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https://www.pc-control.co.uk/Inductive.htm
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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.
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http://www.fargocontrols.com/sensors/inductive_op.html
http://image.slidesharecdn.com/switchesandsensors-131118124337-phpapp01/95/switches-and-sensors-20-638.jpg?cb=1384778720