I would like to point out that there is two methods of interfacing different systems, either make all commons equal to earth ground or attempt to isolate them all.
I favour the first method and have used this extensively.
I only use opto devices to transition between one supply or system and another.
Keep in mind that if you have the usual desk top or tower PC, the PC power supply common is at earth ground potential, as well as any mains driven devices such as 120v/240vac spindles etc, so in effect you do not have isolation anyway between these two, however much isolation you attempt.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.