Serial port - send command respond

[boss-tech]

I made an app, that send command + checksum to the GPS tracker and should return response. But i dont get it... i get only gps data every second - speed, position an so on.

I don't know if something is wrong with my code or GPS has some trick or something...

Code:

// in main class i open port, insert command and calculate checksum
// writer class:
 public void run ()
        {  
            serialPort.notifyOnDataAvailable(true);
              
        	  try {                 
                  serialPort.notifyOnOutputEmpty(true);
               } catch (Exception e) {
                  System.out.println("Error setting event notification: "+ e.toString());         
               }                     
               System.out.println("Writing \""+messageString+"\" to "+serialPort.getName());
               
               try {
                  //write to serial port
            	   
               out.write(messageString.getBytes());
            	
                  out.flush();            
                           
                  
               } catch (IOException e) {
            	   System.out.println("Write error: "+ e);            	   
               }    
        }

 // LISTENING TO SERIALPORT EVENTS
        public void serialEvent(SerialPortEvent event) {
            switch (event.getEventType()) {
            case SerialPortEvent.BI:
            case SerialPortEvent.OE:
            case SerialPortEvent.FE:
            case SerialPortEvent.PE:
            case SerialPortEvent.CD:
            case SerialPortEvent.CTS:
            case SerialPortEvent.DSR:
            case SerialPortEvent.RI:
            case SerialPortEvent.OUTPUT_BUFFER_EMPTY:
               break;
            case SerialPortEvent.DATA_AVAILABLE:
               // we get here if data has been received
           
               byte[] buffer = new byte[1024];
               int len = -1;
               try
               {
                   while ( ( len = this.in.read(buffer)) > -1 )
                   {
                       System.out.print(new String(buffer,0,len));
                   }
               }
               catch ( IOException e )
               {
                   e.printStackTrace();
               }  
         
               break;
            }
       
      }