I am looking at a circuit that contains both a CNY17-3 optocoupler, and a 6N136 opto-coupled logic gate. The specific circuit is from the OpenDCC project (model railways).
I am rather confused as to the reason why there needs to be two different types. I understand that the 6N136 supports 1MBit/s, and the CNY17-3 is much slower (about 6us rise time when saturated).
Why not, for example, choose a faster optotransistor (say, Fairchild's 6M135M and use that for the data reception in this circuit?
I'm probably missing something :) I haven't dealt much with opto-isolation since I used a TLP2200 waaaay back at uni so any and all advice is appreciated....