Freemodbus 源码讲解系列 （二）

这次主要介绍eMBInit(  )函数。该函数是初始化modbus协议栈。其中会把协议的开始、禁止、发送、接收、解析、定时器等操作赋值给相应的函数，这样的好处可以防止底层变量被修改。同时，使得从机的状态固定在一个特定的状态。因为初始化同时初始化了RTU和ASCII两种模式，本例程只讲解RTU的初始化过程。废话不多说，直接交代代码。

 

eMBErrorCode
eMBInit( eMBMode eMode, UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
{
    eMBErrorCode    eStatus = MB_ENOERR;

    /* check preconditions */

   /*防止从机的地址命名不合法*/ 
    if( ( ucSlaveAddress == MB_ADDRESS_BROADCAST ) ||
        ( ucSlaveAddress < MB_ADDRESS_MIN ) || ( ucSlaveAddress > MB_ADDRESS_MAX ) )
    {
        eStatus = MB_EINVAL;
    }
    else
    {
        ucMBAddress = ucSlaveAddress;

        switch ( eMode )
        {
#if MB_RTU_ENABLED > 0
        case MB_RTU:
            pvMBFrameStartCur = eMBRTUStart;
            pvMBFrameStopCur = eMBRTUStop;
            peMBFrameSendCur = eMBRTUSend;
            peMBFrameReceiveCur = eMBRTUReceive;
            pvMBFrameCloseCur = MB_PORT_HAS_CLOSE ? vMBPortClose : NULL;
            pxMBFrameCBByteReceived = xMBRTUReceiveFSM;
            pxMBFrameCBTransmitterEmpty = xMBRTUTransmitFSM;
            pxMBPortCBTimerExpired = xMBRTUTimerT35Expired;

            eStatus = eMBRTUInit( ucMBAddress, ucPort, ulBaudRate, eParity );
            break;
#endif
#if MB_ASCII_ENABLED > 0
        case MB_ASCII:
            pvMBFrameStartCur = eMBASCIIStart;
            pvMBFrameStopCur = eMBASCIIStop;
            peMBFrameSendCur = eMBASCIISend;
            peMBFrameReceiveCur = eMBASCIIReceive;
            pvMBFrameCloseCur = MB_PORT_HAS_CLOSE ? vMBPortClose : NULL;
            pxMBFrameCBByteReceived = xMBASCIIReceiveFSM;
            pxMBFrameCBTransmitterEmpty = xMBASCIITransmitFSM;
            pxMBPortCBTimerExpired = xMBASCIITimerT1SExpired;

            eStatus = eMBASCIIInit( ucMBAddress, ucPort, ulBaudRate, eParity );
            break;
#endif
        default:
            eStatus = MB_EINVAL;
        }

        if( eStatus == MB_ENOERR )
        {
            if( !xMBPortEventInit(  ) )
            {
                /* port dependent event module initalization failed. */
                eStatus = MB_EPORTERR;
            }
            else
            {
                eMBCurrentMode = eMode;
                eMBState = STATE_DISABLED;
            }
        }
    }
    return eStatus;
}

 一、eMBRTUStart（）函数介绍，使能串口和定时器中断。先看代码

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(  );
}

 首先关闭所有中断；

然后给从机的状态赋值，初始化的从机状态为STATE_RX_INIT（从机初始化状态）；

接着初始化串口中断，这里使能了接收中断，失能了发送中断；

再使能定时器中断；

最后使能中断；

二、eMBRTUStop（）函数讲解，该函数eMBRTUStart（）功能正好相反，不再详说。看代码

eMBRTUStop( void )
{
    ENTER_CRITICAL_SECTION(  );
    vMBPortSerialEnable( FALSE, FALSE );
    vMBPortTimersDisable(  );
    EXIT_CRITICAL_SECTION(  );
}

三、eMBRTUSend（）函数讲解，该函数是帧发送函数，通过组帧，再把帧数据发送出去

 

eMBErrorCode
eMBRTUSend( UCHAR ucSlaveAddress, const UCHAR * pucFrame, USHORT usLength )
{
    eMBErrorCode    eStatus = MB_ENOERR;
    USHORT          usCRC16;

    ENTER_CRITICAL_SECTION(  );

    /* Check if the receiver is still in idle state. If not we where to
     * slow with processing the received frame and the master sent another
     * frame on the network. We have to abort sending the frame.
     */
    if( eRcvState == STATE_RX_IDLE )
    {
        /* First byte before the Modbus-PDU is the slave address. */
        pucSndBufferCur = ( UCHAR * ) pucFrame - 1;
        usSndBufferCount = 1;

        /* Now copy the Modbus-PDU into the Modbus-Serial-Line-PDU. */
        pucSndBufferCur[MB_SER_PDU_ADDR_OFF] = ucSlaveAddress;//帧第一位
        usSndBufferCount += usLength;//帧长度

        /* Calculate CRC16 checksum for Modbus-Serial-Line-PDU. */
        usCRC16 = usMBCRC16( ( UCHAR * ) pucSndBufferCur, usSndBufferCount );//CRC校验位 低位在前 高位在后
        ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 & 0xFF );
        ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 >> 8 );

  

      /* Activate the transmitter. */

       //组帧之后，把从机的发送状态改为STATE_TX_XMIT，激活发送
        eSndState = STATE_TX_XMIT;//
        vMBPortSerialEnable( FALSE, TRUE );//串口发送中断
    }
    else
    {
        eStatus = MB_EIO;//如果从机不是接收空闲，返回IO错误类型
    }
    EXIT_CRITICAL_SECTION(  );
    return eStatus;
}

四、eMBRTUReceive（）函数讲解

eMBErrorCode
eMBRTUReceive( UCHAR * pucRcvAddress, UCHAR ** pucFrame, USHORT * pusLength )
{
    BOOL            xFrameReceived = FALSE;
    eMBErrorCode    eStatus = MB_ENOERR;

    ENTER_CRITICAL_SECTION(  );
    assert( usRcvBufferPos < MB_SER_PDU_SIZE_MAX );//使用断言的方式，屏蔽错误。括号为真继续执行，为假直接返回

    /* Length and CRC check */
    if( ( usRcvBufferPos >= MB_SER_PDU_SIZE_MIN )
        && ( usMBCRC16( ( UCHAR * ) ucRTUBuf, usRcvBufferPos ) == 0 ) )//为什么会返回0？？
    {
        /* Save the address field. All frames are passed to the upper layed
         * and the decision if a frame is used is done there.
         */
        *pucRcvAddress = ucRTUBuf[MB_SER_PDU_ADDR_OFF];//提取帧头 从机地址

        /* Total length of Modbus-PDU is Modbus-Serial-Line-PDU minus
         * size of address field and CRC checksum.
         */
        *pusLength = ( USHORT )( usRcvBufferPos - MB_SER_PDU_PDU_OFF - MB_SER_PDU_SIZE_CRC );//计算帧长度

        /* Return the start of the Modbus PDU to the caller. */
        *pucFrame = ( UCHAR * ) & ucRTUBuf[MB_SER_PDU_PDU_OFF];//把接收帧赋给pucFrame指针
        xFrameReceived = TRUE;//接收数据为真
    }
    else
    {
        eStatus = MB_EIO;
    }

    EXIT_CRITICAL_SECTION(  );
    return eStatus;
}

 五、xMBRTUReceiveFSM（）函数讲解。这是串口中断接收数据函数。

因为pxMBFrameCBByteReceived = xMBRTUReceiveFSM;

FreeModbus协议栈通过串口中断接收一帧数据，用户需在串口接收中断中回调prvvUARTRxISR()函数；

prvvUARTRxISR()函数：(portserial.c)

static void prvvUARTRxISR( void )
{
pxMBFrameCBByteReceived( );
}

在第一阶段中eMBInit()函数中赋值pxMBFrameCBByteReceived = xMBRTUReceiveFSM,发生接收中断时,最终调用xMBRTUReceiveFSM函数对数据进行接收；
xMBRTUReceiveFSM()函数：(mbrtu.c)

/*函数功能
*1:将接收到的数据存入ucRTUBuf[]中;
*2:usRcvBufferPos为全局变量，表示接收数据的个数;
*3:每接收到一个字节的数据，3.5T定时器清0
*/

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

    assert( eSndState == STATE_TX_IDLE );

    /* Always read the character. */
    ( void )xMBPortSerialGetByte( ( CHAR * ) & ucByte );//获取接收数据

    switch ( eRcvState )//根据从机的状态执行相应命令
    {
        /* If we have received a character in the init state we have to
         * wait until the frame is finished.
         */

//初始化协议栈时，从机的状态是STATE_RX_INIT，因此在初始化时接收到数据时，会启动定时器中断，定时器清零。 

    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://接收的数据帧有问题状态也会清定时器
        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://接收空闲状态处理，接收首字节
        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://接收其余的字节
        if( usRcvBufferPos < MB_SER_PDU_SIZE_MAX )
        {
            ucRTUBuf[usRcvBufferPos++] = ucByte;
        }
        else
        {
            eRcvState = STATE_RX_ERROR;
        }
        vMBPortTimersEnable(  );
        break;
    }
    return xTaskNeedSwitch;
}
 

 六、xMBRTUTransmitFSM（）函数讲解。pxMBFrameCBTransmitterEmpty = xMBRTUTransmitFSM;

串口发送中断中调用prvvUARTTxReadyISR()函数，继续调用pxMBFrameCBTransmitterEmpty()函数，pxMBFrameCBTransmitterEmpty为函数指针，最终调用xMBRTUTransmitFSM()函数；

BOOL
xMBRTUTransmitFSM( void )
{
    BOOL            xNeedPoll = FALSE;

    assert( eRcvState == STATE_RX_IDLE );

    switch ( eSndState )
    {
        /* We should not get a transmitter event if the transmitter is in
         * idle state.  */
    case STATE_TX_IDLE:
        /* enable receiver/disable transmitter. */
        vMBPortSerialEnable( TRUE, FALSE );
        break;

    case STATE_TX_XMIT:
        /* check if we are finished. */
        if( usSndBufferCount != 0 )
        {
            xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur );
            pucSndBufferCur++;  /* next byte in sendbuffer. */
            usSndBufferCount--;
        }
        else
        {
            xNeedPoll = xMBPortEventPost( EV_FRAME_SENT );
            /* Disable transmitter. This prevents another transmit buffer
             * empty interrupt. */
            vMBPortSerialEnable( TRUE, FALSE );
            eSndState = STATE_TX_IDLE;
        }
        break;
    }

    return xNeedPoll;
}

 七、xMBRTUTimerT35Expired（）函数讲解，初始化

BOOL
xMBRTUTimerT35Expired( void )
{
    BOOL            xNeedPoll = FALSE;

    switch ( eRcvState )
    {
        /* Timer t35 expired. Startup phase is finished. */
    case STATE_RX_INIT://协议栈初始化状态。
        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;//从机状态进入接收空闲状态

    return xNeedPoll;
}

 八、eMBRTUInit( )函数讲解，该函数初始化串口、初始化定时器

eMBErrorCode
eMBRTUInit( UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
{
    eMBErrorCode    eStatus = MB_ENOERR;
    ULONG           usTimerT35_50us;

    ( void )ucSlaveAddress;
    ENTER_CRITICAL_SECTION(  );

    /* Modbus RTU uses 8 Databits. */
    if( xMBPortSerialInit( ucPort, ulBaudRate, 8, eParity ) != TRUE )
    {
        eStatus = MB_EPORTERR;
    }
    else
    {
        /* If baudrate > 19200 then we should use the fixed timer values
         * t35 = 1750us. Otherwise t35 must be 3.5 times the character time.
         */
        if( ulBaudRate > 19200 )
        {
            usTimerT35_50us = 35;       /* 1800us. */
        }
        else
        {
            /* The timer reload value for a character is given by:
             *
             * ChTimeValue = Ticks_per_1s / ( Baudrate / 11 )
             *             = 11 * Ticks_per_1s / Baudrate
             *             = 220000 / Baudrate
             * The reload for t3.5 is 1.5 times this value and similary
             * for t3.5.
             */
            usTimerT35_50us = ( 7UL * 220000UL ) / ( 2UL * ulBaudRate );
        }
        if( xMBPortTimersInit( ( USHORT ) usTimerT35_50us ) != TRUE )
        {
            eStatus = MB_EPORTERR;
        }
    }
    EXIT_CRITICAL_SECTION(  );

    return eStatus;

源码讲解over，我也是第一次这么仔细的分析协议栈，不过还是挺有意思。

下篇讲解从机的接收机制，敬请期待