What are C-Bus Scenes?

What are C-Bus Scenes?

  1. ashleigh

    A scene is simply a collection of Group / Level pairs. When a scene is activated, each Group in the scene is set to its stored Level.

    For example, a scene might be:

    • Kitchen lighting group at 100%
    • Dining lighting group at 70%
    • Hallway lighting group at 50%

    When this scene is activated, the groups will be set so that the Kitchen group is 100%, the dining group is 70% and the hallway group is 50%.

    Sometimes a scene is also known as a PRESET.

    The parts that make up the Scene are known as COMPONENTS.

    In the example above, the scene has 3 components.


    In theory, a scene can be stored anywhere that has enough storage to hold the components.

    A scene can even be split amongst several units, with some of the components stored in one place and some in another. In practice this does not happen very often.


    When a scene has been created and stored in a unit, it is normal for that unit to have a button allocated which will activate the scene.

    When the scene is stored locally in a unit, and one of the buttons of that unit activates the scene, then the process of Activating the scene is referred to as SETTING THE SCENE.

    The unit which stores the scene simply looks up the components, in order, and transmits each onto the bus as a Ramp to Level command.

    Thus, when the above example scene is SET, there will be 3 lighting ramp to level commands transmitted into the C-Bus network.


    Each scene has two magic numbers associated with it.

    These are the TRIGGER GROUP and the ACTION SELECTOR.

    These magic numbers are used to tell one (or more) units to activate a scene they contain, if the scene's associated trigger group and action selector match those which are sent over the bus.

    The process of remotely activating a scene is referred to as TRIGGERING the scene, and the command that sends the trigger group and action selector pair is referred to as a SCENE TRIGGER message.


    The choice of Trigger Group and Action Selector is quite important, because these magical numbers are also used to help work out when the Scene Indicators are ON or OFF.

    (A small aside about indicators and scenes)

    When a button has been set to activate a scene, we want the indicator on that button to be ON when the scene is active, and OFF when the scene is not active.

    Turning the indicator on is the easy part: whenever the scene is SET (locally) or TRIGGERED (remotely) the scene components are played out into the network and the indicator is turned on.

    Turning the indicator off is more difficult, because there are a few different conditions to be considered:

    • When any lighting GROUP changes level, and that group is a component of an active scene, then the scene is said to be BROKEN and the indicator should be turned off;
    • When another Scene is activated, and that other Scene is considered to be MUTUALLY EXCLUSIVE, then the indicator for the currently active scene should be turned off; and
    • When another Scene is activated, and that other Scene is UNRELATED to the currently active scene, then the indicator for the currently active scene should be left alone.

    And just to add a complication, this should all work when for zero or more external units acting as remote triggers, and also for the cases where a single scene has components stored in more than one unit.

    (back to the fun part)

    So... we use the Trigger Group and Action Selector to help us achieve all these results, as follows:

    • Everything that shares the same Trigger Group number is MUTUALLY EXCLUSIVE
    • If scenes do not share the same Trigger Group number they are UNRELATED

    Because there are 256 possible Trigger Groups, it means that in a single network there can be up to 256 UNRELATED scene collections, AND each scene collection can contain up to 256 MUTUALLY EXCLUSIVE scenes, and any combination in any mix of those can be used.


    The behaviour of units when a scene is set is:

    • The unit transmits the lighting groups and levels for the scene; and
    • If you pressed the button on that unit, it transmit the Trigger Group and Action Selector on the Control Trigger Application

    The effect of the transmission of the Trigger Group and Action Selector is twofold:

    1. If there are scene components stored in some other unit, then this other unit will receive the Trigger Group and Action Selector, and if they match it will replay its portion of the scene; and
    2. If we create a scene in a unit, with no components, then we have made a mechanism for remotely triggering a scene (that lives somewhere else).

    As described above, the converse case is where a unit receives a Trigger Group and Action Selector - in which case the scene is activated if both received numbers match those associated with a stored scene. Note that in this case though, the receiving unit does not transmit the Trigger Group and Action Selector, because there was not an associated button being pressed.


    The general conditions for controlling the indicators was described above. When any unit detects that a scene has become broken, that unit transmits a special message (known as an INDICATOR KILLER) into the network. This message includes the associated Trigger Control Group (but not the Action Selector because it is not needed).

    Any unit receiving an Indicator Killer just looks up the Trigger Control Group in its list of known Trigger Control Groups. If a match is found, then any associated scene indicators are turned off.

    This approach allows a flexible, scalable scene operation which always has the indicators showing the right state.


    Wired C-Bus Units
    When you program a standard wired C-Bus unit, you can set a single Trigger Group for that unit, and you can set the Action Selector for each scene.

    This means that wired C-Bus units can only operate in the MUTUALLY EXCLUSIVE mode.

    Wireless C-Bus Units
    The new Wireless C-Bus units have extended the concept, and allow any button to have any combination of Trigger Group and Action Selector assigned.

    This means that the wireless C-Bus units allow any mix of MUTUALLY EXCLUSIVE and UNRELATED scenes on the same unit.

    (The wireless C-Bus units are also genuine multi-application units, so you can have any mix of lighting buttons, scene buttons, and remote scene trigger buttons.)


    If you are using wired C-Bus units, it can be a bit messy. You need to change the unit to application $CA (The Control Trigger application). Then you have all buttons on the unit operating in that application, whether you like that or not.

    Next, the buttons need to be set up to the appropriate Trigger Group by assigning them all to a single block. One button can be set to ON, another to OFF, and two others can be set to RECALL values from memory.

    This rather hacky approach allows up to 4 keys to be used to remotely trigger up to 4 scenes, where the Action Selectors will be 0 (off), 255 (on), and two at the discretion of the installer.

    An alternative method for triggering remote Scenes is to set a key as a Scene key and just have no groups in that Scene. You can still specify an Action Selector for the empty Scene so pressing this key will just send the Action Selector out onto the network on the Trigger Group for that unit.

    For wireless C-Bus, just set a key to have a microfunction of TRIGGER SCENE, enter the Trigger Group and Action Selector, and its done. (And in fact all this does is to make an empty scene as described above).


    If you dig deep enough, you will see that some of the newer units (eg DLT, Wireless) allow additional operations on Scenes. These additional operations CANNOT be remotely operated over the network, but they include:

    • Turn all loads in a scene off
    • Ramp all loads (except those which are defined as off) in a scene up
    • Ramp all loads in a scene down
    • Apply a timer to a Scene

    Scenes are a simple idea that rapidly get complex when the subtleties are worked through. These subtleties include the need to get the indicators to go out under the right circumstances, and the need to remotely trigger a scene.

    By the time the implications of these are worked through we end up with MUTUALLY EXCLUSIVE and UNRELATED scenes, and the method of distinguishing between them.

    There is really only two important things to remember:

    1. The Trigger Group numbers and Action Selectors need to be chosen with a little bit of care; and
    2. When scenes operate with the same Trigger Group number, then only one of those scenes can be active at a time.
    max w likes this.