sub crc16 ($$$)
{
   my ($startadr, $endadr, $crc) = @_;
   my ($cadr, $adr, $i, $u, $d);

   for ($adr = $startadr; $adr <= $endadr; $adr++) {
      if (defined($data[$adr])) {
         $d = $data[$adr] & 0xFF;
      } else {
         $d = 0xFF;
      }
      $i = $crc >> 8;
      $crc <<= 8;
      $crc &= 0xFFFF;
      $crc |= $d;
      $crc ^= $crctable[$i];
   }
   return $crc;
}