var par = parent.document;//not parent.content.document;
var board = par.getElementById("board");
parent.document is the iframe's parent document.From this document, you can access to any object there.
Be careful for the file path, actually the src of iframe must be in the same server and is not open from remote, e.g, http://www.somesit.com is not right, this could cause access denied error. the path must like this src="/fileforler/filename.php"