Compatibility of ballasts is guaranteed by the standard.
For other units, like sensors and controllers the compatibility ensurance is in process and standardization is being achieved.
Compatibility has always to be ensured when a system is being designed.
The maximum number of individual addresses available in the DALI standard is 64.
The maximum DALI supply current is stated as 250mA maximum.
Up to 64 individual ballasts can be connected in a DALI line without exceeding the system node or power supply current limits.
This, of course, does not allow for any DALI control devices to be connected to the system.
If control units using the DALI protocol are to be used in a system then the system size will be limited to either 64 nodes or 250mA total system current (whichever value is reached first).
When a DALI system is to be constructed containing both ballasts and control units it should be ensured that these limits are not exceeded.
If the system size exceeds the limitations there will be problems due to reduced signal integrity. Some devices may fail to communicate or respond to commands and the system operation will become unstable.
The maximum voltage drop on the DALI line may not exceed 2V, resulting in a maximum line length of 300m, between the furthest apart DALI components.
For the above mentioned reasons it is the task of the system planner to consider the power consumption of each component used and to plan the system in such a way that the system limitations are not exceeded.
In the DALI standard all values are specified at the control pins of the ballast.
For the full system the situation is slightly different. In general the control line voltage in a DALI system is normally 16 V (between 22,4 and 9,5 volts) when there is no communication (idle state). This voltage is supplied from an internal DALI power supply.
The digital signal becomes low when the voltage level in the DALI system becomes zero (-6,5V to 6,5V)
In a DALI system the maximum system current is limited to 250mA, which is supplied from additional power supplies. This is to keep the energy consumption low and to ensure digital signal integrity throughout the system.
The smallest possible system, one light fitting and a controller consunes a line current of maximum 2 mA for the digital dimmable ballast and the current required by the control equipment. Thus the power consumed is small.
Since in practice the impedance of different DALI units are not identical, the selection of the correct system power supply not necessarily straight forward.
Good practice is to allow sufficient margins for the supply current. This will guarantee reliable system functionality under different conditions and also allow the flexibility for possible system expansion at a later date. On the other hand, selecting an oversized power supply may cause extra distortion control signals. Power supplies, which have so called dynamic current limitation, will be suitable for use in almost every size of system.
The DALI power supply must limit the supply current to max. 250 mA under all circumstances.
In practical installations the current is good to limit to a lower level in order to maintain the flexibility of changing the layout and increasing the system at later stages.
If the limit is exceeded instability and starting problems may occure in the system. Since the DALI signal varies between 0V and 16V the polarity is important to maintain also with power supply. There is no limitation to having several power supplies on the same DALI control line as long as the current limit is not exceeded and the supply polarity is taken into account.