/**
  * Retrieve data in json format
  *
  **/
 function retrieve($serial_number = '')
 {
     $out = array();
     $Tag = new Tag_model();
     foreach ($Tag->retrieve_records($serial_number) as $obj) {
         $out[] = $obj->rs;
     }
     $obj = new View();
     $obj->view('json', array('msg' => $out));
 }