Ajax Hidden DT with scroller. How to init the length of the ajax post?

jean

I have multiple DT using Scroller extension in a page with multiple jquery tab, DT being in different tabs.
I also use cookie to keep state of the open tab.
what function do I need to call to recompute the length send on an AJAX call when a hidden tab is made active? I already call draw() but the length is sent as NaN which of course doesn't work.
What function do I need to call to recompute those value when the tab is made active or visible?

Thanks