I don't recall ever seeing anyone post diagrams to this forum using ASCII art. When I looked for appropriate options in the Visual and HTML reply windows, I couldn't find anything that would preserve the formatting. 

ASCII art requires that multiple spaces be maintained, and that the font remains at fixed width. However, this forum's software reduces multiple spaces to a single space and converts fixed width fonts to proportional. The way that I get around this problem is to switch to the HTML tab, paste the ASCII diagram (Ctrl-V), and enclose the diagram within the HTML "pre" tag, as follows.

<pre>diagram</pre>

One potential snag occurs when the art includes special HTML characters such as "&<>. These need to be substituted by their HTML name. 

http://www.ascii.cl/htmlcodes.htm

For example, each occurrence of < should be replaced with &lt; (lt = less than). The Search & Replace function of a suitable word processor (eg Microsoft WordPad) should be able to do this.

I use Microsoft's NotePad for generating ASCII art. It uses a fixed width font.

Another useful tool is AACircuit. It generates ASCII circuit diagrams that can be copied to the clipboard and then pasted into your browser.

http://www.tech-chat.de