# Karnaugh loops not easy for me.

Discussion in 'Homework Help' started by Corey Esson, Sep 2, 2015.

1. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
I'm having a heck of a time at this. I think the answer is either A or B but leaning towards B. I've mapped this out, its two variables that require 4 squares. Just can't get it, please any and all help is appreciated.
Thanks in advance for any and all help.
Corey Esson

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2. ### WBahn Moderator

Mar 31, 2012
17,757
4,800
Since this is Homework Help and not Homework Done For You, you need to post YOUR best attempt to solve YOUR homework. That gives us a starting point from which to make observations and suggestions. With nothing to work from, about the best we can do is to tell you to Google "Karnaugh map".

3. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Hi WBahn, sorry but this is my best attempt. I've mapped it, and using another example of a two variable with different minterms I don't understand how that answer is achieved either. I just don't understand it and have Googled till I'm blue in the face. Reading it is tougher than having it explained on a black board as well.
I'll just pick B and see if I'm right tomorrow. Thanks
Corey

4. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Another thing that just came to me. I think that because (in my map) B cancels C-not, therefore, B would be the first Letter.
Then, AB makes D logic 1. Therefore letter B for BD.
Am I making any sense? Or am I still completely out in left field?
Thanks. Corey

5. ### WBahn Moderator

Mar 31, 2012
17,757
4,800
So... are we supposed to use a crystal ball to view this map that you are talking about? Perhaps remote viewing? Or if you leave it out on your front porch we can steal some time on an NRO satellite and snap a picture of your work?

How can we possibly tell you where you went wrong unless you show us what you did?

6. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Sorry, kind of stressed and forgot to show it. I'll take a pic here. I'm. Really trying my best. A simple "need a pic of map would do"
Map coming soon.

7. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
See if I might of mapped this right?

Corey

File size:
4.7 MB
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Feb 19, 2010
3,394
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9. ### WBahn Moderator

Mar 31, 2012
17,757
4,800
How about posting something a bit more reasonable than a 4.7 MB image when a 33 kB file is more than sufficient.

With four inputs, you have sixteen possible input combinations, so your K-map needs 16 boxes. This is normally done as follows:

Now try mapping each box to one of the expressions like you tried to do above.

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10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,394
497
So you have 4 inputs, A, B, C, D.

2^4=16
So your truth table will have 16 rows.

Of those 16 rows only four will have desired output.
A'BCD is same as 0111
ABC'D is same as 1101
ABCD is same as 1111
A'BC'D is same as 0101

This wiki has example of four input karnaugh map: https://en.wikipedia.org/wiki/Karnaugh_map

I hate to ask this, but... Can you even write the truth table using information that is given to you in the exercise?

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11. ### WBahn Moderator

Mar 31, 2012
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I kinda thought, "you need to post YOUR best attempt to solve YOUR homework," was sufficiently suggestive.

12. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Oh my gosh! Sorry to both of you! Didn't mean to, nor realize I sent such a huge file. Sorry about this.
Okay, so WBahn and shteii01, didn't have a clue I needed 16 boxes, wow! So now I'll try to work this out that way and go from here. Thank you for the map to fill in as well.

13. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,394
497
That is why I did the whole step by step setup.
Step 1.
# of inputs.

Step 2.
The size of the truth table.

Step 3.
Fill up the truth table.
(technically you don't need this part)

Step 4.
Draw up the Karnaugh Map and fill it up.

Step 5.
Get/derive the solution.

You can double check your work using Boolean Algebra rules.

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14. ### WBahn Moderator

Mar 31, 2012
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If you get a problem like this on an exam and don't understand how to work it using K-maps, remember that it is sufficient to find the correct answer by eliminating all but one of the choices.

You KNOW that C is incorrect because there are (at least) FOUR possible input combinations that will produce a HI output.

You can determine that D is incorrect as long as you can perform ANY reduction. Well, we can combine the 1st and 3rd terms to get BCD (then ORed with the 2nd and 4th), so that rules out D.

Now you are down to the two you had identified.

The first one, CA, can ONLY be HI if both A is HI and C is HI. Well, look at the very first term, A'BCD. As long as B, C, and D are HI, then the output is HI even A is LO. So that eliminates A from consideration.

That's enough to allow you to choose answer B. But let's look at B anyway. It requires that B and D both be HI. A simple look shows that each term requires that B and D both be HI. So that is necessary -- but it is not sufficient since for that to be the correct answer it also requires that A and C cannot influence the output. If we limit ourselves to when B and D are both HI, we see that each term covers one of the four possible combinations of A and C, so one of them will always be HI.

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15. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Hi shteii01, sort of. But with yours and WBahns help I'm mapping out in 16 squares. I hope I get it in the next while or so.
Thank you
Corey

16. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,394
497
You really only need 4 squares. The equation give in the problem tells you which squares will have 1 in them. The rest are zeros so you don't really care about them.

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17. ### WBahn Moderator

Mar 31, 2012
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Be a bit careful there. A zero is not the same as a don't care.

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18. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Hello again, still working this problem (thanks for helping me with the B answer) but I still need to understand this. Like I said above, without "blackboard" learning, reading is tougher for me and you (WBahn) and shteii01 are my blackboards of sorts. This REALLY helps me! I hope the next file/pic is not too big. In this next one as the example of the book it gives 8 variables and this is where I thought I needed only 4 squares and now I see that I do really only need 8 squares. Pic to follow.

File size:
3.4 MB
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19. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Dang! Still a big file! Sorry.

20. ### Corey Esson Thread Starter Member

Feb 8, 2015
30
1
Okay, I think I'm picking this up now. Now, with more practice I'll get sharper at it. Thank VERY MUCH WBahn and shteii01!!!!
Pic to follow, hope not too big a file.

Corey

File size:
603.6 KB
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