docs: def get_json(self):
          """Create JSON data for slot.
  
          :returns: JSON data for slot as follows:
  
              {
                  "@SlotIdx":0,
                  "OnboardControllers":{
                      "OnboardController": [
                      ]
                  },
                  "AddOnCards":{
                      "AddOnCard": [
                      ]
                  }
              }
          """
          json = self.get_basic_json()
          if self.onboard_cards:
              json['OnboardControllers'] = {
                  'OnboardController':
                      [c.get_json() for c in self.onboard_cards.values()]
              }
          if self.addon_cards:
              json['AddOnCards'] = {
                  'AddOnCard': [c.get_json() for c in self.addon_cards.values()]
              }
          return json