MODBUS TCP not reconnecting

Libraries
1

Hello people,
I have been working on this modbus tcp slave project using freertos and freemodbus library. While it works fine at first, once i disconnect the master software, the tcp connection times out. How can i make it persistent such that i can connect to the slave at anytime and poll fresh data as well.

2

Hi Torpedod,

From your description, it is difficult to diagnose the reason why the TCP connection times out. More information from you would be helpful. Like what is your TCP/IP stack? The log when TCP connection times out. The code you disconnect and reconnect to the master, etc.

3

Most FreeRTOS+TCP API’s return:

  • A negative value (-errno) when someting goes wrong
  • Zero when things went OK
  • A positive value like “number of bytes sent/read”

Hopefully your master software sends a proper FIN as soon as it wants to stop. Check the return values of FreeRTOS_recv() and FreeRTOS_send() because they may return an error.

Note that the following errors are not fatal: pdFREERTOS_ERRNO_EAGAIN and pdFREERTOS_ERRNO_EINTR.

If you want you can post the source code that calls FreeRTOS+TCP APIs, and I will look at it.

Here is a simple function that tells you if a socket is still connected:

    /* Returns pdTRUE if TCP socket is connected. */
    BaseType_t FreeRTOS_issocketconnected( ConstSocket_t xSocket );

Normally you do not need it.

If you do not receive FIN packets, because the server does not disconnect gracefully, you can put a timer on your socket: when no data is received within N seconds, call FreeRTOS_closesocket() and continue with a new socket.

And if you want to close gracefully:

    /* Start closing the connection: */
    FreeRTOS_shutdown( xSocket, FREERTOS_SHUT_RDWR );
    for( ;; )
    {
        BaseType_t xRc = FreeRTOS_recv( xSocket,
                                        pcBuffer,
                                        sizeof pcBuffer,
                                        0 );
        if( xRc > 0 )
        {
            /* Very important: handle the last bytes received. */
        }
        else if( xRc < 0 )
        {
            if( ( xRc != -pdFREERTOS_ERRNO_EAGAIN ) &&
                ( xRc != -pdFREERTOS_ERRNO_EINTR ) )
            {
                printf("The shutdown is ACK'd by the peer (errno %d)\n", -xRc );
                break;
            }
        }
    }
    /* Close ( free ) the socket to free the resources. */
    FreeRTOS_closesocket( xSocket );
4

One more thing to look at are these config options: ipconfigTCP_HANG_PROTECTION,
ipconfigTCP_HANG_PROTECTION_TIME, and ipconfigTCP_KEEP_ALIVE
They may also help to prevent “hanging” sockets.