How does the MBP control the Magsafe power supply?

[WorryWort79]

I have repaired a Magsafe power cord at the Magsafe but the magsafe won't charge the MPB! I understand the center pin is the control pin but I can't understand how the MPB can control the Magsafe with only two wires going to the unit!
I have checked the power at pins 1 and 5 and see about +3.5 V as referenced to pins 2 and 4. I can see no connection between pin 3 and the Magsafe!
I hope someone on this Forum can explain the control function.
Thanks

 

[klamathvx]

This blog post has a pretty good explanation towards the end of how the charger and the computer handshake when plugged in.

 

[WorryWort79]

Thanks for the link klamathvx,
This is the answer I have been looking for!