What advantages/disadvantages do NPN and PNP PLC digital input cards have in comparison with one another?? I am trying to decide between either type of card for an A-B Flex I/O application and the other engineers have their preferences but don't have any hard facts to support their case. Does anyone out there have any comments??
The ones that match your sensors. If you're doing a new system and can select both sensors and inputs from scratch, use PNP (positive logic). This prevents an accidental grounding of the input from reading a logic true (especially important in critical applications). If you do decide to use NPN, I'll expect you to rewire all the lights in your house so the switch is in the neutral leg.
Most users prefer to use PNP (sourcing) sensors into a NPN sinking input card. Reason being if wire falls off and touches ground it will short and open DC power supply. The NPN (sinking) sensor into PNP input card if wire falls off and touches ground it will not open DC power supply and could (depending on which wire) turn input on.
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Actually the ones that are strappable both ways are the handiest because no matter how you plan, there will always be something that's the wrong sense. For what we do, integrating existing machinery, I can confidently say that there is no discernable standard whatsoever and many machines use both. I have also heard quite plausible reasons for both, one of which is ignored whichever way you go. PNP and NPN are more applicable for outputs where it's understood you are talking about an open collector. Sinking and sourcing are better and if you want mass confusion simply add positive and negative logic.
It depends on the type of sensor you use. I have found that PNP type sensors are less confusing for the electrician on the floor. When checking the voltage of the input with respect to ground, an on condition is 24 volts. With an NPN sensor, the on condition is 0 volts. Somehow, this confuses the average electrician, who is used to 110 vac I/O. They say, the input LED is on and I got 0 volts--the card must be bad.
Some dc input cards are "i don't care" or can be wired two different ways to accomodate both types of sensors.(i.e. ab's 1771-iq16) These card's individual input channels are usually isolated from one another.
That's the best way to go if you're not sure of the sensors beforehand.