What are the differences between DMX and DALI – and does it matter?

This question has been answered by Bas Hoksbergen, architectural market manager, Pharos Architectural Controls.

DMX has its roots in theatrical and entertainment lighting, but is increasingly being used in architectural settings. It is a streaming protocol, with each stream – called a ‘universe’ – carrying up to 512 ‘channels’ of data. All fixtures receive data for all channels, which is sent by the controller at a specific refresh rate (such as 33 times per second).

The protocol does not assign any specific meaning to the channels, so the controller must be aware of what is expected by the fixtures. Each fixture listens for a specific number of channels (such as three for an RGB fixture), starting at a particular channel number (the fixture’s ‘address’). The number of fixtures supported per universe therefore depends on the channel footprint of the fixtures in use.

DMX assumes a centralised control system, which receives input from any sensors or control interfaces and sends data to all fixtures. DMX is transmitted over shielded twisted pair cable, with all fixtures connected in series. DMX over Ethernet protocols are also available, for transmission of many universes over a single cable.

DALI is a command-based protocol that specifies a set of messages that controllers can send to fixtures. Instructions include scene recall, setting a fade time and setting a level. DALI has free typology, it can be used with any standard mains cable, it is polarity independent and can be installed in a mains channel. DALI can support up to 64 ballasts per bus, 16 groups and 16 scenes per ballast, and 16 fade times that range between 0-90s. One or more control elements (such as sensors or user interfaces) can be present on the DALI bus to send DALI instructions. As such, DALI does not require a centralised control system.

Is this important?

Yes – DMX cannot be connected directly to DALI or vice versa. However, there are ways to use both protocols in the same installation (see How can DMX and DALI luminaires be combined in a single system? for more information on how to achieve this).

Each protocol has its strengths and weaknesses, and the exact same results cannot be achieved with both. The following points may help you in your decision.

Synchronised changes. The streaming nature of DMX means level changes are always synchronised. With DALI, changes will only be synchronised if they result from the same instruction, for example the recall of previously programmed scenes. However, because of the 16 scene limitation in DALI, many control systems rely on sending individual set-level commands, which will not be synchronised and can create a staggered ‘popcorn’ effect.

Dynamic effects. DMX allows the colour and intensity of a luminaire to be changed every frame, for example 33 times per second. This means that if your controller can create dynamic effects, such as a wave, fades or subtle sparkles, they will be displayed exactly as intended. The command-based nature of DALI can be very limiting for dynamic effects. You might need to settle for low refresh rates, staggered changes, or accept less control freedom, such as broadcasting to all fixtures.

Fading control. Fades in DALI are defined within the fixtures using steps, up to a maximum of 90 seconds. Not all DALI fixtures succeed in breaking up the required fade time into a smooth transition. In DMX, fades are determined by the controller, with advanced DMX control systems allowing fades ranging from zero seconds to 24 hours, and supporting 16-bit control for smoother fades (65,535 intensity steps, rather than 255)

Number of scenes. DALI fixtures support a maximum of 16 scenes. An advanced DALI system can effectively increase this number by remembering extra scenes and sending those to fixtures individually when required, but this results in unsynchronised changes. DMX scenes are stored in the controller, with advanced systems supporting hundreds of scenes.

Emergency lighting. The DALI protocol, unlike DMX, includes specific emergency lighting functions, including testing and ‘level on signal loss’ features.

Daylight harvesting. Several DALI manufacturers have created systems to fulfil this requirement using sensors connected directly to the DALI bus. Similar results can be achieved with DMX using a controller capable of receiving input from a sensor via a separate cable and processing of this PID loop.

Luminaire feedback. The DALI protocol is bidirectional and includes messages like a lamp or ballast error. Some DMX products support RDM (remote device management) commands for reporting status back to the controller on the DMX cable. In both cases, it is important to verify that both the fixtures and control system support the desired feedback messages.

System integration, user interfaces and wall stations. DALI offers the ability to connect control devices to the DALI bus and control fixtures directly. This is useful where the desired control logic is relatively simple. DMX/RDM does not typically support additional control devices on the DMX bus, but many controllers can receive input over a separate connection. Control systems are available for both protocols that can offer sophisticated control logic and system integration via a centralised controller.

Ease of installation. DALI is very convenient for installers because all devices and control elements can sit on the same bus, with free typology. DALI is also mains tolerant, and can be installed in the same conduits as the main cabling. A DMX installation does not need to be difficult, but it does require a bit more knowledge, care, and planning, especially if it includes sensors and user interfaces via separate cabling.

There are many aspects where the quality, flexibility and power of the control system are more important than the choice between DMX and DALI. Both protocols can support installations up to many thousands of fixtures. Tuneable white, RGB and other colour fixtures can be supported by both protocols, providing both controller and fixtures support the same modes. Similarly, many control systems can support numerous independent zones of control.

Conclusion

DALI is great due to its installer friendliness, and certain features (daylight harvesting, emergency lighting, multiple sensors) are easier to execute using DALI. When using dynamic effects, or if synchronised playback or specific fade times are required, then DMX will be a better choice for your project. Most importantly, select a control system that matches the features you need. A good controller or control system might natively support both DALI and DMX to utilise the advantages of both systems.

For more information on Pharos Controls, please visit pharoscontrols.com

 

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