code: def _add_crc(self, frame):
          
          crc = struct.pack('<H', crc16(frame))
          return frame + crc
docs: Add CRC to modbus frame (for RTU mode)
  
          :param frame: modbus RTU frame
          :type frame: str (Python2) or class bytes (Python3)
          :returns: modbus RTU frame with CRC
          :rtype: str (Python2) or class bytes (Python3)