PLC questions

[mallorz]

write the Boolean expression and draw the gate logic diagram and typical PLC ladder logic diagram for a control system wherein a fan is to run only when all of the following conditions are met:

*input A is off
*input B is On or input C is On, or both B and C are on
*inputs D and E are both On
*One or more of inputs F,G or H are On.

Draw the framework for a ladder diagram, then start from the output and work your way towards the inputs according to the logic.

*Note, the example below is random. The "inputs" are not to demonstrate your problem*

-                 +

| --?--?-- --( )--|

|                 |

|                 |             

|                 |

|                 |

 

[DwSoFt]

write the Boolean expression and draw the gate logic diagram and typical PLC ladder logic diagram for a control system wherein a fan is to run only when all of the following conditions are met:

*input A is off
*input B is On or input C is On, or both B and C are on
*inputs D and E are both On
*One or more of inputs F,G or H are On.

hope u all can help me.

i can help you, i can give you a diagram. but guess what, you wont learn as well. so im not gonna do it all for you, so it will not have labels. im gonna guid you through it

!                                                              !

!---!\!---!---! !---!---! !---! !---!---! !---!--------( )-----!

!         !         !               !         !                !

!         !---! !---!               !---! !---!                !

!                                   !         !                !

!                                   !---! !---!                !

A must be off, and B or C must be on and D and E must be on and E or F or G must be on, to turn output on. that is the boolean writing, and there is a diagram above. all you need to do is write the labels in. with the boolean here, it should be easy for you to figure out.

 

[sufi85]

Start small. Show all three methods of control for just the first condition:

input A is off

i need to ignore the Input A if i want to draw the ladder logic diagram?i mean that i have to start with input B till the input H?

what i understood is input A is a Load Not (LD NOT)

 

[DwSoFt]

You can't just ignore it, otherwise when it is on the otput could still come on.

So, mostr of them u want to be true, when they turn on. This is represented by a NO contact ! ! Now let's say we won't it to be true when its off. Well thenn we need the opposit instruction A NC contact !/! So when the input is true it will open the contact in the ladder diagram

In other words, if all your devices in the field come in as NO then NO on the ladder diagram can mean AND or OR, and inputs like. inputA there are either an AND NOT. Or an OR NOT. Which is boolean logic

This make sense to you?

Give u a hint, the first input on my drawing for you is input A. This allowes the logic to be true when A is off but prevents the logic from being true when A is on. And that's the AND NOT in boolean. Lol

 

[Lancie1]

I need to ignore the Input A if I want to draw the ladder logic diagram?

Remember that you are working with Boolean logic, where every device has two states - ON or OFF, 1 or 0, True or False. So if a switch is OFF, what state does it have and should you ignore that state? How is one state less valuable than the other state, and could it ever be safely ignored?

Because Input A is some type of two-state device (an external switch or contact) most likely at some point in time, someone will turn it ON. Your program must know what to do in that case (NOT turn on the fan).

 

[DwSoFt]

Thank you lancie for explaining it better than I could

 

[sufi85]

to all friends, thanks a lot. i very appreciated it. now i understood and i have to study hard right now.

 

[[email protected]]

Home work

I wish I knew you people would do my home work when I was in school!

 

[uptown47]

This is represented by a NO contact ! ! ... Well thenn we need the opposit instruction A NC contact !/!

Just in case you didn't know..

NO stands for Normally Open

NC stands for Normally Closed.

I'm sure you'd figured it out but thought I'd mention it as you'll see that written a lot as you learn ladder. Good luck.

;-)

 

[Lancie1]

I wish I knew you people would do my home work when I was in school!

Would you have been able to tell the wrong solutions posted here from the correct solutions, and explain how to solve the problem to your instructor? If not, you would not have been any better off.

 

[CCZ]

PLCs.net perhaps?

Hey everyone,

It looks like we are forgetting what website we are on. Perhaps the students can go to http://www.plcs.net/contents.shtml and read through the exercises there. That might do a better job of explaining the basics.

CCZ

 

[Lancie1]

Just in case you didn't know..

NO stands for Normally Open

NC stands for Normally Closed.

Yes, and the greatest confusion for beginners is not realizing that there are TWO sets of these NO and NC contacts - the first set in the field devices (for example a NC Stop Pushbutton), and the second set in the PLC instructions (for example a NO PLC contact that would represent the NC state of the external Stop pushbutton).

Until you get that straight (that a PLC NO always shows the state of an external input, and a PLC NC always shows the OPPOSITE state of an external input), there will be many errors in your program logic.

 

[uptown47]

Yes, and the greatest confusion for beginners is not realizing that there are TWO sets of these NO and NC contacts - the first set in the field devices (for example a NC Stop Pushbutton), and the second set in the PLC instructions (for example a NO PLC contact that would represent the NC state of the external Stop pushbutton).

Until you get that straight (that a PLC NO always shows the state of an external input, and a PLC NC always shows the OPPOSITE state of an external input), there will be many errors in your program logic.

Agreed. Especially when it comes to a normally closed stop button in a start/stop latch. And then having to explain that the button is wired normally closed...

;-)

 

[Lancie1]

Agreed. Especially when it comes to a normally closed stop button in a start/stop latch. And then having to explain that the button is wired normally closed...

I assume that you meant "when it comes to a normally OPEN stop button in a PLC program start/stop latch. And then having to explain that the actual button is wired normally closed..."

 

[uptown47]

I assume that you meant "when it comes to a normally OPEN stop button in a PLC program start/stop latch. And then having to explain that the actual button is wired normally closed..."

Yep, that's exactly what I meant... brain not in gear!