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My best guess is that it's used to identify the type of power supply (65W vs 90W) that's connected to the system. Given the adapter that's described in this thread: http://forum.thinkpads.com/viewtopic.php?f=18&t=91992 , not connecting anything to that center pin might mean that the laptop will assume that the 65W supply is being used. I say that since to me that's the logical choice; you wouldn't want the system to think it's using a 90W supply and configure itself to use more power than what's available from the adapter.
My X60 systems report correctly that a 65W supply is connected (under the Battery tab of the ThinkVantage Power Manager). I don't seem to have a 90W supply around here but I'd imagine that if one were used, the Power Manager would "see" it. I've measured the center pin with respect to ground; the metal on the outside of the DC plug. There's no voltage present when the supply is plugged in but there is a resistance of 10Kohms when it's unplugged. Maybe if somebody has a 90W supply they can measure the resistance and report.
X22 X24 X31 X41 X41T X60 X60s X61 X61s X200 X200s X300 X301 Z60m Z61t Z61p 560 560Z 600 600E 600X T21 T22 T23 T41 T60p T410 T420 T520 W500 W520 R50 A21p A22p A31 A31p
NOTE: All links to PC-Doctor software hosted by me are dead. Files removed 8/28/12 by manufacturer's demand.
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