One more thing for folks trying to talk to digital sensors like this. I started to make a lot of progress when I worked out a way to actually look at what was going on over the digital lines. I connected a separate DAQ card reading at 10 Khz to the digital lines and read the voltages going out. This worked well for this application where everything was very slow (lines on for 2-3 ms). It let me see that lines weren't going low when I was expecting them to. If you have troubles with this sort of digital interface project, I highly recommend this.
If you're lucky enough to have one, a digital storage oscilloscope is a wonderful tool for exactly this troubleshooting (so I'm told). If you've got one, however, you probably know a lot more about troubleshooting such circuits than I do... But students might glance around the teaching lab to see if there's one sitting somewhere.
