USB读的问题求教

在GPRS通信模块中使用USB,固件和驱动都已经做好了，现在做上位机程序执行AT命令实现数据的收发,在发送AT+ISSND命令发送数据都成功，可是要从USB驱动上读取模块返回来的数据好像不是很顺利,我用的是异步读写的方法:
不知道周立功的usbdebug是怎么实现USB数据的正确读写的，请论坛上的高手能指导一番，我的源代码如下:
husb=CreateFile(symLink,GENERIC_WRITE|GENERIC_READ,0,NULL,OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL|FILE_FLAG_OVERLAPPED,NULL);

读代码：
if (!ReadFile(hUsb,pData,dwMaxReadLen,dwRealReadLen,&m_ovRead)){          
______________________long LastError=::GetLastError();
______________________if (iLastError==ERROR_IO_PENDING)
______________________{
________________________switch(::WaitForSingleObject(m_ovRead.hEvent,dwTimeOut))
_________________________{
____________________________case WAIT_OBJECT_0:
_______________________________if (::GetOverlappedResult(hUsb,&m_ovRead,dwRealReadLen,true))
__________________________________{iReturn=0;} break;
_____________________________case WAIT_TIMEOUT:
__________________________________::CancelIo(hUsb);
__________________________________break;
_____________________________default:;
_____________________________}
__________________________}
_______________________}