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)

def _add_crc(self, frame):
        """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)
        """
        crc = struct.pack('<H', crc16(frame))
        return frame + crc