0. 协议栈接收状态的变化图

 1.协议栈的初始化和使能

void
eMBRTUStart( void )
{
    ENTER_CRITICAL_SECTION(  );
    /* Initially the receiver is in the state STATE_RX_INIT. we start
     * the timer and if no character is received within t3.5 we change
     * to STATE_RX_IDLE. This makes sure that we delay startup of the
     * modbus protocol stack until the bus is free.
     */
    eRcvState = STATE_RX_INIT;
    vMBPortSerialEnable( TRUE, FALSE );
    vMBPortTimersEnable(  );

    EXIT_CRITICAL_SECTION(  );
}

 2.协议栈接收状态机

BOOL
xMBRTUReceiveFSM( void )
{
    BOOL            xTaskNeedSwitch = FALSE;
    UCHAR           ucByte;

    assert( eSndState == STATE_TX_IDLE );

    /* Always read the character. */
    ( void )xMBPortSerialGetByte( ( CHAR * ) & ucByte );

    switch ( eRcvState )
    {
        /* If we have received a character in the init state we have to
         * wait until the frame is finished.
         */
    case STATE_RX_INIT:
        vMBPortTimersEnable(  );
        break;

        /* In the error state we wait until all characters in the
         * damaged frame are transmitted.
         */
    case STATE_RX_ERROR:
        vMBPortTimersEnable(  );
        break;

        /* In the idle state we wait for a new character. If a character
         * is received the t1.5 and t3.5 timers are started and the
         * receiver is in the state STATE_RX_RECEIVCE.
         */
    case STATE_RX_IDLE:
        usRcvBufferPos = 0;
        ucRTUBuf[usRcvBufferPos++] = ucByte;
        eRcvState = STATE_RX_RCV;

        /* Enable t3.5 timers. */
        vMBPortTimersEnable(  );
        break;

        /* We are currently receiving a frame. Reset the timer after
         * every character received. If more than the maximum possible
         * number of bytes in a modbus frame is received the frame is
         * ignored.
         */
    case STATE_RX_RCV:
        if( usRcvBufferPos < MB_SER_PDU_SIZE_MAX )
        {
            ucRTUBuf[usRcvBufferPos++] = ucByte;
        }
        else
        {
            eRcvState = STATE_RX_ERROR;
        }
        vMBPortTimersEnable(  );
        break;
    }
    return xTaskNeedSwitch;
}

3.时间中断函数

BOOL
xMBRTUTimerT35Expired( void )
{
    BOOL            xNeedPoll = FALSE;

    switch ( eRcvState )
    {
        /* Timer t35 expired. Startup phase is finished. */
    case STATE_RX_INIT:
        xNeedPoll = xMBPortEventPost( EV_READY );
        break;

        /* A frame was received and t35 expired. Notify the listener that
         * a new frame was received. */
    case STATE_RX_RCV:
        xNeedPoll = xMBPortEventPost( EV_FRAME_RECEIVED );
        break;

        /* An error occured while receiving the frame. */
    case STATE_RX_ERROR:
        break;

        /* Function called in an illegal state. */
    default:
        assert( ( eRcvState == STATE_RX_INIT ) ||
                ( eRcvState == STATE_RX_RCV ) || ( eRcvState == STATE_RX_ERROR ) );
    }

    vMBPortTimersDisable(  );
    eRcvState = STATE_RX_IDLE;

    return xNeedPoll;
}