How would you do this?

[MikeWinston]

If the grounds are not isolated, can someone explain to me why an optocoupler is more reliable?

I agree Charles. Isolated grounds would be best. But if thats not practical, than keeping circuit neighborhoods isolated would be next best. At least thats the way I understand it and have seen in practice.

Meaning of neighborhood. If you already know this than I apologize. One neighborhood would be the analog section. Another would be the digital. All neighborhoods should have their own ground that leads back to the point where power ground comes into the circuit. This point will have the lowest impeadance to power/earth ground. In other words, all neighborhoods share their ground at only one point. Doing this keeps noise created in one neighborhood from effecting another.

This does not work for all applications but using an optocoupler will keep this idea in place.

Also this idea relies on the use of the right capacitors in the right place. I still don't fully understand how caps fixed certain high speed trigger problems i had.
...maybe it was inductance or quantum interactions ... i don't know ???