Didn't do any of your own searching, did you?

However, as I've just done this very search myself, and others might find them of interest/use, try these:

http://www.xjtag.com/documents.htm

http://www.engr.udayton.edu/faculty/j...jtag1.html

http://www.inaccessnetworks.com/projects/...intro.html

http://www-ee.eng.hawaii.edu/~msmith/A...CH14.2.htm