"Learn PLCs" Question

"A Level Application" in the interactive "Learn PLCS" states that it is using normally closed sensors. Why do the diagrams indicate normally open sensors?

Funny you should ask... We just finished discussing this question.

See THIS LINK <--- click me!

beerchug

-Eric

The term 'sensor' refers to the physical device doing the sensing. There is nothing in the picture that indicates the type of sensor. There is only the text that states that the sensors are both wired as normally closed.

You are confusing the ladder diagram NO and NC contacts with the wiring of the physical sensors. Think of a NO contact in a ladder diagram as asking the question "Is this signal ON?". Think of a NC contact in a ladder diagram as asking the question "Is this signal OFF?". In both cases, when the answer to the question is 'yes', the NO or NC element in the ladder diagram is TRUE.

Sensors

I posted this before reading Eric's link otherwise I would have kept shut. I guess each of us will find our way to understand ---]/[--- and ---] [---.

For those that think we should not be comparing these logic instructions to real world relays/contacts, I see your point and had not thought of it that way and it may be less confusing but now I find myself comparing the real world relay/contacts to the logic instructions. (i.e. relay coil=Bit, -] [- / -]/[- =Bit statements.)

I'M SOOOOOOOOOOO CONFUSED!!!!!!!!!!!!!!!!
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I view a set of contacts as a statement being made by the relay coil and I ask is the relay ( ---] [--- energized) or ( ---]/[--- energized not),and you are always looking for the TRUTH.

To throw another hat into the ring, you have sensors that have "LIGHT/DARK" settings. This combined with N.O./ N.C. logic and N.O./ N.C. physically can really get confusing.
All schematics are drawn in the de-energized state.

Roger

Its best to follow Eric's link, it may alleviate some of the confusion.