Sergegsx:
thanks dc42, so I dont need both?
Is my description of the function of each diode correct? or am I missing something?
Your description is correct, and you don't need both diodes.
btw you can also get opto isolators that have two back-to-back LEDs on the input side. If you used on of those, then it wouldn't matter which way round you connected the 80V input, because it would work either way. Here's an example: http://www.farnell.com/datasheets/54263.pdf.