Thanks guys! Answers were truly helpful!
I have another situation - isolating power electronic circuit from control electronic (PIC18F). I have 4 optocouplers 6N136, 6N137, 6N138, and 6N139 as choise. Little confusing fact that I'm not completely informed about is output type - TTL/TTL, TTL/CMOS,... TC427 input is TTL/CMOS compatible, and i'm littlebit unshure about connecting TTL/TTL optocoupler output with TTL/CMOS TC427 input. Is that kind of connection correct? Or it has to be TTL/CMOS compatible output connected with TTL/CMOS compatible input? :-?
Thanks in advance!