https://www.pc-control.co.uk/Inductive.htm
http://engineeronadisk.com/V2/book_implementation/engineeronadisk-380.gif
3 & 4 wire DC: These amplified D.C. sensors contain an output amplifier. They are supplied as 3 wire with function N.O. or NC and as 4 wire with complementary outputs (NO + NC) in the types NPN and PNP. Standard version include protected against short circuit, protected against polarity and peaks created by the disconnection of inductive loads. They are compatible with P.L.C. Units.
http://www.fargocontrols.com/sensors/inductive_op.html
http://image.slidesharecdn.com/switchesandsensors-131118124337-phpapp01/95/switches-and-sensors-20-638.jpg?cb=1384778720