code: def _SendStruct(self, fmt, *args):
          
          data = struct.pack(fmt, *args)
          data_len = len(data) + 1
          checksum = (data_len + sum(bytearray(data))) % 256
          out = struct.pack("B", data_len) + data + struct.pack("B", checksum)
          self.ser.write(out)
docs: Pack a struct (without length or checksum) and send it.