So I unhooked the raspberry pi, and then borrowed it's ground and 3.3v and ran them through a dip switch bank into my input pins and then out to a multimeter. I did not like the results. My DAC isn't just a little nonlinear, the LSB run 1.9v and the MSB runs 2.3, and any two pins 3.3. There's no way to make a number from that little difference. There's obviously something unstated about this design and it's pissing me off. I went into panic mode attempting to find someplace I could buy a parallel DAC (not Frys as far as I could tell) in town so I could get it without shipping time. Then I vaguely remembered buying a DAC breakout from Sparkfun, but when I looked in my kit there wasn't one. I was just about to give up and went digging through the project parts box for an op-amp I bought so I could try an example circuit for R-2R DACs that used one, when I spotted the breakout board. I had put it in at the start of the project in case my own DAC was crap and I desperately needed a replacement. It's a little more complicated to operate, but it's not like I have shortage of GPIOs