Ikegami, Thanks for the response, its just what i was after =). I realize the short comings of the version I wrote, but had to give it a go for the mental exercise, and I guess you'd have to add binary trees to your storage mechanism so that you can do anything useful like you say rather that just being able to know if there is a fault in an entire line (not that my C compiler is any more useful at times). That can wait though I've got the doco of a whole new module to wade through. Thanks again, I appreciate your insight. Regards Paul