Has anyone worked with the HP HCMS-29XX series of alphanumeric displays?

I am trying to hook up three (cascaded) to a PIC, and am having some problems.
I have successfully driven two displays, but the third is causing me some
troubles.  (Nothing lights is one failure)

After playing around, it seems I dont understand the displays as well as
I thought.  When I am driving a single display, all is well unless I send
more data to the display than the dot registers will hold.  I was under the
impression that it clocked "off the end" if you will.  Is this not the case?

I am close to my wits end with this one.   Any help would be greatly
appreciated.

Jon Doran