In this article on Instructable is explained how to turn a ti calculator into an intervalometer for a DLSR, by simply using its communication cable and writing a little script: Turn a TI Graphing Calculator into an Intervalometer and Create Time Lapse Videos.

The author tested it with a ti-83+ and a Canon Rebel, but ti-84, ti-89 and other cameras are also discussed in the comments. Other combinations can be found all over the web.