the MagSafe connector pins allow for the adapter to be inserted in either of two orientations. The first and second pins on each side of the tiny central pin have continuity with their mirror pins.
-The inner large pins are V+ (14.5 / 16.5 / 18.5 / 20 V DC). Measuring with no load will give 6.86 V DC for MagSafe and about 3 V DC for MagSafe 2; the full voltage is provided after a ~40 kOhm load is applied for one second.
-The outer large pins are ground.
-The tiny center pin is a data pin using the 1-Wire protocol. The computer uses this pin to change the LED’s color and retrieve the serial number and wattage of the power supply.
-Only two wires – power and ground – go to the charger unit. There is no data communication via the adapter sense pin with the charger unit itself.
-The maximum voltage supplied is as follows:
14.5 V DC for the 45 W units supplied with MacBook Air
16.5 V DC for the 60 W units supplied with MacBook and 13" MacBook Pro
18.5 V DC for the 85 W units supplied with 15" and 17" MacBook Pro
20 V DC for the 85 W units supplied with 15" MacBook Pro Retina
-The rectangular metal shroud surrounding the pins acts as shielding for the electrical pins and a ferrous attractor for the magnet in the laptop.