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Karnaugh Maps

4.2.4 simplifying -
Alternative approach





If you followed closely the algebraic stuff you would probably spot that the first two terms simplify to not A and the last two to B. Since we can repeat the middle term, we get the solution not A + B.




In the horizontal, B is eliminated because it is both a zero and a one within the pairing.

not A is left because the row is A = 0

The vertical pair work in a similar way.




This special type of binary sequence is known as a Grey code.


Oops - this is a three - variable map and Grey is mis-spelt.

Apart from that the diagram is perfect.





Sharp observers, such as yourself, will notice that this is the car alarm system again.

Therefore we are not surprised to see it simplify to the same expression.

Next we see how these expressions can be converted to circuits using logic gates.


A Karnaugh map is a visual representation of an algebraic expression which allows us to easily spot the patterns we were looking for above, using algebra. They can typically use 2, 3 or 4 variables - but since the IB Guide limits truth tables to 3 inputs we need not consider 4-variable K-maps.

This first example shows a K-map with two variables:

On the map, we try to find pairs of 1's that are horizontal or vertical (we have one of each pair in the above map).

K map with adjacent pairs of ones circled

When the map is expanded to 3 variables, then one edge has 2 of them and they are given in the sequence 00 01 11 and 10 (or any other sequence where only 1 bit changes at a time) - not the more usual binary "counting" sequence.

three-variable K-Map

We can also pair by "wrapping around" the edges of the map - effectively the k-map is on a cylinder.

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related: [ Topic 4 home | previous:algebra | next: circuits ]

K-map is an abbreviation for Karnaugh map (you probably spotted that already).


Here is a link to a site about K-maps and one of my all time favourite computer books.

(Click on the images for greyscale versions if you need them).

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