the old NPN/PNP question for a SLC500 1747-L40E

[Earl29]

Hi guys,
I have SLC500 1747-L40E brick with sinking inputs. If I understood Terry Woods NPN/PNP posting correctly (image #9) would'nt I use NPN sensors & proxes as input devices? What confuses me, and frankly what doesn't, is I'm used to seeing NPN as sinking and in conflict with Mr. Woods' posting.
Thanks

 

[elevmike]

Lets make it simple: If your inputs are Sinking, then your field devices connected to the inpust are to be Sourcing devices.

 

[Steve Etter]

Before I commit fully to my answer (which would be "no, you will need to use PNP sensors with sinking inputs" if I weren't such a coward), to what post of Terry's are you refering?

Steve

 

[Ken Moore]

Elevmike is correct.
Allen-Bradley sinking inputs require sourcing devices.

 

[rsdoran]

to what post of Terry's are you refering?

http://www.patchn.com/npnpnp.htm

The PLC BOOK also has a chapter on sensors which explains npn & pnp: http://www.patchn.com/plc_disc_sens.pdf

 

[Earl29]

I was referring to http://www.patchn.com/npnpnp.htm #9 of the npn-pnp examples.
Thanks
Earl29

 

[elevmike]

If I understood Terry Woods NPN/PNP posting correctly (image #9) would'nt I use NPN sensors & proxes as input devices? What confuses me, and frankly what doesn't, is I'm used to seeing NPN as sinking and in conflict with Mr. Woods' posting.
Thanks

Earl,

Scroll down to the bottom of the Patch.net link. You will see in the last example that NPN or PNP transistors could be used as Sinking or Sourcing devices depending on their oriantation in the device. Just because a particular device uses a NPN transistor output, it does not necessarly mean it's a Sinking or Sourcing Device.

Just keep in mind the following: If the input or output on the PLC is a Sinking I/O then the device connected to it will be a Sourcing Device; and visa versa.

 

[Rod]

Maybe this will help confuse you.

Rod (The CNC dude)

 

[Terry Woods]

My drawings reflected all of the possibilities. There was a time when the selection of available sensors was very small... in those days we made our own. The drawing is valid in terms of possibilities.

However, since sensor manufacturers appear to have settled on a particular configuration for sourcing (the PNP form... think Peein'-N-Peein' as in sourcing... think of that kid peein' on the Ford or Chevy Logo) and sinking (NPN... Not PeeiN), you are most likely (these days) to use a PNP (Peein'-N-Peein') Sourcing Sensor to feed an NPN (Not PeeiN) Sinking Input.