sink vs source 1

[montech]

Greetings From Colorado.
Can anyone tell me the if there is seriously an advantage to sourcing IO current vs sinking it?I have an old design here at the plant that sinks(pnp)and wouldnt be that hard to change the wiring to accomodate sinking(NPN).I heard PNP is faster but noisier.
Thanks for the input
Montech

 

[roydm]

Are you referring to Analog or Discrete?
PLC inputs that are referenced to ground are usually easier to deal with.
I see no reason to change from one method to the other if it's working.
Roy

 

[itsmoked]

There is no inherent superiority of one over the other.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[montech]

Thanks for the input fellas no pun intented.
Happy coding!
Montech

 

[controlsdude]

There is an advantage to one versus the other.

Most PLC manufacturers usually only produce dc input card where it is looking for 24vdc or sourcing outputs and inputs. If you look at lead times it is hard to get any quantities for sinking or npn type cards. Most PLC manufacturers have sourcing or PNP type cards on the shelf.

 

[dV8r]

It's like this, in today's "PC" world sinking is so negative and sourcing is so positive, why would anyone want to be 'different'?

 

[Skogsgurra]

Actually, there are at least two reasons to choose sourcing instead of sinking outputs.

The first one is that it is more fail-safe to have a signal that doesnt turn into a True signal when the wire is shorted to GND, which was almost universally the case with old sinking (NPN) I/O.

The second reason is that it has been a safety standard to use nothing but sourcing I/O for longer than I can remember. There is even a standard prescribing sourcing I/O in machine tools and some other equipment. I have forgotten that number, too. It is so taken for granted that no one remembers there is a standard any more.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[controlsdude]

i have only run into NPN type io only when this engineer in product developement was using components that came from Japan. I am not sure why Japan is hung up on NPN type IO. When I have run into Japanese engineering the thought put into safety is not up to European or North American standards.

 

[karaokeking]

Agree with all Actually all of the equip I work with from Japan has the +24 grounded. Try that for going back to the 'ol 6 volt positive ground, huh. And why does AB wire their cards backwards from all the rest? Check out the difference between convention sinking and sourcing and AB's sinking and sourcing (discrete) cards. Only in America:::
(I enjoyed the input statement by the way)

 

[montech]

Well I want to thank all for the help.I wil continue in the future to wire everything,especially from a saftey input Gunnar thank you,as sourcing(pnp)types.Good Stuff

 

[OldPfart]

I use PNP inputs (positive going high to turn on). I se NPN outputs (positive going low to turn on). I also use switch mode power supplies 240VAC/24VDC or 24VDC/24VDC for isolation. I do NOT ground the negative output from the switchmode or I would lose my isolation. Works fine. Never had a problem.

 

[mugsmugs]

I'm not sure there is much of a difference anymore, but I believe initially the difference was speed, as an NPN transistor was much faster then a PNP, so if you needed faster response time, the NPN transistor circuits were more desirable.

 

[OldPfart]

Cost was another - when transistors were expensive NPN was much cheaper than PNP - long time ago though.

 

[ScottyUK]

While it is certainly true that PNP transistors are slower than their NPN duals, in terms of PLC I/O they are both 'instantaneous'. Processor scan rate is several orders of magnitude slower than any effect the transistor could have, so the difference would be invisible. Cost or availability is a much more likely explanation.


----------------------------------

If we learn from our mistakes I'm getting a great education!