I have been trying to use my TI-84 calculator to do time lapse videos. I have tried a number of programs in the calculator, but none of them make the camera fire. I am using a Rebel XSi. Has anyone had any luck using an XSi? I would really, really appreciate any help. Thanks.

Check ebay for an "intervalometer." Hopefully yours can accept one. I'm a Nikon shooter and not up on Canon equipment.

How are you connecting the calculator to the camera? If you are trying to use the USB port on the camera I think it will be difficult. You'll need to program something on the calculator that can talk to the camera like the EOS Utility does. A more simple approach would be to wire the USB port on the calculator to the remote switch port on the camera such that sending a certain signal from the calc closes the switch and fires the camera.