I want to write my own software code for your ED moving message signs (i.e. ED100-2411-N1-SR2). Where can I find the protocol?

You can download the protocol .pdf (HC11 Scroller Protocol) on the "ED Products" section of the "Downloads" section of this Support Site.

http://edisupport.helpserve.com/index.php?_m=downloads&_a=viewdownload&downloaditemid=112&nav=0,7