I designed my own serial connection protocol in Snap!
Nice project. I have a couple of suggestions.
In converting to binary, 
is log₂(num). Also, I'm kinda surprised it works without calling either FLOOR or CEILING to make it an integer. That would at least make it more self-documenting.
I didn't notice the bits flashing until after I read the code, and even then I had to be looking for it. I suggest
instead of 50.
It doesn't really affect the running of the program, but that FOREVER in the DRAW sprite bothers me. Maybe make the RECEIVED variable global instead of sprite-local and then change the FOREVER in the DRAW script to REPEAT UNTIL (NOT (RECEIVED)) or something?
I made those changes.
Thoughts, anyone?
Cool project, I made something similar but it models internet connections. Created it a while ago but never knew what to do with it