Bottom Line Up Front

Use the right serial adaptor; one that supports the prerequisite voltage in your project. Or in my cast, grab an LM311 and stick it up front.  

TL;DR

Please understand that this was a rapid prototype. I have spent a factor more documenting what I did than actually getting it to work. Inserting a voltage comparator in front of the ccTalk interface kept the adventure going forward versus buying then wait for a dual voltage USB serial adaptor. Conceptually, this is what I did:

And we can see on the oscilloscope that the 3.3V signal (an ascii “A”) is normalized to 5V:

Getting closer to actually sending ccTalk commands to my coin acceptor!