3 Contact Laptop Power Supply
The protocol is 1-wire; it's used to read a DS2501 write-once memory chip:
http://www.laptop-junction.com/toast/content/inside-dell-ac-power-adapter-mystery-revealed

I have no idea what data is in the chip, but you can probably read the data out of a working power supply and see if you can decipher it.

The data might contain information about the max current the supply can output.