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What's the difference between Z-Wave Association and Scenes, when should I use them?
In Z-Wave systems, Associations and Scenes are used to enable one device to control other devices in the system. They do behave differently and there are reasons and times to use either.
An Association is made directly between one device and another - it can control one device or a small 'group' of other devices (typically 5 or 6 devices).
The advantage of using Association is that commands are sent directly from the 'controlling' device to the 'controlled' device(s), the commands are not sent via the central Z-Wave controller.
This method helps reduce unnecessary network traffic and is good for simple On/Off, non-conditional commands.
For instance, a Z-Wave motion sensor could be associated with several Z-Wave dimmers, when the motion sensor detects movement (tripped), it can immediately send a BASIC Set command (ON) to the associated dimmers - turning those lights on. Normally this type of sensor will also send an OFF command a set amount of time after it has last detected motion. This time would be set in the sensor's configuration settings.
Most devices have 1 or 2 'Association Groups' and send different types of commands to each group - check the device's user manual for details.
Associations are typically configured in the central controller. The 'controlling' device is 'associated' with the devices that it will control. More information is in APNT-19: Associating Devices in Z-Wave Controllers.
Scenes give you much greater control of your system. They can send different types of commands to multiple devices, they can use sensors as inputs, and can make conditional decisions.
This is easier to explain with a few examples.
Scenes are created in the central Z-Wave controller, you select the triggers, schedules and how the scene will control devices - most controllers have very good 'scene editors' to help you create your scenes.
In summary, Associations are easy to configure, but offer limited control of other devices. Scenes are more complex to create, however, they offer infinite control of devices in the system.