This is a private utility method.
        This method sends a sysex command to Firmata.

        :param sysex_command: sysex command

        :param sysex_data: data for command

        :returns : No return value.

async def _send_sysex(self, sysex_command, sysex_data=None):
        """
        This is a private utility method.
        This method sends a sysex command to Firmata.

        :param sysex_command: sysex command

        :param sysex_data: data for command

        :returns : No return value.
        """
        if not sysex_data:
            sysex_data = []

        # convert the message command and data to characters
        sysex_message = chr(PrivateConstants.START_SYSEX)
        sysex_message += chr(sysex_command)
        if len(sysex_data):
            for d in sysex_data:
                sysex_message += chr(d)
        sysex_message += chr(PrivateConstants.END_SYSEX)

        for data in sysex_message:
            await self.write(data)