The below link appears to have many F/X circuits/schematics etc...
Building a multiple effect unit is simple combining a group into a controller.
Regardless, as to viewing the effect as pure digital is subjective, generally somewhere in the path a D/A - A/D is occuring.
Eventide along with Lexicon may claim pure digital but practically any audio engineer will cater to some analog mix within the path; pure digital can sound very synthesized.
http://www.tonepad.com/projects.asp?projectType=fx