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Modbus TCP class

, 23 May 2014 CPOL 314.8K 37.1K 131
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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
Product Manager
United States United States
No Biography provided

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Comments and Discussions

 
GeneralRe: values of message Pin
Stephan Stricker24-May-13 8:54
memberStephan Stricker24-May-13 8:54 
QuestionRe: values of message Pin
vinnie_j26-Jul-13 3:11
membervinnie_j26-Jul-13 3:11 
QuestionSiemens S1200 Pin
Andrea Feduzzi14-May-13 6:09
memberAndrea Feduzzi14-May-13 6:09 
AnswerRe: Siemens S1200 Pin
Stephan Stricker22-May-13 7:13
memberStephan Stricker22-May-13 7:13 
GeneralRe: Siemens S1200 Pin
Andrea Feduzzi31-May-13 6:19
memberAndrea Feduzzi31-May-13 6:19 
GeneralMy vote of 5 Pin
JongchanAhn24-Mar-13 15:03
memberJongchanAhn24-Mar-13 15:03 
GeneralRe: My vote of 5 Pin
Andrea Feduzzi14-May-13 6:10
memberAndrea Feduzzi14-May-13 6:10 
Questionproblem with writing to Siemens PLC holding register with MODBUS TCP Pin
Member 941116121-Dec-12 9:14
memberMember 941116121-Dec-12 9:14 
AnswerRe: problem with writing to Siemens PLC holding register with MODBUS TCP Pin
Stephan Stricker21-Dec-12 22:55
memberStephan Stricker21-Dec-12 22:55 
GeneralRe: problem with writing to Siemens PLC holding register with MODBUS TCP Pin
Member 94111612-Jan-13 9:16
memberMember 94111612-Jan-13 9:16 
GeneralRe: problem with writing to Siemens PLC holding register with MODBUS TCP Pin
Stephan Stricker3-Jan-13 4:12
memberStephan Stricker3-Jan-13 4:12 
GeneralRe: problem with writing to Siemens PLC holding register with MODBUS TCP Pin
Member 94111612-Jan-13 12:16
memberMember 94111612-Jan-13 12:16 
AnswerRe: problem with writing to Siemens PLC holding register with MODBUS TCP Pin
Andrea Feduzzi14-May-13 6:12
memberAndrea Feduzzi14-May-13 6:12 
QuestionCan't Write Coils Pin
KitaMenang22-Nov-12 3:08
memberKitaMenang22-Nov-12 3:08 
Hi thanks for the code but i am having a slight problem

Connection to the PLC is succesful but i cant seem to write coils.
I am using your test application.

Problem:

The Start address is at 0
The Size is 8 Bytes

I can write the first coil but when i try to write the second one it does not work
I have tried writing multiple coils but still it does not work. May I no why this is happening and how i can solve it
AnswerRe: Can't Write Coils Pin
Stephan Stricker22-Nov-12 3:17
memberStephan Stricker22-Nov-12 3:17 
Questionconvert error Pin
tomasz25616-Nov-12 2:28
membertomasz25616-Nov-12 2:28 
AnswerRe: convert error Pin
Stephan Stricker18-Nov-12 8:44
memberStephan Stricker18-Nov-12 8:44 
QuestionCan't open with free ms tools Pin
chris wedgwood26-Oct-12 4:09
memberchris wedgwood26-Oct-12 4:09 
AnswerRe: Can't open with free ms tools Pin
Stephan Stricker18-Nov-12 8:45
memberStephan Stricker18-Nov-12 8:45 
GeneralRe: Can't open with free ms tools Pin
chris wedgwood18-Nov-12 23:11
memberchris wedgwood18-Nov-12 23:11 
QuestionUnable to Read, able to write Pin
alexamex16-Oct-12 6:38
memberalexamex16-Oct-12 6:38 
AnswerRe: Unable to Read, able to write Pin
Stephan Stricker16-Oct-12 7:15
memberStephan Stricker16-Oct-12 7:15 
QuestionAny VB.Net synchronous mode examples available? Pin
Member 90645551-Jul-12 2:25
memberMember 90645551-Jul-12 2:25 
QuestionIt works Pin
Member 862908814-May-12 6:06
memberMember 862908814-May-12 6:06 
QuestionDisconnects and holding registers value displacement Pin
harizhent16-Mar-12 4:31
memberharizhent16-Mar-12 4:31 
AnswerRe: Disconnects and holding registers value displacement Pin
Stephan Stricker20-Mar-12 2:51
memberStephan Stricker20-Mar-12 2:51 
GeneralMy vote of 5 Pin
Nikolay Ugorliev24-Feb-12 1:27
memberNikolay Ugorliev24-Feb-12 1:27 
QuestionModbus says error: Illegal function! Pin
xeroxvrey6-Feb-12 5:57
memberxeroxvrey6-Feb-12 5:57 
AnswerRe: Modbus says error: Illegal function! Pin
Stephan Stricker6-Feb-12 6:07
memberStephan Stricker6-Feb-12 6:07 
GeneralRe: Modbus says error: Illegal function! Pin
xeroxvrey6-Feb-12 10:27
memberxeroxvrey6-Feb-12 10:27 
QuestionModbus TCP Class not working Pin
Joseph Frippiat23-Jan-12 1:50
memberJoseph Frippiat23-Jan-12 1:50 
AnswerRe: Modbus TCP Class not working Pin
Stephan Stricker23-Jan-12 1:56
memberStephan Stricker23-Jan-12 1:56 
GeneralRe: Modbus TCP Class not working Pin
Joseph Frippiat23-Jan-12 2:13
memberJoseph Frippiat23-Jan-12 2:13 
GeneralRe: Modbus TCP Class not working Pin
Joseph Frippiat23-Jan-12 22:24
memberJoseph Frippiat23-Jan-12 22:24 
QuestionCould you help me Pin
Dantty27-Dec-11 4:42
memberDantty27-Dec-11 4:42 
BugTCP Network Bug Pin
mrraphi16-Nov-11 21:20
membermrraphi16-Nov-11 21:20 
GeneralRe: TCP Network Bug Pin
Member 841032817-Nov-11 2:07
memberMember 841032817-Nov-11 2:07 
GeneralRe: TCP Network Bug Pin
mrraphi17-Nov-11 3:43
membermrraphi17-Nov-11 3:43 
QuestionNo connection could be made because the target machine actively refused it Pin
ChiWaiLam6-Oct-11 1:00
memberChiWaiLam6-Oct-11 1:00 
AnswerRe: No connection could be made because the target machine actively refused it Pin
Member 841032817-Nov-11 2:09
memberMember 841032817-Nov-11 2:09 
QuestionMODBUS TCP Server in ASP.net Pin
arulchris8-Sep-11 21:50
memberarulchris8-Sep-11 21:50 
GeneralHelp with address mapping Pin
sandeepbaynes17-May-11 19:17
membersandeepbaynes17-May-11 19:17 
Questionnewbie interested in Modbus TCP/IP Pin
autocatch3-May-11 16:27
memberautocatch3-May-11 16:27 
AnswerRe: newbie interested in Modbus TCP/IP Pin
Stephan Stricker4-May-11 8:36
memberStephan Stricker4-May-11 8:36 
GeneralRe: newbie interested in Modbus TCP/IP Pin
autocatch4-May-11 13:30
memberautocatch4-May-11 13:30 
GeneralBig Bug Pin
mrraphi29-Mar-11 3:36
membermrraphi29-Mar-11 3:36 
GeneralRe: Big Bug Pin
Stephan Stricker6-Apr-11 5:05
memberStephan Stricker6-Apr-11 5:05 
GeneralMy vote of 5 Pin
JongchanAhn24-Mar-11 22:26
memberJongchanAhn24-Mar-11 22:26 
Generalconnection lost error Pin
ttusharbb23-Jan-11 18:28
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Generalerror Pin
ttusharbb23-Jan-11 1:08
memberttusharbb23-Jan-11 1:08 

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