I decided to make my own GPS for my Nikon D90. Nikon wants well over $200 for a GPS unit. The parts I used cost roughly $40 in total, much less expensive than Nikon’s solution. The D90 accessory plug outputs +5V and expects 5V TTL logic. I was able to output +3.3V logic from my Arduino and it worked, but the GPS unit’s threshold must have been lower and the D90 didn’t accept the data. Below are the details on how I completed the task. I have left out the tracing and testing (e.g. checking to make sure I didn’t connect two pins or ground anything out) phases of the project in this writeup, but needless to say you should test and check your modifications with a multimeter.