This is for anyone contemplating using opto couplers or similar devices in
projects.

    "USE ONLY AS A LAST RESORT!"

Why?  They are SLOOOOOOOOOOW!

So before you design them into a project, go to SIEMENS web site
<http://www.siemens.com>, find the opto app notes section and get app notes
1, 2, 4 and 5 as a minimum. They are in .PDF format and although small,
ther is enough info there to put you on the right track.

Good luck with your projects,
Bill