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socketsnetwork-programmingtcpmodbus

Modbus TCP/IP device communication problems

I'm trying to communicate with a device using Modbus TCP/IP. I'm using C# 4.0 .NET Sockets and have run into trouble getting responses back from the device. It will have to be asynchronous communication due to the nature of the device/network.

Right now I can connect to the device. When the Connect command executes, the device LED lights up that represents that there is an active connection. Then I execute the Send command and the light turns off (meaning I've lost connection) and when I receive, I get nothing. I've tried a bunch of different data packets but haven't been successful in any instance. I am thinking, is it something with the SocketType when I initialize the Socket? Could it be the port (currently using port 4) I'm trying to use?

Here is the Modbus TCP/IP document I've tried to use: Modbus Documentation

and here is my code:

sockPort = new Socket(IPAddress.Parse(_commInfo.IPAddress).AddressFamily, SocketType.Stream, ProtocolType.Tcp);   
IPEndPoint m_localhost = new IPEndPoint(IPAddress.Parse(addr.Address.ToString()), 4);    
sockPort.Bind(m_localhost);
sockPort.Connect(new IPEndPoint(IPAddress.Parse(_commInfo.IPAddress), 502));
byte[] rx = new byte[260];

byte[] data = { 0x03, 0x00, 0x20, 0x00, 0x04, 0x45, 0xf0 };

sockPort.Send(data);
var asf = sockPort.Receive(rx);
Solution

  • The problem is most probably with your interpretation of the application (modbus) protocol - endianness, packet layout, etc. Use wireshark or tcpdump(1) to figure out what is sent to and from the device. Some other points:

    • You don't need to bind(2) the local end of the socket, kernel will select an ephemeral local port for you.
    • You should be all set for bi-directional communication on the socket once the connect(2) is complete.
    • Don't ignore return values from send and receive calls - they can be less then you expected.

    As for workings of the device itself it's probably best to look for manufacturer documentation and search for device-specific mailing lists, user groups, and forums.

    Hope this helps.