docs: def sendcommand(self, command, datalength=0, data=None):
          '''send a packet in the vendapin format'''
          packet = chr(Vendapin.STX) + chr(Vendapin.ADD) + chr(command) + chr(datalength)
          if datalength > 0:
              packet += chr(data)
          packet += chr(Vendapin.ETX)
          sendpacket = packet + chr(self._checksum(packet))
          self._printpacket(sendpacket)
          self.serial.write(sendpacket)