Group Objects

Group objects are the values your application exchanges over the KNX bus. The stack gives you typed C++ objects that map to the group addresses configured in ETS. This page is fully portable — nothing here depends on a specific MCU.

Typed object aliases

Every group object is a DeviceObject under the hood. You use one of the typed aliases, which carry their own storage:

Declaring objects

Declare each object as a member field, constructed with its objectId. The id is what ties the object to its group address in the ETS project.

class MyLogic : public KnxLogicBase {
  // ...
 private:
  BitObject       onOff_{Object_OnOff_Bit};                    // incoming on/off
  BitObject       onOffFeedback_{Object_OnOffFeedback_Bit};    // outgoing status
  ByteObject      ledState_{Object_LedState_Byte};             // incoming state
  ByteObject      ledStateFeedback_{Object_LedStateFeedback_Byte};
  BitObject       alive_{Object_AliveNotification_Bit};        // outgoing alive
};

Registering: Tables().Push

An object only participates in bus traffic once it is pushed into the database. Register every object in the constructor:

MyLogic::MyLogic(IKnxStack& stack) : KnxLogicBase(stack) {
  this->stack.Tables().Push(&onOff_);
  this->stack.Tables().Push(&onOffFeedback_);
  this->stack.Tables().Push(&ledState_);
  this->stack.Tables().Push(&ledStateFeedback_);
  this->stack.Tables().Push(&alive_);
}

See Database for capacity limits and lookups.

Accessors

Read and write an object's value through its typed accessors. Note that SetValueAsFloat / GetValueAsFloat work on the raw 16-bit word — encode the DPT 9.x float yourself.

onOffFeedback_.SetValueAsBit(true);
uint8_t state = ledState_.GetValueAsByte();

Reacting: onDifference

When a group write arrives from the bus, the stack calls your onDifference handler with a Difference. Route on difference.objectId, then read the new value from difference.newData (a KnxData):

void MyLogic::OnDifference(Difference& difference) {
  switch (difference.objectId) {
    case Object_OnOff_Bit: {
      bool on = difference.newData.GetDataBit();
      // apply on/off ...
      break;
    }
    case Object_LedState_Byte: {
      uint8_t state = difference.newData.GetDataByte();
      // apply state ...
      break;
    }
    default:
      break;   // an object this logic does not care about
  }
}

KnxData exposes GetDataBit(), GetDataByte(), and GetDataWord() (plus GetDataSpan() for raw bytes). difference.oldData holds the previous value if you need to compare.

Sending feedback

To publish a value onto the bus, set it on the object, then call SendTelegramGroupValueWrite:

onOffFeedback_.SetValueAsBit(on);
this->stack.SendTelegramGroupValueWrite(onOffFeedback_);

Group object flags

Each object's communication behaviour is controlled by config flags. The named constants live in GroupObjectFlag.h.