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Thermistors are just resistors, which are non-polar. They can be wired in either direction and will still measure the same resistance.
It looks like Printrbot made some changes to the firmware and do not support board 81 in the current version of the firware; at least it's not defined in the pins.h file. Here's a link to a recent change that might account for the removal of the definition: https://github.com/Printrbot/Marlin/comm...
You'll want to contact the Printrbot development team for more information.
The NTC3950 is a different thermistor from the ATC Semitec 104GT-2 and is not one of the predefined models available in Repetier, so you'll need to use a user-defined table and enter the specs manually from a datasheet.
This topic on the SeeMeCNC forum should have the answers you're looking for: http://forum.seemecnc.com/viewtopic.php?...