def connect(self):
        """
        establish a connection to the Instrument
        """
        if not driver_ok:
            logger.error("Visa driver NOT ok")
            return False
        visa_backend = '@py'  # use PyVISA-py as backend
        if hasattr(settings, 'VISA_BACKEND'):
            visa_backend = settings.VISA_BACKEND

        try:
            self.rm = visa.ResourceManager(visa_backend)
        except:
            logger.error("Visa ResourceManager cannot load resources : %s" %self)
            return False
        try:
            resource_prefix = self._device.visadevice.resource_name.split('::')[0]
            extras = {}
            if hasattr(settings, 'VISA_DEVICE_SETTINGS'):
                if resource_prefix in settings.VISA_DEVICE_SETTINGS:
                    extras = settings.VISA_DEVICE_SETTINGS[resource_prefix]
            logger.debug('VISA_DEVICE_SETTINGS for %s: %r'%(resource_prefix,extras))
            self.inst = self.rm.open_resource(self._device.visadevice.resource_name, **extras)
        except:
            logger.error("Visa ResourceManager cannot open resource : %s" %self._device.visadevice.resource_name)
            return False
        logger.debug('connected visa device')
        return True
establish a connection to the Instrument