The Bell Example

Guys

I have a question regarding the BELL example in the tutorial. SW1 is normally open and SW2 is normally closed. Now when the supply is turned on SW1 will be normallyu closed and SW2 will be normally open then how will the Bell ever ring?

Jay

Please identify which chapter of the tutorial you're referring to. Use the chapter headings from the table of contents like 'Basic program example' or 'A level application'.

I'm not sure exactly which example you're having trouble with. I quickly checked a few of the chapters and I didn't run across any bell example with one switch normally closed and another normally open. That's not to say that there isn't such an example, just that my attention span is short.

The Bell Example Details

Hi

The example is mentioned in the Relay chaper and its modified in the chapter PLC registers. My question is related to the modified example in the chapter of PLC registers.

Jay

Nc Normally close..No..normally open

Please concern...and check
Nc= Normally close but if not normal..will open
No= Normally open but if not normal.. will close

platootod
Mech.Rit>non

The Q still remains???

Hi

I think i get it. Let me explain the scenario. When SW1 is pressed it becomes normally closed and SW2 is already closed so the bell rings and then when SW2 is pressed it becomes normally open and the bell stops. Am i right?

Jay

Your question is the classic question of understanding the relationship between the physical devices and the instructions in the PLC program.

Think of the NO and NC contact instructions in the PLC program as questions. In this case, we're talking about a switch wired to an input module. The NO contact instruction (--| |--) asks the question "Is there current flow at the input terminal"? If the answer to the question is yes, the NO contact instruction is TRUE. If the answer is no, the instruction is FALSE. The NC contact instruction
(--|/|--) asks the question "Is there no current flow at the input terminal"? If the answer to the question is yes, the NC contact instruction is TRUE. If the answer is no, the instruction is FALSE.

Most of use the terms 'normally open' and 'normally closed' to denote the 'rest' state of devices. We would not say something like 'when you press the button it becomes normally closed'. Instead, we would say "When you press the button wired normally open, the contacts close". Do you see the difference?

Let me come again

Steve

So according to the question theory, When SW1(NO) is pressed?

Is there a current flowing?
Yes
So, NO contact is TRUE

SW2(NC) asks at the same time

Is there a no current flowing?
No
So, NC contact is False

Then how will the bell ever ring?

I am sorry but this is a bit confusing.

Jay

Remember, both physical devices are wired normally open. That didn't change in the example. The only thing that changed was the program.

The program is asking two questions to decide whether or not to turn the output on.

The first question: "Is there current flow at the 0000 terminal?
The second question: "Is there no current flow at the 0001 terminal?

If the answer to both questions is yes, turn the output on.

If you leave both switches in their 'normal' state, the answer to the first question is no, the answer to the second question is yes, so the output remains off.

If you physically close the first switch, but leave the second switch in its 'normal' state, the answer to the first question is yes, the answer to the second question is yes, so the output gets turned on.

If you physically close the second switch, but leave the first switch in its 'normal' state, the answer to the first question is no, the answer to the second question is no, so the output remains off.

If you physically close both switches, the answer to the first question is yes, the answer to the second question is no, so the output remains off.

New Q???

Steve

Now i understood what it means. You said that both the switches are physically attached in NO and the program changes. How will you mention in the program that you want this SW NC and the other one as NO. I have done ladder programming but that was years back.

Jay

It's up to the control system designer to determine if a particular input is to be wired normally open or normally closed.

The two major factors in the decision are safety, and the behavior of the system under the most likely failure conditions.

Once that decision is made, you can make your program easier to understand by the name you assign to the signal.

Let's use an example of a sensor that detects when a box reaches the end of a conveyor. The sensor is wired to a PLC input. You could wire the sensor so that there is current flow at the PLC input terminal when the box is present or when the box is absent.

If you wire it so that there is current flow when the box is present, you should name the input 'Box is present'. Then when you look at the ladder logic and you see a NO contact instruction you should think "That instruction will be true when the box is present". When you see a NC contact instruction you should think "That instruction will be true when a box is not present.

If you wire it so that there is no current flow when the box is present, you should name the input 'Box is absent'.

Thank you

Steve

I have understood it completely. Thank you very much for your help. I am a beginner with PLC. I had worked with the Ladder diagrams years back with Messung PLC in India (I am sure you have not heard of this name). I want to get familiar with the current PLCs as they are many jobs in this field. I am a student. So i cant afford to spend to by an equipment. So i am looking for somebody to help me with this one. Let me know if you can help me.

Thanks again,

Jay

Well, ya know, ya take the time to build up a reply only to find good answers posted in the meantime...
So... maybe this will help.

Actually, jayshah11, the Register Chapter refers to the program example in the “Basic Program Example” (the previous chapter). It does not refer directly to the Relay Chapter.

Your primary problem is that you are trying to associate the --| |-- and --|/|-- symbols in the program example directly to the contacts in the hardware... DON’T DO THAT!



The figure on the left is supposed to represent real hardware. The figure on the left shows the hardware WITHOUT a PLC. Once a PLC is used, the figure on the left is only a conceptual expression of the operation provided by the PLC. The Ladder Code DOES NOT directly represent the figure on the left. The figure on the left represents how the code works!

Once a PLC is brought into the picture, the actual hardware configuration is quite different. The figure below shows the hardware WITH a PLC.



Now, in the first code example, the first --| |-- is simply asking if SW-1 is ON. The second --| |-- is simply asking if SW-2 is ON. If the answer to BOTH questions is YES (TRUE), then the Output will be energized. If the answer to either question is NO (FALSE), then the Output will NOT turn ON.

In the second code example, the --| |-- is simply asking if SW-1 is ON. The --|/|-- is simply asking if SW-2 is OFF. If the answer to BOTH questions is YES (TRUE), then the Output will be energized. If the answer to either question is NO (FALSE), then the Output will NOT turn ON.

Pay attention...

As far as the code is concerned, it doesn’t give a hoot whether either switch is "Normally Open" or "Normally Closed". All the code wants to know is... --| |-- “Is it ON?” or --|/|-- “Is it OFF?”

As far as the code is concerned, it doesn’t give a hoot whether either switch is "Normally Open" or "Normally Closed".

Click to expand...

Someone call?

No, there was no "hollering" involved...

beerchug

-Eric

(Quote
As far as the code is concerned, it doesn’t give a hoot whether either switch is "Normally Open" or "Normally Closed".


Terry is correct, the actual "normal" state of the switche(s) is not important to the end result of the operation. The code or Ladder Logic will instruct the PLC how to use the inputs or lack of inputs from those switches.


Roger