freemodbus-v1.5.0 源码分析

　注：转载请注明出处   http://www.cnblogs.com/wujing-hubei/p/5935142.html 

　FreeModbus协议栈作为从机，等待主机传送的数据，当从机接收到一帧完整的报文后，对报文进行解析，然后响应主机，发送报文给主机，实现主机和从机之间的通信。

1、初始化协议栈---eMBInit函数（mb.c中），以RTU为例

eMBErrorCode  eMBInit( eMBMode eMode, UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
{

/*函数功能：
*1:实现RTU模式和ASCALL模式的协议栈初始化;
*2:完成协议栈核心函数指针的赋值，包括Modbus协议栈的使能和禁止、报文的接收和响应、3.5T定时器中断回调函数、串口发送和接收中断回调函数;
*3:eMBRTUInit完成RTU模式下串口和3.5T定时器的初始化，需用户自己移植;
*4:设置Modbus协议栈的模式eMBCurrentMode为MB_RTU，设置Modbus协议栈状态eMBState为STATE_DISABLED;
*/
 　　eMBErrorCode    eStatus = MB_ENOERR;

/* check preconditions */
    if( ( ucSlaveAddress == MB_ADDRESS_BROADCAST ) ||
        ( ucSlaveAddress < MB_ADDRESS_MIN ) || ( ucSlaveAddress > MB_ADDRESS_MAX ) )   //验证从机地址
    {
        eStatus = MB_EINVAL;                                  //地址错误
    }
    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;         //接收状态机，串口接受中断最终调用此函数接收数据
            pxMBFrameCBTransmitterEmpty = xMBRTUTransmitFSM;    //发送状态机，串口发送中断最终调用此函数发送数据
            pxMBPortCBTimerExpired = xMBRTUTimerT35Expired;     //报文到达间隔检查，定时器中断函数最终调用次函数完成定时器中断

            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
            {//设定当前状态
                eMBCurrentMode = eMode;           //设定RTU模式
                eMBState = STATE_DISABLED;        //modbus协议栈初始化状态,在此初始化为禁止
            }
        }
    }
    return eStatus;
}

　　eMBInit函数中pvMBFrameStartCur、pvMBFrameStartCur等为协议栈函数的接口，对于不同的通信模式使用不同的函数进行初始化！！！此编程模式可以借鉴学习！！！

　　eMBInit函数对底层驱动（串口和定时器）进行初始化。初始化完成并且成功之后对事件也进行了初始化，完成后全局变量eMBState=STATE_DISABLED。

 2、启动协议栈----eMBEnable函数（mb.c函数）

eMBErrorCode
eMBEnable( void )
{
    /*函数功能：
     *1:实现RTU模式和ASCALL模式的协议栈初始化;
     *2:完成协议栈核心函数指针的赋值，包括Modbus协议栈的使能和禁止、报文的接收和响应、3.5T定时器中断回调函数、串口发送和接收中断回调函数;
     *3:eMBRTUInit完成RTU模式下串口和3.5T定时器的初始化，需用户自己移植;
     *4:设置Modbus协议栈的模式eMBCurrentMode为MB_RTU，设置Modbus协议栈状态eMBState为STATE_DISABLED;
     */
    eMBErrorCode    eStatus = MB_ENOERR;

    if( eMBState == STATE_DISABLED )
    {
        /* Activate the protocol stack. */
        pvMBFrameStartCur(  );       //激活协议栈
        eMBState = STATE_ENABLED;    //设置Modbus协议栈工作状态eMBState为STATE_ENABLED
    }
    else
    {
        eStatus = MB_EILLSTATE;
    }
    return eStatus;
}

---eMBRTUStart函数 （mbrtu.c）

void
eMBRTUStart( void )
{
    /*函数功能
     * 1:设置接收状态机eRcvState为STATE_RX_INIT；
     * 2:使能串口接收,禁止串口发送,作为从机,等待主机传送的数据;
     * 3:开启定时器，3.5T时间后定时器发生第一次中断,此时eRcvState为STATE_RX_INIT,上报初始化完成事件,然后设置eRcvState为空闲STATE_RX_IDLE;
     * 4:每次进入3.5T定时器中断,定时器被禁止，等待串口有字节接收后，才使能定时器;
     * */
    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 );   //开启串口接收，发送未开启
    vMBPortTimersEnable(  );              //启动定时器

    EXIT_CRITICAL_SECTION(  );
}

　　eMBEnable函数启动协议栈pvMBFrameStartCur，对于RTU模式，pvMBFrameStartCur函数指针指向 eMBRTUStart，在eMBRTUStart函数中，全局变量eRcvState=STATE_RX_INIT,并使能串口和定时器。注意！！！此时定时器将开始工作！！！。eMBEnable函数中将把全局变量改为 eMBState=STATE_ENABLED。

3、状态机轮训---eMBPoll函数（mb.c）

eMBErrorCode eMBPoll( void )
{
　　/*函数功能:
　　*1:检查协议栈状态是否使能，eMBState初值为STATE_NOT_INITIALIZED，在eMBInit()函数中被赋值为STATE_DISABLED,在eMBEnable函数中被赋值为STATE_ENABLE;
　　*2:轮询EV_FRAME_RECEIVED事件发生，若EV_FRAME_RECEIVED事件发生，接收一帧报文数据，上报EV_EXECUTE事件，解析一帧报文，响应(发送)一帧数据给主机;
　　*/

static UCHAR   *ucMBFrame;          //接收和发送报文数据缓存区
    static UCHAR    ucRcvAddress;       //modbus从机地址
    static UCHAR    ucFunctionCode;     //功能码
    static USHORT   usLength;           //报文长度
    static eMBException eException;     //错误码响应  枚举

    int             i;
    eMBErrorCode    eStatus = MB_ENOERR;
    eMBEventType    eEvent;                 //错误码

    /* Check if the protocol stack is ready. */
    if( eMBState != STATE_ENABLED )           //检查协议栈是否使能
    {
        return MB_EILLSTATE;                  //协议栈未使能，返回协议栈无效错误码
    }

    /* Check if there is a event available. If not return control to caller.
     * Otherwise we will handle the event. */

    if( xMBPortEventGet( &eEvent ) == TRUE )   //判断事件是否发生
    {
        switch ( eEvent )          //查询哪个事件发生
        {
        case EV_READY:
            break;

        case EV_FRAME_RECEIVED:       //接收到一帧数据，此事件发生
            eStatus = peMBFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength );  //接收数据，并检验报文长度和CRC校验是否正确
            /*
             * ucRcvAddress 主站要读取的从站的地址
             * ucMBFrame  指向PDU的头部
             * usLength  PDU的长度
             */
            if( eStatus == MB_ENOERR )
            {
                /* Check if the frame is for us. If not ignore the frame. */
                if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST ) )
                {
                    ( void )xMBPortEventPost( EV_EXECUTE );        //修改事件标志为EV_EXECUTE执行事件
                }
            }
            break;

        case EV_EXECUTE:                             //修改事件标志为EV_EXECUTE执行事件
            ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF];  //提取功能码
            eException = MB_EX_ILLEGAL_FUNCTION;       //赋错误码初值为无效的功能码
            for( i = 0; i < MB_FUNC_HANDLERS_MAX; i++ )
            {
                /* No more function handlers registered. Abort. */
                if( xFuncHandlers[i].ucFunctionCode == 0 )
                {
                    break;
                }
                else if( xFuncHandlers[i].ucFunctionCode == ucFunctionCode )    //根据报文中的功能码，处理报文
                {
                    eException = xFuncHandlers[i].pxHandler( ucMBFrame, &usLength ); //对接收到的报文进行解析
                    break;
                }
            }

            /* If the request was not sent to the broadcast address we
             * return a reply. */
            if( ucRcvAddress != MB_ADDRESS_BROADCAST )
            {
                if( eException != MB_EX_NONE )    //接收到的报文有错误
                {
                    /* An exception occured. Build an error frame. */
                    usLength = 0;                      //响应发送数据的首字节为从机地址
                    ucMBFrame[usLength++] = ( UCHAR )( ucFunctionCode | MB_FUNC_ERROR );  //响应发送数据帧的第二个字节，功能码最高位置1
                    ucMBFrame[usLength++] = eException;          //响应发送数据帧的第三个字节为错误码标识
                }
                if( ( eMBCurrentMode == MB_ASCII ) && MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS )
                {
                    vMBPortTimersDelay( MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS );
                }                
                eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength );  //modbus从机响应函数,发送响应给主机
            }
            break;

        case EV_FRAME_SENT:
            break;
        }
    }
    return MB_ENOERR;
}

 　　

　　在eMBPoll函数中，首先由 xMBPortEventGet( &eEvent ) == TRUE 判断时间是否发生，若无事件发生则不进入状态机；若有时间发生则进入状态机开始轮询。状态机的时间转换在定时中断服务函数中实现。

　　在eMBEnable函数中启动定时器后，定时器开始工作（见 eMBRTUStart函数）， 在定时器第一次超时之后将会发送xNeedPoll = xMBPortEventPost( EV_READY ) 事件，然后关闭定时器，接收机状态（全局变量）eRcvState为STATE_RX_IDLE。此时，主循环eMBPoll中将执行一次EV_READY下的操作。至此，完成Modbus协议栈的初始化准备工作，协议栈开始运行，eMBPoll()函数轮询等待接收完成事件发生。

---定时器中断服务函数

void BOARD_GPTA_HANDLER()
{
//    BOOL            bTaskWoken = FALSE; 

    PRINTF("
Timer Expired:


");
    vMBPortSetWithinException( TRUE );
    GPT_ClearStatusFlag(BOARD_GPTA_BASEADDR, gptStatusFlagOutputCompare1);  //关闭定时器 

( void )pxMBPortCBTimerExpired( ); //事件转换  vMBPortSetWithinException( FALSE );
}

---xMBRTUTimerT35Expired  T3.5超时函数

BOOL  xMBRTUTimerT35Expired( void )
{
    /* 函数功能
     * 1:从机接受完成一帧数据后，接收状态机eRcvState为STATE_RX_RCV；
     * 2:上报“接收到报文”事件(EV_FRAME_RECEIVED);
     * 3:禁止3.5T定时器，设置接收状态机eRcvState状态为STATE_RX_IDLE空闲;
     */
    BOOL            xNeedPoll = FALSE;

    switch ( eRcvState )          //上报modbus协议栈的事件状态给poll函数
    {
        /* Timer t35 expired. Startup phase is finished. */
    case STATE_RX_INIT:
        xNeedPoll = xMBPortEventPost( EV_READY );   //初始化完成事件
        break;

        /* A frame was received and t35 expired. Notify the listener that
         * a new frame was received. */
    case STATE_RX_RCV:                        //一帧数据接收完成
        xNeedPoll = xMBPortEventPost( EV_FRAME_RECEIVED );    //上报协议栈事件,接收到一帧完整的数据
        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(  );  //当接收到一帧数据后，禁止3.5T定时器，直到接受下一帧数据开始，开始计时
    eRcvState = STATE_RX_IDLE;  //处理完一帧数据，接收器状态为空闲

    return xNeedPoll;
}

4、报文接收

　　在定时器第一次中断之后，状态机为eRcvState=STATE_RX_IDLE，即读空闲状态，eMBPoll也阻塞在等待接收完成事件发生。而在eMBPoll之前的eMBRTUStart函数中已经开启了串口中断，因此在接收到数据之后，串口中断将会响应，在串口中断服务函数中将调用接收状态机函数xMBRTUReceiveFSM来接收数据。

---xMBRTUReceiveFSM函数

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

    assert( eSndState == STATE_TX_IDLE );   //确保没有数据在发送

    /* Always read the character. */
    ( void )xMBPortSerialGetByte( ( CHAR * ) & ucByte ); //从串口数据寄存器读取一个字节数据

    switch ( eRcvState )      //根据不同的状态转移
    {
        /* If we have received a character in the init state we have to
         * wait until the frame is finished.
         */
    case STATE_RX_INIT:
        vMBPortTimersEnable(  );       //开启3.5T定时器
        break;

        /* In the error state we wait until all characters in the
         * damaged frame are transmitted.
         */
    case STATE_RX_ERROR:            //数据帧被损坏，重启定时器，不保存串口接收的数据
        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:           // 接收器空闲，开始接收，进入STATE_RX_RCV状态
        usRcvBufferPos = 0;
        ucRTUBuf[usRcvBufferPos++] = ucByte;    //保存数据
        eRcvState = STATE_RX_RCV;

        /* Enable t3.5 timers. */
        vMBPortTimersEnable(  );          //每收到一个字节，都重启3.5T定时器
        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;   //接收数据
        }
        else
        {
            eRcvState = STATE_RX_ERROR;     //一帧报文的字节数大于最大PDU长度，忽略超出的数据
        }
        vMBPortTimersEnable(  );            //每收到一个字节，都重启3.5T定时器
        break;
    }
    return xTaskNeedSwitch;
}

　　在串口中断前，状态机为eRcvState=STATE_RX_IDLE，接收状态机开始后，读取uart串口缓存中的数据，并进入STATE_RX_IDLE分支中存储一次数据后开启定时器，然后进入STATE_RX_RCV分支继续接收后续的数据，直至定时器超时！如果没有超时的话，状态不会转换，将还可以继续接收数据。超时之后，在T3.5超时函数xMBRTUTimerT35Expired 中将发送EV_FRAME_RECEIVED事件。然后eMBPoll函数将会调用eMBRTUReceive函数。

 /* A frame was received and t35 expired. Notify the listener that
         * a new frame was received. */
    case STATE_RX_RCV:                        //一帧数据接收完成
        xNeedPoll = xMBPortEventPost( EV_FRAME_RECEIVED );    //上报协议栈事件,接收到一帧完整的数据
        break;

---eMBRTUReceive函数

eMBErrorCode
eMBRTUReceive( UCHAR * pucRcvAddress, UCHAR ** pucFrame, USHORT * pusLength )
{
    /*eMBPoll函数轮询到EV_FRAME_RECEIVED事件时,调用peMBFrameReceiveCur()，
     * 此函数是用户为函数指针peMBFrameReceiveCur()的赋值,
     * 此函数完成的功能：
     * 从一帧数据报文中，取得modbus从机地址给pucRcvAddress、PDU报文的长度给pusLength，
     * PDU报文的首地址给pucFrame，函数*形参全部为地址传递，
     */
    BOOL            xFrameReceived = FALSE;
    eMBErrorCode    eStatus = MB_ENOERR;

    ENTER_CRITICAL_SECTION(  );
    assert( usRcvBufferPos < MB_SER_PDU_SIZE_MAX );  //断言宏，判断接收到的字节数<256，如果>256，终止程序

    /* Length and CRC check */
    if( ( usRcvBufferPos >= MB_SER_PDU_SIZE_MIN )
        && ( usMBCRC16( ( UCHAR * ) ucRTUBuf, usRcvBufferPos ) == 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]; //保存从站地址

        /* 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 ); //PDU长度

        /* Return the start of the Modbus PDU to the caller. */
        *pucFrame = ( UCHAR * ) & ucRTUBuf[MB_SER_PDU_PDU_OFF];  //pucFrame指向PDU起始位置
        xFrameReceived = TRUE;
    }
    else
    {
        eStatus = MB_EIO;
    }

    EXIT_CRITICAL_SECTION(  );
    return eStatus;
}

　　eMBRTUReceive函数完成了CRC校验、帧数据地址和长度的赋值，便于给上层进行处理！之后eMBPoll函数发送 ( void )xMBPortEventPost( EV_EXECUTE )事件。在EV_EXECUTE 事件中，从站对接收到的数据进行处理，包括根据功能码寻找功能函数处理报文和调用eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength ) 发送应答报文。

--- eMBRTUSend函数

eMBErrorCode  eMBRTUSend( UCHAR ucSlaveAddress, const UCHAR * pucFrame, USHORT usLength )
{
    /*函数功能
     * 1:对响应报文PDU前面加上从机地址;
     * 2:对响应报文PDU后加上CRC校;
     * 3:使能发送，启动传输;*/
    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;   //在协议数据单元前加从机地址
        usSndBufferCount = 1;

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

        /* Calculate CRC16 checksum for Modbus-Serial-Line-PDU. */
        usCRC16 = usMBCRC16( ( UCHAR * ) pucSndBufferCur, usSndBufferCount );
        ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 & 0xFF );
        ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 >> 8 );

        /* Activate the transmitter. */
        eSndState = STATE_TX_XMIT;   //发送状态
                                                          //以下为新添加
        xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur );   //发送一个字节的数据，进入发送中断函数，启动传输
        pucSndBufferCur++;    /* next byte in sendbuffer. */
        usSndBufferCount--;

        vMBPortSerialEnable( FALSE, TRUE );   //使能发送，禁止接收
    }
    else
    {
        eStatus = MB_EIO;
    }
    EXIT_CRITICAL_SECTION(  );
    return eStatus;
}

在 eMBRTUSend函数中会调用串口发送数据，在进入串口发送中断后会调用xMBRTUTransmitFSM发送状态机函数发送应答报文。

---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 );        //发送器处于空闲状态，使能接收，禁止发送
        break;

    case STATE_TX_XMIT:               //发送器处于发送状态,在从机发送函数eMBRTUSend中赋值STATE_TX_XMIT
        /* check if we are finished. */
        if( usSndBufferCount != 0 )        //发送数据
        {
            xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur );
            pucSndBufferCur++;  /* next byte in sendbuffer. */
            usSndBufferCount--;
        }
        else              //传递任务，发送完成
        {
            xNeedPoll = xMBPortEventPost( EV_FRAME_SENT );   //协议栈事件状态赋值为EV_FRAME_SENT,发送完成事件,eMBPoll函数会对此事件进行处理
            /* Disable transmitter. This prevents another transmit buffer
             * empty interrupt. */
            vMBPortSerialEnable( TRUE, FALSE );   //使能接收，禁止发送
            eSndState = STATE_TX_IDLE;    //发送器状态为空闲状态
        }
        break;
    }

    return xNeedPoll;
}

至此：协议栈准备工作，从机接受报文，解析报文，从机发送响应报文四部分结束。

参考：

freemodbus库函数详解 FreeModbus源码详解-程序设计  

Freemodbus完全分析_百度文库  

FreeModbus学习笔记_百度文库