Thanks guys for all your suggestions.

Sorry to be secretive over this project guys, but I'm being mentored / tutored and most of the code so far (99%) has been supplied by that Friend and Mentor so I don't feel that it's appropriate for me to list it here. Hopefully once the festive holiday is over he will be able to assist me further. I'm hoping that once we've got the fundamentals tested and the project is completed we may be able to release the code to the forum community.

His code produces the variable that I have used in my output routine, which I've tested in the real world and now need to replicate 4 times.

I'll take the suggestions and search the forum for information on interrupts.

Many thanks