Quote Originally Posted by irving2008 View Post
. . . . . When you say a 3.5v swing, between what and what? 3.5v is a good logic one level...
Many thanks for sticking with this one. My 'scope is years out of cal' but I'd say the swing on the modified channel is roughly between +1.6v and +4.8v which suggests the ON resistance of the opto is the limitation.