20-Mar-12 2:51 
GeneralMy vote of 5 PinmemberNikolay Ugorliev24-Feb-12 1:27 
QuestionModbus says error: Illegal function! Pinmemberxeroxvrey6-Feb-12 5:57 
AnswerRe: Modbus says error: Illegal function! PinmemberStephan Stricker6-Feb-12 6:07 
GeneralRe: Modbus says error: Illegal function! Pinmemberxeroxvrey6-Feb-12 10:27 
QuestionModbus TCP Class not working PinmemberJoseph Frippiat23-Jan-12 1:50 
AnswerRe: Modbus TCP Class not working PinmemberStephan Stricker23-Jan-12 1:56 
Joseph,
could it be that the data simply is zero? You can double check with Wireshark what
data you request and what data you receive in the response. Also keep in mind that
coils are output data. If you want to read digital input data you have to use
discrete inputs.
 
Stephan
GeneralRe: Modbus TCP Class not working PinmemberJoseph Frippiat23-Jan-12 2:13 
GeneralRe: Modbus TCP Class not working PinmemberJoseph Frippiat23-Jan-12 22:24 
QuestionCould you help me PinmemberDantty27-Dec-11 4:42 
BugTCP Network Bug Pinmembermrraphi16-Nov-11 21:20 
GeneralRe: TCP Network Bug PinmemberMember 841032817-Nov-11 2:07 
GeneralRe: TCP Network Bug Pinmembermrraphi17-Nov-11 3:43 
QuestionNo connection could be made because the target machine actively refused it PinmemberChiWaiLam6-Oct-11 1:00 
AnswerRe: No connection could be made because the target machine actively refused it PinmemberMember 841032817-Nov-11 2:09 
QuestionMODBUS TCP Server in ASP.net Pinmemberarulchris8-Sep-11 21:50 
GeneralHelp with address mapping Pinmembersandeepbaynes17-May-11 19:17 
Questionnewbie interested in Modbus TCP/IP Pinmemberautocatch3-May-11 16:27 
AnswerRe: newbie interested in Modbus TCP/IP PinmemberStephan Stricker4-May-11 8:36 
GeneralRe: newbie interested in Modbus TCP/IP Pinmemberautocatch4-May-11 13:30 
GeneralBig Bug Pinmembermrraphi29-Mar-11 3:36 
GeneralRe: Big Bug PinmemberStephan Stricker6-Apr-11 5:05 
GeneralMy vote of 5 PinmemberJongchanAhn24-Mar-11 22:26 
Generalconnection lost error Pinmemberttusharbb23-Jan-11 18:28 

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