FreeModBus源码解析（1）---开篇

 1   /* Check if the protocol stack is ready. */
     //判断协议栈硬件环境有没有准备。好比如ModBusTCP协议里基于W5500以太网芯片的初始化，sockt通信建立提供TCP服务。
 2     if( eMBState != STATE_ENABLED )
 3     {
 4         return MB_EILLSTATE;
 5     }
 6 
 7     /* Check if there is a event available. If not return control to caller.
 8      * Otherwise we will handle the event. */
　　　　//通过xMBPortEventGet函数处理如果有事件发生得到状态机的状态值并赋值给eEvent,
　　　　//然后通过switch语句运行状态机调用相应函数进行处理，当然这里用了函数指针、回调。
 9     if( xMBPortEventGet( &eEvent ) == TRUE )
10     {
　　　　　　//
11         switch ( eEvent )
12         {
13         case EV_READY:
14             break;
15 
16         case EV_FRAME_RECEIVED:
17             eStatus = peMBFrameReceiveCur( &ucRcvAddress, &ucMBFrame,      &usLength );
18             if( eStatus == MB_ENOERR )
19             {
20                 /* Check if the frame is for us. If not ignore the frame. */
21                 if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST ) )
22                 {
23                     ( void )xMBPortEventPost( EV_EXECUTE );
24                 }
25             }
26             break;
27 
28         case EV_EXECUTE:
29             ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF];
30             eException = MB_EX_ILLEGAL_FUNCTION;
31             for( i = 0; i < MB_FUNC_HANDLERS_MAX; i++ )
32             {
33                 /* No more function handlers registered. Abort. */
34                 if( xFuncHandlers[i].ucFunctionCode == 0 )
35                 {
36                     break;
37                 }
38                 else if( xFuncHandlers[i].ucFunctionCode == ucFunctionCode )
39                 {
40                     eException = xFuncHandlers[i].pxHandler( ucMBFrame, &usLength );
41                     break;
42                 }
43             }
44 
45             /* If the request was not sent to the broadcast address we
46              * return a reply. */
47             if( ucRcvAddress != MB_ADDRESS_BROADCAST )
48             {
49                 if( eException != MB_EX_NONE )
50                 {
51                     /* An exception occured. Build an error frame. */
52                     usLength = 0;
53                     ucMBFrame[usLength++] = ( UCHAR )( ucFunctionCode | MB_FUNC_ERROR );
54                     ucMBFrame[usLength++] = eException;
55                 }
56                 if( ( eMBCurrentMode == MB_ASCII ) && MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS )
57                 {
58                     vMBPortTimersDelay( MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS );
59                 }                
60                 eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength );
61             }
62             break;
63 
64         case EV_FRAME_SENT:
65             break;
66         }
67     }