why bytesAvailable() returns '0' when there are answer data from serial port monitor

[to_guliang]

hello, everyone. i use qextserialport in my application to realize serial communication.
i can write data using writeData(const char * data, qint64 maxSize). But the function bytesAvailable () returns '0' while reading, so i can't get any data through readData(char * data, qint64 maxSize). But i can see the answer data from serial port monitor. WHY??
my code:

if(RS232->open(QIODevice::ReadWrite))
   {
       RS232->writeData(Data,13);
      //usleep(500);
 
      int i= RS232->QIODevice::bytesAvailable();
      if(i > 0)
      {
         char *pStr=NULL;
         RS232->readData(pStr,25);     
      }
      RS232->close(); 
   }

if(RS232->open(QIODevice::ReadWrite))
   {
       RS232->writeData(Data,13);
      //usleep(500);
 
      int i= RS232->QIODevice::bytesAvailable();
      if(i > 0)
      {
         char *pStr=NULL;
         RS232->readData(pStr,25);     
      }
      RS232->close(); 
   }

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adding the "usleep(500);" has no use.

thanks very much for any advice!