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An introduction to DALI

The Digital Addressable Lighting Interface (DALI) is a standard that provides a single standard interface methodology for the digital control of lighting systems.

Figure 1: Typical analogue dimmable ballast¹

Figure 2: Manchester Coding

This standard allows for the seamless integration of dimmable ballasts, LED drivers, relays and control panels from various manufacturers into one extremely versatile control system. DALI was established as a successor to current analogue dimmable systems and as an open standard alternative to the Digital System Interface.

Analogue dimming
The dimmable electronic ballasts of existing analogue dimming systems typically consist of an Application Specific Integrated Circuit (ASIC) to take control of the dimming functionality along with a number of other essential blocks (ref IR). Firstly, an EMI filtering block is required to prevent the noise generated by the ballast from being returned to the mains. The mains voltage is then rectified and power factor corrected on its way to the ballast output driving stage.

The required dimming level of the lamp is set by the application of an external analogue control voltage which is typically in the range of 1 to 10 volts DC where 1 volt represents the lowest possible intensity of the lamp and 10 volts represents the highest. This voltage must then be galvanically isolated from the potentially dangerous voltages within the ballast and serves as a reference to the ASIC to set the required current level in the lamp.

Digital Signal Interface (DSI)
The DSI is a proprietary standard for the digital dimming of lighting equipment and is a precursor of DALI. Although it uses a similar system of data transmission to DALI, DSI is a much more limited system that uses a transmission wire for each device in the system to transmit a byte which communicates the required lighting level to the device. A big disadvantage of this type of system over a DALI installation is that since no assignable addresses exist, any changes to the behaviour of the system will necessitate physical wiring changes. Also, since DSI uses one way communication, it is incapable of providing any of the monitoring functionality that is available with DALI.

DALI architecture
A DALI network consists of one DALI controller, one DALI power supply unit, and one or more DALI lighting devices. DALI signals are transmitted over the DALI network by means of a two wire, asynchronous, half-duplex, differential bus. The bus is supplied with a 16 volts potential from the single power supply in the system. The bus is manipulated by the individual DALI components through the application and removal of short circuits across the bus voltage.

Data in DALI is transmitted using a biphase form of encoding known as Manchester Coding.
This is a popular form of digital coding in widespread use due to the many advantages it brings over simpler non return to zero schemes (NRZ). NRZ is a very basic scheme of coding in which the transmitted message structure is an analogue of the input data signal and therefore the DC content of the transmission along with the regularity of state transitions within the data, are highly dependent upon the actual data to be transmitted. For example, a long string of sequential logical ones in the input data might cause the DC content of the message to increase significantly but more critically might cause the receiver to lose synchronisation. For this reason, NRZ coding schemes are inherently reliant on a combination of low data rates, short transmission lines, extensive error correction and separate clock signals. Biphase schemes such as Manchester Coding overcome all of these shortcomings by enforcing symmetry and state changes in the transmission signal, regardless of the shape of the input data signal.

The coding scheme dictates that a state transition must always occur in the midpoint of the bit being transmitted. Logical zeroes are transmitted using a high to low transition in the midpoint and logical ones use a low to high transition. This ensures a signal transition at least once per data bit ensuring simple clock recovery for any data pattern.

DALI utilises a differential form of this coding scheme, which brings some further advantages. The differential nature of the transmission in combination with the requirement for diode bridges in the interface circuitry of DALI devices ensures that the polarity of the bus connection to a DALI device becomes irrelevant. The high noise immunity of differential coding also allows the DALI control bus to be wired in very close proximity to or even within the same sheath as high voltage cable including mains cabling. The topology for interconnection of devices within a DALI network is not dictated by the specification as the standard is flexible enough to accommodate parallel connections, serial connections or even a combination of both, affording flexibility to the system designer.

DALI groups and scenes
Each device in a DALI network is assigned a unique address in the range 0 to 63. This enforces a limit of 64 individual devices within any DALI subsystem. However, more complex systems can easily be implemented through the interconnection of multiple DALI subsystems using gateways. These addresses can be changed or grouped together at any time simply through a setting change on a DALI control panel. This allows lighting scenes to be created and assigned to groups of lighting devices. Examples of these scenes might be, for example, to dim all lighting in a room to 10% intensity at the touch of a button or to immediately illuminate all lighting assigned as being emergency lighting upon detection of an alarm. The possibilities for power savings are tremendous simply through automatic dimming of non-essential lighting outside of peak hours or even automatic reduction of lighting intensity as the ambient light level permits. These groups and scenes can then easily be adjusted to align with future building infrastructure changes.

DALI dimming
DALI is designed to be used with all types of lighting technologies of various efficiencies which must all appear to respond uniformly. For this reason, the dimming response of DALI devices is tightly defined. DALI provides for 256 levels of dimming between the maximum and minimum available intensities from any individual lighting device. The levels are distributed on a logarithmic dimming curve. This results in larger absolute intensity steps at higher intensities and finer adjustments at lower intensities in an attempt to produce a dimming range that appears linear to the human eye.

DALI command structure
As DALI is based on two-way communication, two different data frame structures exist; forward frames (from the master to any slave device) and backward frames (the response from any slave device to the master) each of which have a slightly different structure. Forward frames consist of one start bit, one address byte, one data byte and two stop bits. Backward frames are similar in structure but have no address byte as all backward frames are destined for the master device.

The address byte can indicate either a broadcast message, a group specifier or an individual device address. Broadcast messages are processed by all slave devices in the system and group messages are processed by all devices that have already been assigned to the specified group.

The data byte contains the command to be carried out by the addressed device(s). The DALI standard contains a pre-defined set of available commands which can be loosely grouped together by type. Different types of commands and examples of each follow.²

(i) Arc power control commands
a. Direct Arc Power Control: Set the arc power level directly with a fade
b. Off: Immediately extinguish the lamp without fading
c. Up or Down: Dim up or down the lamp intensity for a period of 200ms
d. Step Up or Step Down: Adjust the lamp intensity up or down by one step
e. Recall Max Level: Set the intensity to maximum without fading
f. Recall Min Level: Set the intensity to minimum without fading
g. Go To Scene: Set the intensity to the level specified in the selected scene

(ii) Configuration commands
a. Reset: Reset all values in persistent memory to their default values. Devices receiving this command are permitted to be non-responsive for a period of 300ms after reception of the command
b. Store Actual Level in the DTR: Store the current arc power level in the Data Transfer Register

(iii) Arc Power Parameter Settings
a. Store the DTR as Max Level: Set the value currently in the data transfer register as the new maximum level
b. Store the DTR as Min Level: Set the value currently in the data transfer register as the new minimum level
c. Store the DTR as System Failure Level: Set the value currently in the data transfer register as the new system failure level
d. Store the DTR as Power On Level: Set the value currently in the data transfer register as the new power on level
e. Store the DTR as Fade Time: Set the fade time using the value currently in the data transfer register
f. Store the DTR as Scene X: Set the new level of the scene X to the value currently in the data transfer register

(iv) System parameter settings
a. Remove from scene: Remove the specified device from the specified scene
b. Add to group: Add the specified device to the specified group
c. Remove from group: Remove the specified device from the specified group

(v) Query commands
a. Query Device: Ask if there is a device with the specified address that is available to respond. Response shall be ‘yes’ or ‘no’
b. Query Lamp Failure: Ask if there is a lamp problem at the specified address. Response shall be ‘yes’ or ‘no’
c. Query Lamp Power On: Ask if there is a lamp lighting at the specified address. Response shall be ‘yes’ or ‘no’
d. Query Version Number: Ask for the version number of the IEC standard document met by the hardware and software. Response shall be the version number as an 8 bit number
e. Query Physical Minimum Level: Ask for the version number of the IEC standard document met by the hardware and software. Response shall be the version number as an 8 bit number

Although DALI has not yet found its way into widespread use, it is slowly emerging as an excellent choice for lighting management due to the many advantages it brings over current dimming solutions. In times of ever increasing emphasis on energy efficiency, DALI offers huge potential for global energy savings through efficient management of resources. Another major advantage is the flexibility offered by the interoperability of DALI devices from all manufacturers in combination with the on-the-fly configurability of DALI networks. This offers peace of mind to the infrastructure designer that once adopted, DALI will offer future upgradability and access to a wide selection of replacement devices. Installation is also simplified with a single run of five-core cable capable of supplying power and DALI signals for the network with no restrictions placed on interconnection topology.

References
1 Analogue and Digital Fluorescent Lighting Dimming Systems Thomas J Ribarich and Cecilia Contenti, International Rectifier
2 IEC 60929 Annex E Control interface for controllable ballasts (2003)

Phelim Bradley, Design Engineer, Excelsys Technologies

Figure 3: Typical DALI system in parallel arrangement

Figure 4: DALI logarithmic dimming levels


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