The middle pin on a magsafe power supply is used to send data two ways. It uses the 1-wire protocol and passes along (among other things), the serial number of the charger, the amperage of the charger, the power type (so the computer won't try and charge using an airplane adapter). Negotiation happens between the charger and computer for power and the LED color.
The middle pin on a magsafe power supply is used to send data two ways. It uses the 1-wire protocol and passes along (among other things), the serial number of the charger, the amperage of the charger, the power type (so the computer won't try and charge using an airplane adapter). Negotiation happens between the charger and computer for power and the LED color.