def read_data(self, variable_instance):
        """
        read values from the device
        """
        if self.inst is None:
            return
        if variable_instance.visavariable.device_property.upper() == 'PRESENT_VALUE':
            return self.parse_value(self.inst.query('?U6P0'))
        elif variable_instance.visavariable.device_property.upper() == 'PRESENT_VALUE_MANUAL_C_FREQ':
            freq = VariableProperty.objects.get_property(variable=variable_instance, name='VISA:FREQ')
            if freq is None:
                freq = 500
            return self.parse_value(self.inst.query('?MAM1SR9HT3ST2SM%dE'%freq))
        return None
read values from the device