DC input question on the PLCS.net Tutorial

On the plcs.net tutorial, at the DC Inputs page...
http://www.plcs.net/chapters/dcin25.htm
... I'm trying to figure out the relationship between
the PNP and NPN graphics vs the graphic at the bottom
of the page.

Does this mean that the sensor is the same in either
case - except for how you connect it?

The pnp and npn graphics show an output transistor, yet
there is no transistor in the bottom graphic. Does this
mean that, functionally, it's the same as a transistor
switch, but that's not actually what it is?

Why are there two inputs on the bottom graphic?

Thanks for your help...

Hello, and welcome.
Generally, sensors are either PNP or NPN. This is stated on the sensor literature. Some sensors are available with a PNP AND an NPN output, though.

It looks like there are two outputs shown in the bottom graphic just for info, and demostrating that, in that case, the inputs can be configured for either PNP or NPN. Some PLC input cards are dedicated to one or the other.

pp