DALI schematic: help to analize reception circuit and limited current supply
Let's start with the easy bit.

2. Limited current source:

The current limiter circuit is this one







simulate this circuit &ndash; Schematic created using CircuitLab

D2 stabilizes a constant voltage on the base of the transistor, so the voltage on the base is VB=VD1 then the voltage on the emitter is the voltage on the base minus the base-emitter diode-voltage (VBE) so VE=VB-VBE. Now VE is also the voltage across the resistor R1 so the current in the resistor must be IR1=(VD1-VBE)/R1. it is obvious ofcause from this equation that since VD1 AND VBE are both constant that the current in R1 (IR1) must be constant.

More coming up on the "1. ..reception mode" part thingie..

1. In reception mode:


  ..With such current value, the U2 optocoupler transistor is not saturated and therefore, DALI_IN is not close to 0 V.


Where do you get that idea?

If you look at the output side of U2 it is pulled up to 5v by a 10k resistor, if we calculate the current through the transistor in order to pull the output low we get that 5v/10k=0.5mA thats a very low current. Now we look at the datasheet of the TCLT1000 and we take a look at the graph of the collector-emitter saturation-voltage (y-axis) compared to the collector-current (x-axis). We see from this that if the current is below 1mA then the saturation voltage is below 0.2v what that means is that if the collector pulles 0.5 mA which is enough to pull the output completely low then the voltage across the output is going to be &lt;0.2v so in other words it is going to be pulled to GND.
Now we can estimate how much current that takes throught the led because we can look at the graphs of the CTR (current-transfer-ratio) compared to forward current in the led and from this we see that if the forward current in the diode is 1.1mA then the CTR is aprox. 50%, what that means is that 1.1mA throught the led = 0.55mA through the transistor. In other words the output is pulled low both in the case of 9.5v and 22.5v, but below 9.5v/1.1mA I would expect the output of the opto-coupler to start going high.