freemodbus-v1.5.0 源码分析 (转)

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

 1 eMBErrorCode  eMBInit( eMBMode eMode, UCHAR ucSlaveAddress, UCHAR ucPort, ULONG ulBaudRate, eMBParity eParity )
 2 {
 3 
 4 ?1/*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;
 5 /* check preconditions */
 6     if( ( ucSlaveAddress == MB_ADDRESS_BROADCAST ) ||
 7         ( ucSlaveAddress < MB_ADDRESS_MIN ) || ( ucSlaveAddress > MB_ADDRESS_MAX ) )   //验证从机地址
 8     {
 9         eStatus = MB_EINVAL;                                  //地址错误
10     }
11     else
12     {
13         ucMBAddress = ucSlaveAddress;
14 
15         switch ( eMode )
16         {
17 #if MB_RTU_ENABLED > 0
18         case MB_RTU:
19             pvMBFrameStartCur = eMBRTUStart;
20             pvMBFrameStopCur = eMBRTUStop;
21             peMBFrameSendCur = eMBRTUSend;
22             peMBFrameReceiveCur = eMBRTUReceive;
23             pvMBFrameCloseCur = MB_PORT_HAS_CLOSE ? vMBPortClose : NULL;
24             pxMBFrameCBByteReceived = xMBRTUReceiveFSM;         //接收状态机，串口接受中断最终调用此函数接收数据
25             pxMBFrameCBTransmitterEmpty = xMBRTUTransmitFSM;    //发送状态机，串口发送中断最终调用此函数发送数据
26             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
27 
28             eStatus = eMBRTUInit( ucMBAddress, ucPort, ulBaudRate, eParity );
29             break;
30 #endif
31 #if MB_ASCII_ENABLED > 0
32         case MB_ASCII:
33             pvMBFrameStartCur = eMBASCIIStart;
34             pvMBFrameStopCur = eMBASCIIStop;
35             peMBFrameSendCur = eMBASCIISend;
36             peMBFrameReceiveCur = eMBASCIIReceive;
37             pvMBFrameCloseCur = MB_PORT_HAS_CLOSE ? vMBPortClose : NULL;
38             pxMBFrameCBByteReceived = xMBASCIIReceiveFSM;
39             pxMBFrameCBTransmitterEmpty = xMBASCIITransmitFSM;
40             pxMBPortCBTimerExpired = xMBASCIITimerT1SExpired;
41 
42             eStatus = eMBASCIIInit( ucMBAddress, ucPort, ulBaudRate, eParity );
43             break;
44 #endif
45         default:
46             eStatus = MB_EINVAL;
47         }
48 
49         if( eStatus == MB_ENOERR )
50         {
51             if( !xMBPortEventInit(  ) )
52             {
53                 /* port dependent event module initalization failed. */
54                 eStatus = MB_EPORTERR;
55             }
56             else
57             {//设定当前状态
58                 eMBCurrentMode = eMode;           //设定RTU模式
59                 eMBState = STATE_DISABLED;        //modbus协议栈初始化状态,在此初始化为禁止
60             }
61         }
62     }
63     return eStatus;
64 }

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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）

 1 eMBErrorCode
 2 eMBEnable( void )
 3 {
 4     /*函数功能：
 5      *1:实现RTU模式和ASCALL模式的协议栈初始化;
 6      *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;
 7      *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;
 8      *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;
 9      */
10     eMBErrorCode    eStatus = MB_ENOERR;
11 
12     if( eMBState == STATE_DISABLED )
13     {
14         /* Activate the protocol stack. */
15         pvMBFrameStartCur(  );       //激活协议栈
16         eMBState = STATE_ENABLED;    //设置Modbus协议栈工作状态eMBState为STATE_ENABLED
17     }
18     else
19     {
20         eStatus = MB_EILLSTATE;
21     }
22     return eStatus;
23 }

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---eMBRTUStart函数（mbrtu.c）

 1 void
 2 eMBRTUStart( void )
 3 {
 4     /*函数功能
 5      * 1:设置接收状态机eRcvState为STATE_RX_INIT；
 6      * 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;
 7      * 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;
 8      * 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;
 9      * */
10     ENTER_CRITICAL_SECTION(  );
11     /* Initially the receiver is in the state STATE_RX_INIT. we start
12      * the timer and if no character is received within t3.5 we change
13      * to STATE_RX_IDLE. This makes sure that we delay startup of the
14      * modbus protocol stack until the bus is free.
15      */
16     eRcvState = STATE_RX_INIT;
17     vMBPortSerialEnable( TRUE, FALSE );   //开启串口接收，发送未开启
18     vMBPortTimersEnable(  );              //启动定时器
19 
20     EXIT_CRITICAL_SECTION(  );
21 }

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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）

 1 eMBErrorCode eMBPoll( void )
 2 {
 3 　　/*函数功能:
 4 　　*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;
 5 　　*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;
 6 　　*/
 7 static UCHAR   *ucMBFrame;          //接收和发送报文数据缓存区
 8     static UCHAR    ucRcvAddress;       //modbus从机地址
 9     static UCHAR    ucFunctionCode;     //功能码
10     static USHORT   usLength;           //报文长度
11     static eMBException eException;     //错误码响应  枚举
12 
13     int             i;
14     eMBErrorCode    eStatus = MB_ENOERR;
15     eMBEventType    eEvent;                 //错误码
16 
17     /* Check if the protocol stack is ready. */
18     if( eMBState != STATE_ENABLED )           //检查协议栈是否使能
19     {
20         return MB_EILLSTATE;                  //协议栈未使能，返回协议栈无效错误码
21     }
22 
23     /* Check if there is a event available. If not return control to caller.
24      * Otherwise we will handle the event. */
25 
26     if( xMBPortEventGet( &eEvent ) == TRUE )   //判断事件是否发生
27     {
28         switch ( eEvent )          //查询哪个事件发生
29         {
30         case EV_READY:
31             break;
32 
33         case EV_FRAME_RECEIVED:       //接收到一帧数据，此事件发生
34             eStatus = peMBFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength );  //接收数据，并检验报文长度和CRC校验是否正确
35             /*
36              * ucRcvAddress 主站要读取的从站的地址
37              * ucMBFrame  指向PDU的头部
38              * usLength  PDU的长度
39              */
40             if( eStatus == MB_ENOERR )
41             {
42                 /* Check if the frame is for us. If not ignore the frame. */
43                 if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST ) )
44                 {
45                     ( void )xMBPortEventPost( EV_EXECUTE );        //修改事件标志为EV_EXECUTE执行事件
46                 }
47             }
48             break;
49 
50         case EV_EXECUTE:                             //修改事件标志为EV_EXECUTE执行事件
51             ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF];  //提取功能码
52             eException = MB_EX_ILLEGAL_FUNCTION;       //赋错误码初值为无效的功能码
53             for( i = 0; i < MB_FUNC_HANDLERS_MAX; i++ )
54             {
55                 /* No more function handlers registered. Abort. */
56                 if( xFuncHandlers[i].ucFunctionCode == 0 )
57                 {
58                     break;
59                 }
60                 else if( xFuncHandlers[i].ucFunctionCode == ucFunctionCode )    //根据报文中的功能码，处理报文
61                 {
62                     eException = xFuncHandlers[i].pxHandler( ucMBFrame, &usLength ); //对接收到的报文进行解析
63                     break;
64                 }
65             }
66 
67             /* If the request was not sent to the broadcast address we
68              * return a reply. */
69             if( ucRcvAddress != MB_ADDRESS_BROADCAST )
70             {
71                 if( eException != MB_EX_NONE )    //接收到的报文有错误
72                 {
73                     /* An exception occured. Build an error frame. */
74                     usLength = 0;                      //响应发送数据的首字节为从机地址
75                     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
76                     ucMBFrame[usLength++] = eException;          //响应发送数据帧的第三个字节为错误码标识
77                 }
78                 if( ( eMBCurrentMode == MB_ASCII ) && MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS )
79                 {
80                     vMBPortTimersDelay( MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS );
81                 }                
82                 eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength );  //modbus从机响应函数,发送响应给主机
83             }
84             break;
85 
86         case EV_FRAME_SENT:
87             break;
88         }
89     }
90     return MB_ENOERR;
91 }

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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

1 void BOARD_GPTA_HANDLER()
2 {
3 //    BOOL            bTaskWoken = FALSE; 
4 
5     PRINTF("\r\nTimer Expired:\n\n\r");
6     vMBPortSetWithinException( TRUE );
7     GPT_ClearStatusFlag(BOARD_GPTA_BASEADDR, gptStatusFlagOutputCompare1);  //关闭定时器 
8 ( void )pxMBPortCBTimerExpired( ); //事件转换  vMBPortSetWithinException( FALSE );
9 }

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---xMBRTUTimerT35Expired T3.5超时函数

 1 BOOL  xMBRTUTimerT35Expired( void )
 2 {
 3     /* 函数功能
 4      * 1:从机接受完成一帧数据后，接收状态机eRcvState为STATE_RX_RCV；
 5      * 2:上报“接收到报文”事件(EV_FRAME_RECEIVED);
 6      * 3:禁止3.5T定时器，设置接收状态机eRcvState状态为STATE_RX_IDLE空闲;
 7      */
 8     BOOL            xNeedPoll = FALSE;
 9 
10     switch ( eRcvState )          //上报modbus协议栈的事件状态给poll函数
11     {
12         /* Timer t35 expired. Startup phase is finished. */
13     case STATE_RX_INIT:
14         xNeedPoll = xMBPortEventPost( EV_READY );   //初始化完成事件
15         break;
16 
17         /* A frame was received and t35 expired. Notify the listener that
18          * a new frame was received. */
19     case STATE_RX_RCV:                        //一帧数据接收完成
20         xNeedPoll = xMBPortEventPost( EV_FRAME_RECEIVED );    //上报协议栈事件,接收到一帧完整的数据
21         break;
22 
23         /* An error occured while receiving the frame. */
24     case STATE_RX_ERROR:
25         break;
26 
27         /* Function called in an illegal state. */
28     default:
29         assert( ( eRcvState == STATE_RX_INIT ) ||
30                 ( eRcvState == STATE_RX_RCV ) || ( eRcvState == STATE_RX_ERROR ) );
31     }
32 
33     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
34     eRcvState = STATE_RX_IDLE;  //处理完一帧数据，接收器状态为空闲
35 
36     return xNeedPoll;
37 }

View Code

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

 1 BOOL  xMBRTUReceiveFSM( void )
 2 {
 3     /*函数功能
 4      *1:将接收到的数据存入ucRTUBuf[]中;
 5      *2:usRcvBufferPos为全局变量，表示接收数据的个数;
 6      *3:每接收到一个字节的数据，3.5T定时器清0
 7      */
 8 
 9     BOOL            xTaskNeedSwitch = FALSE;
10     UCHAR           ucByte;
11 
12     assert( eSndState == STATE_TX_IDLE );   //确保没有数据在发送
13 
14     /* Always read the character. */
15     ( void )xMBPortSerialGetByte( ( CHAR * ) & ucByte ); //从串口数据寄存器读取一个字节数据
16 
17     switch ( eRcvState )      //根据不同的状态转移
18     {
19         /* If we have received a character in the init state we have to
20          * wait until the frame is finished.
21          */
22     case STATE_RX_INIT:
23         vMBPortTimersEnable(  );       //开启3.5T定时器
24         break;
25 
26         /* In the error state we wait until all characters in the
27          * damaged frame are transmitted.
28          */
29     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
30         vMBPortTimersEnable(  );
31         break;
32 
33         /* In the idle state we wait for a new character. If a character
34          * is received the t1.5 and t3.5 timers are started and the
35          * receiver is in the state STATE_RX_RECEIVCE.
36          */
37     case STATE_RX_IDLE:           // 接收器空闲，开始接收，进入STATE_RX_RCV状态
38         usRcvBufferPos = 0;
39         ucRTUBuf[usRcvBufferPos++] = ucByte;    //保存数据
40         eRcvState = STATE_RX_RCV;
41 
42         /* Enable t3.5 timers. */
43         vMBPortTimersEnable(  );          //每收到一个字节，都重启3.5T定时器
44         break;
45 
46         /* We are currently receiving a frame. Reset the timer after
47          * every character received. If more than the maximum possible
48          * number of bytes in a modbus frame is received the frame is
49          * ignored.
50          */
51     case STATE_RX_RCV:
52         if( usRcvBufferPos < MB_SER_PDU_SIZE_MAX )
53         {
54             ucRTUBuf[usRcvBufferPos++] = ucByte;   //接收数据
55         }
56         else
57         {
58             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
59         }
60         vMBPortTimersEnable(  );            //每收到一个字节，都重启3.5T定时器
61         break;
62     }
63     return xTaskNeedSwitch;
64 }

View Code

　　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。

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

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---eMBRTUReceive函数

 1 eMBErrorCode
 2 eMBRTUReceive( UCHAR * pucRcvAddress, UCHAR ** pucFrame, USHORT * pusLength )
 3 {
 4     /*eMBPoll函数轮询到EV_FRAME_RECEIVED事件时,调用peMBFrameReceiveCur()，
 5      * 此函数是用户为函数指针peMBFrameReceiveCur()的赋值,
 6      * 此函数完成的功能：
 7      * 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，
 8      * PDU报文的首地址给pucFrame，函数*形参全部为地址传递，
 9      */
10     BOOL            xFrameReceived = FALSE;
11     eMBErrorCode    eStatus = MB_ENOERR;
12 
13     ENTER_CRITICAL_SECTION(  );
14     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
15 
16     /* Length and CRC check */
17     if( ( usRcvBufferPos >= MB_SER_PDU_SIZE_MIN )
18         && ( usMBCRC16( ( UCHAR * ) ucRTUBuf, usRcvBufferPos ) == 0 ) )
19     {
20         /* Save the address field. All frames are passed to the upper layed
21          * and the decision if a frame is used is done there.
22          */
23         *pucRcvAddress = ucRTUBuf[MB_SER_PDU_ADDR_OFF]; //保存从站地址
24 
25         /* Total length of Modbus-PDU is Modbus-Serial-Line-PDU minus
26          * size of address field and CRC checksum.
27          */
28         *pusLength = ( USHORT )( usRcvBufferPos - MB_SER_PDU_PDU_OFF - MB_SER_PDU_SIZE_CRC ); //PDU长度
29 
30         /* Return the start of the Modbus PDU to the caller. */
31         *pucFrame = ( UCHAR * ) & ucRTUBuf[MB_SER_PDU_PDU_OFF];  //pucFrame指向PDU起始位置
32         xFrameReceived = TRUE;
33     }
34     else
35     {
36         eStatus = MB_EIO;
37     }
38 
39     EXIT_CRITICAL_SECTION(  );
40     return eStatus;
41 }

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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

 1 eMBErrorCode  eMBRTUSend( UCHAR ucSlaveAddress, const UCHAR * pucFrame, USHORT usLength )
 2 {
 3     /*函数功能
 4      * 1:对响应报文PDU前面加上从机地址;
 5      * 2:对响应报文PDU后加上CRC校;
 6      * 3:使能发送，启动传输;*/
 7     eMBErrorCode    eStatus = MB_ENOERR;
 8     USHORT          usCRC16;
 9 
10     ENTER_CRITICAL_SECTION(  );
11 
12     /* Check if the receiver is still in idle state. If not we where to
13      * slow with processing the received frame and the master sent another
14      * frame on the network. We have to abort sending the frame.
15      */
16     if( eRcvState == STATE_RX_IDLE )
17     {
18         /* First byte before the Modbus-PDU is the slave address. */
19         pucSndBufferCur = ( UCHAR * ) pucFrame - 1;   //在协议数据单元前加从机地址
20         usSndBufferCount = 1;
21 
22         /* Now copy the Modbus-PDU into the Modbus-Serial-Line-PDU. */
23         pucSndBufferCur[MB_SER_PDU_ADDR_OFF] = ucSlaveAddress;
24         usSndBufferCount += usLength;
25 
26         /* Calculate CRC16 checksum for Modbus-Serial-Line-PDU. */
27         usCRC16 = usMBCRC16( ( UCHAR * ) pucSndBufferCur, usSndBufferCount );
28         ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 & 0xFF );
29         ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 >> 8 );
30 
31         /* Activate the transmitter. */
32         eSndState = STATE_TX_XMIT;   //发送状态
33                                                           //以下为新添加
34         xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur );   //发送一个字节的数据，进入发送中断函数，启动传输
35         pucSndBufferCur++;    /* next byte in sendbuffer. */
36         usSndBufferCount--;
37 
38         vMBPortSerialEnable( FALSE, TRUE );   //使能发送，禁止接收
39     }
40     else
41     {
42         eStatus = MB_EIO;
43     }
44     EXIT_CRITICAL_SECTION(  );
45     return eStatus;
46 }

View Code

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

 1 BOOL xMBRTUTransmitFSM( void )
 2 {
 3     BOOL            xNeedPoll = FALSE;
 4 
 5     assert( eRcvState == STATE_RX_IDLE );
 6 
 7     switch ( eSndState )
 8     {
 9         /* We should not get a transmitter event if the transmitter is in
10          * idle state.  */
11     case STATE_TX_IDLE:
12         /* enable receiver/disable transmitter. */
13         vMBPortSerialEnable( TRUE, FALSE );        //发送器处于空闲状态，使能接收，禁止发送
14         break;
15 
16     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
17         /* check if we are finished. */
18         if( usSndBufferCount != 0 )        //发送数据
19         {
20             xMBPortSerialPutByte( ( CHAR )*pucSndBufferCur );
21             pucSndBufferCur++;  /* next byte in sendbuffer. */
22             usSndBufferCount--;
23         }
24         else              //传递任务，发送完成
25         {
26             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
27             /* Disable transmitter. This prevents another transmit buffer
28              * empty interrupt. */
29             vMBPortSerialEnable( TRUE, FALSE );   //使能接收，禁止发送
30             eSndState = STATE_TX_IDLE;    //发送器状态为空闲状态
31         }
32         break;
33     }
34 
35     return xNeedPoll;
36 }

View Code

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。

原贴链接： http://www.cnblogs.com/wujing-hubei/p/5935142.html

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

freemodbus-v1.5.0 源码分析 (转)

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