eMbPoll()的作用是FreeMod协议通信过程中不断查询事件对列有无完速数据桢，并进行地址和CRD验证，最后运行和回复主机。
为了减小代码尺寸对eMbPoll进行改进：
原版：
1:
2: eMBErrorCode
3: eMBPoll( void )
4: {
5: static UCHAR *ucMBFrame;
6: static UCHAR ucRcvAddress;
7: static UCHAR ucFunctionCode;
8: static USHORT usLength;
9: static eMBException eException;
10:
11: int i;
12: eMBErrorCode eStatus = MB_ENOERR;
13: eMBEventType eEvent;
14:
15: /* Check if the protocol stack is ready. */
16: if( eMBState != STATE_ENABLED )
17: {
18: return MB_EILLSTATE;
19: }
20:
21: /* Check if there is a event available. If not return control to caller.
22: * Otherwise we will handle the event. */
23: if( xMBPortEventGet( &eEvent ) == TRUE )
24: {
25: switch ( eEvent )
26: {
27: case EV_READY:
28: break;
29:
30: case EV_FRAME_RECEIVED:
31: eStatus = peMBFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength );
32: if( eStatus == MB_ENOERR )
33: {
34: /* Check if the frame is for us. If not ignore the frame. */
35: if( ( ucRcvAddress == ucMBAddress ) || ( ucRcvAddress == MB_ADDRESS_BROADCAST ) )
36: {
37: ( void )xMBPortEventPost( EV_EXECUTE );
38: }
39: }
40: break;
41:
42: case EV_EXECUTE:
43: ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF];
44: eException = MB_EX_ILLEGAL_FUNCTION;
45: for( i = 0; i < MB_FUNC_HANDLERS_MAX; i++ )
46: {
47: /* No more function handlers registered. Abort. */
48: if( xFuncHandlers[i].ucFunctionCode == 0 )
49: {
50: break;
51: }
52: else if( xFuncHandlers[i].ucFunctionCode == ucFunctionCode )
53: {
54: eException = xFuncHandlers[i].pxHandler( ucMBFrame, &usLength );
55: break;
56: }
57: }
58:
59: /* If the request was not sent to the broadcast address we
60: * return a reply. */
61: if( ucRcvAddress != MB_ADDRESS_BROADCAST )
62: {
63: if( eException != MB_EX_NONE )
64: {
65: /* An exception occured. Build an error frame. */
66: usLength = 0;
67: ucMBFrame[usLength++] = ( UCHAR )( ucFunctionCode | MB_FUNC_ERROR );
68: ucMBFrame[usLength++] = eException;
69: }
70: if( ( eMBCurrentMode == MB_ASCII ) && MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS )
71: {
72: vMBPortTimersDelay( MB_ASCII_TIMEOUT_WAIT_BEFORE_SEND_MS );
73: }
74: eStatus = peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength );
75: }
76: break;
77:
78: case EV_FRAME_SENT:
79: break;
80: }
81: }
82: return MB_ENOERR;
83: }
改进后的eMbPoll():
1:
2: void eMBPoll( void ){
3:
4: static UCHAR *ucMBFrame;
5: static UCHAR ucFunctionCode;
6: static USHORT usLength;
7: static eMBException eException;
8: eMBEventType eEvent;
9: UCHAR i;
10: USHORT usCRC16;
11: if(xMBPortEventGet( &eEvent) == TRUE ){ //桢事件判断
12: if(eEvent == EV_FRAME_RECEIVED){
13: if(usRcvBufferPos < MB_SER_PDU_SIZE_MIN) //最小桢判断
14: return;
15: if(usMBCRC16((UCHAR *)ucRTUBuf, usRcvBufferPos ) != 0) //CRC判断
16: return;
17: if(IS_VALID_ADD){ //地址
18: ucMBFrame = (UCHAR *) &ucRTUBuf[MB_SER_PDU_PDU_OFF];
19: usLength = (USHORT)( usRcvBufferPos - MB_SER_PDU_PDU_OFF - MB_SER_PDU_SIZE_CRC);
20: ucFunctionCode = ucMBFrame[MB_PDU_FUNC_OFF];
21: eException = MB_EX_ILLEGAL_FUNCTION;
22: for(i = 0; i < MB_FUNC_HANDLERS_MAX; i++ ){ //执行功能码
23: if( xFuncHandlers[i].ucFunctionCode == 0 ){
24: return;
25: }
26: else if( xFuncHandlers[i].ucFunctionCode == ucFunctionCode ){
27: eException = xFuncHandlers[i].pxHandler( ucMBFrame, &usLength );
28: break;
29: }
30: }
31: if(IS_NOT_BROADCAST){ //回复主机
32: if( eException != MB_EX_NONE ){ //错误码
33: usLength = 0;
34: ucMBFrame[usLength++] = ( UCHAR )( ucFunctionCode | MB_FUNC_ERROR );
35: ucMBFrame[usLength++] = eException;
36: }
37: if(eRcvState == STATE_RX_IDLE){ //发送
38: pucSndBufferCur = ( UCHAR * ) ucMBFrame - 1;
39: pucSndBufferCur[MB_SER_PDU_ADDR_OFF] = ucMBAddress;
40: usSndBufferCount = usLength + 1;
41: usCRC16 = usMBCRC16( ( UCHAR * ) pucSndBufferCur, usSndBufferCount );
42: ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 & 0xFF );
43: ucRTUBuf[usSndBufferCount++] = ( UCHAR )( usCRC16 >> 8 );
44: eSndState = STATE_TX_XMIT;
45: vMBPortSerialEnable( FALSE, TRUE );
46: }//发送结束
47: }//回复结束
48: }//地址判断
49: }//桢事件判断
50: }
51: }
改进说明：
1、eMbPoll（）调用一次即可运行功能码和回复主机；
2、省去独立的接收函数peMBFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength ); 而直接操作，（其实里面对算出数据桢的启始位置、和长度）；
3、省去发送函数peMBFrameSendCur( ucMBAddress, ucMBFrame, usLength ); 而直接操作；
4、省去返回值，因为调用处没有使用；
5、对功能的遍历i改成unsigned char类型，省去ucRcvAddress和eMBErrorCode eStatus = MB_ENOERR; 变量，
6、功能兼容原版本。
eMbPoll的经典之处在于功能的运行，——》函数指针，这部分在其它笔记中记录。