code: def read_data(self, variable_instance):
          
          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
docs: read values from the device