It's purely text based using the symbols "_" (underscore), "-" (hyphen), "/" (forward slash), "\" (backslash), "|" (pipe), etc.
I tend to use this quite a bit on this forum as it's easier than using Visio, but you have to set the font to a fixed width font such as "Courier New" otherwise the figures and timing diagrams are distorted by the proportional spaced fonts.
Copy the image in post #2 into a fixed width font (programmers) text editor and you'll see how it's done. You can use GVIM or any other text editor to produce stuff like that (plus a lot of time ;-))