DALI transmit circuit

[treez]

Hello,
We are getting lots of problems with our DALI system….its like on page 3 of the AN1465…
https://ww1.microchip.com/downloads/en/AppNotes/01465A.pdf
It seems that the bits being transmitted from the our micro (via the opto as you see on page 3) are not being received correctly. We didn’t write the software so we don’t even know what bits are being sent.
The problem with the circuit on page 3 is that the rail that drives Q2 base is actually collapsed by Q2 itself. Also, the opto base-collector capacitance means it’s a bit slow in terms of rise and fall times.

Therefore, we are going to put the isolation barrier further upstream and do DALI transmission as in the attached.
Do you know why Microchip did not suggest doing this in AN1465

 

[volker@muehlhaus]

Where is your TX isolation?

 

[treez]

Thanks, there is an isolation transformer (not shown on schem) producing the 3v3.

 

[volker@muehlhaus]

Have you checked the DALI standard documents if that is ok?

 

[treez]

Thanks, yes as long as the isolation is there its ok.
I certainly think, and i am sure you agree?...that the darlingotn arrangement shown is better than the circuit in pg3 of AN1465.