Ethernet Developers Guide
One of the parameters to the WSAAsyncSelect is a user defined message the window socket DLL sends to the window. The sample application user message is WM_TCPIP_EVENT and is defined in the file wn_msh.h. MFC architectural framework calls the CSample_View tcpip_event member function to process this message. Like all functions which process messages, tcpip_event parameters are a word and a long word. The word parameter is the socket, and the long word parameter contains the network event , and an error code. Tcpip_event examines the network event and calls the member function indicated in the following table.
Network Event | Member Function |
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FD_READ | OnTcpIpRead() |
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FD_WRITE | OnTcpIpWrite() |
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FD_OOB | OnTcpIpOob() |
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FD_CONNECT | ⁄ OnTcpIpConnect |
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FD_CLOSE | OnTcpIpClose() |
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B.7.2 Application Message Format
TCP/IP transmits a message as a stream. There is no indication of the start of a message nor the end of the message. The NOE option module adds a header to determine the message boundaries. The message is a Modbus message. The header contains the following fields.
Invoke Identifier. This two byte field associates a request with the response. The client application picks the invoke identifier, and server returns the same invoke identifier in the response.
Protocol Type. This two byte field identifies the protocol type. Currently, the only protocol supported is Modbus.
zCommand Length. This two byte field is the size of the rest of the message. Destination Identifier. This one byte field is reserved for future use.
The Modbus message follows the header. The message does not contain the address field, instead, the first byte is the Modbus function code.
The data structure for the header is declared in modbus.h and the CSample_View encode_header function encodes the header. The member functions encode_clear_stats, encode_read_stats, encode_read_rq, and encode_write_rq encode the corresponding Modbus messages.
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