Modbus TCP class

Stephan Stricker

Rate me:

Please Sign up or sign in to vote.

4.89/5 (84 votes)

23 May 2014CPOL

1.7M

60.4K

154

171

A Modbus TCP class.

The project has moved to GIT

https://github.com/stephan1827/modbusTCP

Please download and ask questions from there.

License

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

Written By

Stephan Stricker

Systems Engineer

Germany

This member has not yet provided a Biography. Assume it's interesting and varied, and probably something to do with programming.

Comments and Discussions

First Prev Next

Re: Modbus Framework for .NET

Flavi18-Nov-13 1:51

Flavi1

8-Nov-13 1:51

I didn't want to share my stuff yet as I'm not 100% happy with it yet, let me know if you're waiting for it (in which case I'll share an unstable version).

Illegal Data Address

softwaremonkey3-Oct-13 3:43

softwaremonkey

3-Oct-13 3:43

First of all, thanks for the code, it works great and is a real help to a Modbus noob like me. I have a small problem though...

I am requesting diagnostic data from a Wago 750 bus coupler at address 0x1050 and I get the reply "Illegal Data Address". The Wago documentation lists this address so I have no idea where to look next.

Could anyone suggest what might be the cause?

Confused | :confused:

Re: Illegal Data Address

softwaremonkey3-Oct-13 4:16

softwaremonkey

3-Oct-13 4:16

Ah-ha!

I found that if I enter the address as a hex number in the Modbus TCP Tester, it works but if I enter it as a decimal number, it doesn't.

The tester app doesn't say what base to use so I guessed - wrongly!

D'Oh! | :doh:

Re: Illegal Data Address

Mehmet Ali Dalaklı16-Jan-14 7:53

Mehmet Ali Dalaklı

16-Jan-14 7:53

Would you please tell us how dıd you entered the proper data to read ınputs.

I have to read the value of X0. Whıch type of entrıes need to whıch textboxes?

modbus message response is not receiving in same order

Uma Mahes17-Sep-13 20:10

Uma Mahes

17-Sep-13 20:10

Hello,

I'm using synchronous sockets and tested with the simulator. In my test application created thread and timer both reads different registers and coils.The reason for the timer is to read the status of motor,target reached etc...this will continuously poll the coils. The thread is used to control the actual motion.

When i request to read for coil but I received the register output in synchronous sockets.
Has any one encountered this kind of issue?

I understood that it may be due to calling in timer and thread. Any suggestion to overcome this issue?

Thanks

Re: modbus message response is not receiving in same order

Stephan Stricker18-Sep-13 3:30

Stephan Stricker

18-Sep-13 3:30

I am not sure if I understand correct what the problem is. Keep in mind that coils are actually digital outputs. Discrete inputs are digital inputs. So when you read coils you read back digital outputs.

Stephan

Re: modbus message response is not receiving in same order

Uma Mahes21-Jan-14 1:48

Uma Mahes

21-Jan-14 1:48

The issue is in WriteSyncData method, sending command function and received buffer function command is not matching. How do we ensure that the command we send is actually what we receive?

byte sendFunc = write_data[7];
tcpSynCl.Send(write_data, 0, write_data.Length, SocketFlags.None);
result = tcpSynCl.Receive(tcpSynClBuffer, 0, tcpSynClBuffer.Length, SocketFlags.None);
if (sendFunc != tcpSynClBuffer[7])
System.Diagnostics.Trace.WriteLine("Command and Response are not matching inside Master.WriteSyncData" + "Send Func : " + sendFunc + "Response : " + tcpSynClBuffer[7]);

It happens that we wanted to readsinglecoil but what we receive is previous call's(writesingleregister) output. It looks TCP synchronization issue. Is there any time delay to send the next command?

Can not read/write Address more than 32767. Edited conversion

hadihidayat9-Sep-13 15:52

hadihidayat

9-Sep-13 15:52

Hello..

I have a problem when trying to Read and Write address more than Integer limit (32767).

On function CreateReadHeader and CreateWriteHeader There is a problem when convert Start Address to byte, in this line of code :

(this is VB conversion code. from here: http://hadiscada.blogspot.com/2013/07/modbus-sample-vb-engllish-version.html[^])

Dim _adr As Byte() = BitConverter.GetBytes(CShort(IPAddress.HostToNetworkOrder(CShort(startAddress))))
data(8) =  _adr(0)
' Start address
data(9) = _adr(1)
' Start address

I solved my problem with this edited code :

'Dim _adr As Byte() = BitConverter.GetBytes(CShort(IPAddress.HostToNetworkOrder(CShort(startAddress))))
data(8) = startAddress \ 256 ' _adr(0)
' Start address
data(9) = startAddress Mod 256 ' _adr(1)
' Start address

Thanks..

modified 17-Sep-13 5:14am.

My vote of 5

陳志杰25-Aug-13 17:49

陳志杰

25-Aug-13 17:49

great implementation

Some improvements necessary

muharrem27-Jul-13 13:17

muharrem

27-Jul-13 13:17

First of all, thanks for such a good working and kind sharing it with the community.

I had a quick investigation on the class file, saw some things that are needed to be considered
more precisely.

First one:
What MSDN says is "The asynchronous BeginReceive operation must be completed by calling the EndReceive method. Typically, the method is invoked by the callback delegate." Check it from http://msdn.microsoft.com/en-us/library/dxkwh6zw.aspx

There is no such call to EndReceive in OnReceive callback.

Second one:
This design pattern will work perfectly, if you receive the whole message from the slave at once (in one transaction).
But what if you don't get the whole message from the slave at once? This is very common situation in
tcp/ip communication that you can get your data, lets say 100 bytes data as 40,40 and 20 bytes, in two or three or more parts.
So in OnReceive, received length should be checked against to requested or sent length from slave then BegingReceive should be recalled
for the remaining part of the message. So you can be sure that received message is in completed status.

Or am I mistaken?

Re: Some improvements necessary

Stephan Stricker29-Jul-13 7:10

Stephan Stricker

29-Jul-13 7:10

I agree with the first point and I added EndSend and EndReceive to the code. Fragmentation appears mostly when you exceed the Ethernet MTU size of 1500 bytes. Since modbusTCP only allows 255 bytes of data we should be well below this. I ran a quick test and after several hours and more than a thousand frames there was not a single fragmentation. I know its not 100% clean but I would have to change code quite a bit for this change.

Stephan

Re: Some improvements necessary

muharrem30-Jul-13 4:51

muharrem

30-Jul-13 4:51

To make the class more flexible for all kind of physical connection types, not only for ethernet,
I still think that fragmentation should be considered too.
Please imagine that, if application would be required to gather information scattered on a very large field connected with
gprs based network or even through adsl line. I can not imagine all right now. But since it is a tcp then why not?

Cheer

asp.net and ModbusTCP class

enfrost15-Jul-13 8:57

enfrost

15-Jul-13 8:57

Hi Stephan, so i got your example to work great and I am trying to use the dll for my own project.

I am trying to use asp.net and make a website I can go to that will essentially do exactly what your application does (just the read registers portion) but for the web. I have gotten to the point where it connects and I use the method ReadHoldingRegister which works fine but I have no idea how to get the data sent back. I'm not sure I can use your custom event handler to detect if data is sent back. I am pretty new to C# so maybe it is simple and I don't know yet but thought I would ask this question to see if you have a good idea on how I might resolve this issue.

Thanks so much!

Jeff

Re: asp.net and ModbusTCP class

Stephan Stricker15-Jul-13 11:10

Stephan Stricker

15-Jul-13 11:10

Jeff,
I am sorry but I dont know how to handle events in ASP. The issue is that you load the ASP page and then it just sits there. The modbus driver will send a callback when new data arrives but I dont know how an ASP web page can receive that.

Stephan

More simple sample, please!)

Voodoo902-Jul-13 1:57

Voodoo90

2-Jul-13 1:57

Hi, Stephan!
I'm not good in c#, but I need some programm for write/read data to my PLC Siemens Simatic S7 1200. Now I use your sample. But I need program, where I can just enter IP and read first two holding registers and write only first holding register (in "word" format). Can you help me?

Best regards!

p.s. Sorry for my english, I'm from Russia

Modbus TCP Sample (VB.net)

hadihidayat2-Jul-13 0:36

hadihidayat

2-Jul-13 0:36

Thanks Stephen.

I am not good in C#, so I convert your project to VB.net

please visit my blog (http://hadiscada.blogspot.com/2013/07/modbus-tcp-samplevb.html[^]) for more information.

modified 3-Jul-13 22:56pm.

Not able to read from Modbus TCP simulator

MayurShah.in28-Jun-13 21:51

MayurShah.in

28-Jun-13 21:51

Hi,

I am trying to read data from Modbus TCP simulator but it gives me an error like

Modbus says error: Illegal function!

I am able to write data.

Please help.

Thank you,
Mayur Shah

Re: Not able to read from Modbus TCP simulator

Stephan Stricker29-Jun-13 20:55

Stephan Stricker

29-Jun-13 20:55

This is a response from your slave device. You probably call a function to read coils to an address where you have registers or you read from an address that does not exist. If you can not find the issue please download wireshark and add some network traces so I can see the request and response.

Stephan

Modbus RTU over TCP

JosepLluis17-Jun-13 10:18

JosepLluis

17-Jun-13 10:18

Hello,

Great example!

Do you think you could show an example on how to encapsulate Modbus RTU or Modbus ASCII messages over TCP?

I think the differences between RTU and ASCII are interesting code wise, and encapsulating those "older" Modbus variants into TCP is also interesting: using modern Ethernet to communicate with legacy serial devices.

Thank you in advance

--
Josep

Re: Modbus RTU over TCP

Stephan Stricker17-Jun-13 13:33

Stephan Stricker

17-Jun-13 13:33

Hi,
I have not seen a device that would use a modbusRTU frame embedded directly into TCP. I found the term when I search for it but I dont think that this is in any spec. ModbusTCP and modbusRTU are very close in their telegram structure. There are minor changes like for example the CRC does not exist in modbusTCP because the error detection is covered by the TCP frame. The function codes are exactly the same. Even function calls like read/write single coil still exist in modbusTCP also they dont make much sense. I guess for compatibility reasons it is nice to have all functions.

Best regards

Stephan

Re: Modbus RTU over TCP

JosepLluis18-Jun-13 2:31

JosepLluis

18-Jun-13 2:31

Hi there,

Legacy devices using Modbus RTU (or ASCII) are now being interfaced through Ethernet, as less and less computers come with COM ports. Thus using an Ethernet to Serial converter lets you interface in a modern way to a legacy device.

You are 100% right in the functions and CRC checks. A difference would be having as mandatory the node ID.

As per coding learning, the LRC check-sum method employed by Modbus ASCII makes an interesting and not too difficult exercise Smile | :)

Thank you for your time.

All the best,
Josep

Re: Modbus RTU over TCP

kenselvia@gmail.com21-Jun-13 23:21

kenselvia@gmail.com

21-Jun-13 23:21

Thanks for the very useful code!

This describes the difference between modbus TCP and RTU;

http://www.simplymodbus.ca/TCP.htm[^]

ModbusUtility.cs found here calculates the correct CRC;

http://code.google.com/p/nmodbus[^]

This will generate correct RTU MBAP header;

private byte[] CreateReadHeaderRTU(ushort id, byte unit, ushort startAddress, ushort length, byte function)
 {
     byte[] data = new byte[6];
     byte[] crc = new byte[2];
     data[0] = unit; // Was 0 Slave address
     data[1] = function; // Function code
     byte[] _adr = BitConverter.GetBytes((short) IPAddress.HostToNetworkOrder((short) startAddress));
     data[2] = _adr[0]; // Start address
     data[3] = _adr[1]; // Start address
     byte[] _length = BitConverter.GetBytes((short) IPAddress.HostToNetworkOrder((short) length));
     data[4] = _length[0]; // Number of data to read
     data[5] = _length[1]; // Number of data to read
     crc = ModbusUtil.CalculateCrc(data);
     byte[] ret = new byte[data.Length + crc.Length];
     Buffer.BlockCopy(data, 0, ret, 0, data.Length);
     Buffer.BlockCopy(crc, 0, ret, data.Length, crc.Length);
     return ret;
 }

But I did not finish the response OnReceive() code

//TCP 00000000 | 00 00 00 00 00 07 01 03 04 42 A0 9B 1C          | .........B...
//RTU 00000000 | 03 03 04 44 C2 41 88 5C C9                      | ...D.A.\.
ushort rtu = BitConverter.ToUInt16(tcpAsyClBuffer, resultptr+2); // Zero for TCP, bytecount for RTU

because I realized ModbusRTU has no transaction ID, therefore it can't be done non-sequentially asynchronously since nothing in the response identifies the reqeust. But I did verify a single request/response was correct. I will eventually have to finish the synchronous RTU support.

I also added support for Enron (Modicon) and Omni extended ASCII read buffer (report) function 65

I created an Excel RTD (real-time-data) add-in using your code-base with a few modifications. I'll post it when I finish.

Ken

Slave_ID and Slave_Address

SBerube13-Jun-13 2:04

SBerube

13-Jun-13 2:04

Thanks for the class, it's clean, simple and works very well.

However, I noticed that in "CreateReadHeader" and "CreateWriteHeader", you save the message ID/save Id with an inverted bit order, my device complained about this and I checked it with WireShark

byte[] _id = BitConverter.GetBytes((short)id);
data[0] = _id[0];               // Slave id high byte -- Fixed _id order.
data[1] = _id[1];               // Slave id low byte

Basically, _id[0] is your low-byte and vise-versa.

Also as a general note/recommendation some Modbus/TCP devices (i.e. Schneider) use the Slave Address for different messaging methods, so it is convenient to add the possibility of changing this as part of the read/write headers. The modifications are easy enough to make on your own but perhaps this can help some other potential users.

Thanks again, really like the code.

Re: Slave_ID and Slave_Address

Stephan Stricker14-Jun-13 3:51

Stephan Stricker

14-Jun-13 3:51

Hi,
thank you very much for the recommendations. I have updated the code and will post a new version shortly.

Stephan

values of message

monkar22-May-13 7:08

monkar

22-May-13 7:08

Hi first of all thx a lot for this wonderfull applicatio but i have a question how to save values of message in case of coill inputs for further usage<

Last Visit: 31-Dec-99 18:00 Last Update: 3-May-24 10:42

Refresh

ᐊ Prev 1 234 5 6 7 Next ᐅ

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