I am new to PLCs and I am using "All about ckts" as one of the sources to learn more about PLCs. I really like the explanation you have here. I have been trying to figure this out for a while and thought would just ask you guys if it's me or is it really an error.

Here's the link to the page -
http://www.allaboutcircuits.com/vol_4/chpt_6/6.html

Here's the text I am refering to -

When the normally-closed "Stop" pushbutton switch is unactuated (not pressed), the PLC's X2 input will be energized, thus "closing" the X2 "contact" inside the program. This allows the motor to be started when input X1 is energized, and allows it to continue to run when the "Start" pushbutton is no longer pressed. When the "Stop" pushbutton is actuated, input X2 will de-energize, thus "opening" the X2 "contact" inside the PLC program and shutting off the motor. So, we see there is no operational difference between this new design and the previous design.

However, if the input wiring on input X2 were to fail open, X2 input would de-energize in the same manner as when the "Stop" pushbutton is pressed. The result, then, for a wiring failure on the X2 input is that the motor will immediately shut off. This is a safer design than the one previously shown, where a "Stop" switch wiring failure would have resulted in an inability to turn off the motor.

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I really don't understand the first line where he says, "NORMALLY CLOSED STOP PUSH BUTTON" and the picture above this text shows otherwise. The picture shows a normally-open STOP pushbutton. (Please go to the link and find the picture right above this text). Is that an error? or Is he referring to something else. How would the PLC design be safe if both start and stp pushbutton are NORMALLY-OPEN.

Thanks.

 

Remember in a PLC a P.B. can be input in a normally open fashion, but can be inverted in the logic if desired.
But all the diagram shows is the difference between using a N.C. and N.O. and the status effect (on/off) in the normal state.

PLC logic is just a electronic representation of hard wired ladder.
The advantage is the contact can be used over and over and with either state.
http://static.schneider-electric.us/docs/Machine%20Control/0140CT9201.pd
Max.

 

Either the picture has been fixed since you posted this, or you're looking at the wrong diagram. The diagram above the text shows a NC stop pushbutton.

 

It shows an NC stop pushbutton in the picture, but it shows an NO switch in the ladder logic diagram.

 

It shows an NC stop pushbutton in the picture, but it shows an NO switch in the ladder logic diagram.

Right. As it should be.

 

Okay, then when would you use a NC switch in the ladder logic?

 

Okay, then when would you use a NC switch in the ladder logic?

If you used a NO stop pushbutton. But as the article says, this is not the optimal solution as it does not fail safe.

With the NC pushbutton, the NO contact in the program can be thought of like a "normally open, held closed" limit switch.

 

Thanks for the input. Looks like not action is required on my part.