I have 2 9V power supplies with barrel plugs one with the center post positive the other with the center post Neg. I know that usualy pos is the center post but either i missed it on the schematic or it was unclear to me, either way. I am unsure which supply to use in the off chance it is different. if someone could point out the answer or point me at the answer i would be greatful.
The arduino external power connector uses positive center.
However if you were to plug in a negative center connector there would be no damage as the arduino board has a series polarity protection diode, so no damage, it just wouldn't work.
Thanks i figured that was the case i just didn't feel comfortable trial and erroring it.