Quine McCluskey Method

The Quine McCluskey method, also called the tabulation method, is a very useful and convenient method for simplification of the Boolean functions for a large number of variables (greater than 4).

• This method is useful over K-map when the number of variables is larger, for which K-map formation is difficult.
• This method simplifies functions by systematically identifying and selecting prime implicants.
• Majorly, this method includes the use of minterms and prime implicants and obtains essential prime implicants, which are further used in the simplified Boolean functions. 

Terminologies

• Implicant:Implicant is defined as a group of 1s(for minterm).  
• Prime implicant: It is the largest possible group of 1s(for minterm).
• Essential Prime implicant: These are groups that cover at least one minterm which cannot be covered by other implicants.

Note: The method requires converting decimal minterms to their binary representation for grouping.

Steps for Quine McCluskey Method

1. Arrange the given minterms according to the number of ones present in their binary representation in ascending order.
2. Take the minterms from the continuous group if there is only a one-bit change to make their pair. 
3. Place the '-' symbol where there is a bit change accordingly and keep the remaining bits the same.
4. Repeat steps 2 to 3 until we get all prime implicants (when all the bits present in the table are different).
5. Make a prime implicant table that consists of the prime implicants (obtained minterms) as rows and the given minterms (given in problem) as columns.
6. Place '1' in the cells corresponding to the minterms that are covered by each prime implicant
7. Observe the table, if a minterm is covered by only one prime implicant then it is essential to the prime implicant.
8. Add the essential prime implicants to the simplified Boolean function. 

Solved Examples of Quine McCluskey Method

Example: Simplify using tabulation method : F(A,B,C,D) =∑ m(0,1,2,4,6,8,9,11,13,15)

Solution: Convert the given minterms into their binary representation and arrange them according to the number of ones present in the binary representation. 

As 0 has no 1 in its representation it is kept in one group(0). Similarly, 1 2 4, and 8 contain one 1 in their representation so it is kept in the next group(1). 6 and 9 in the next group(2), 11, and 13 in the next group(3), 15 in the last group(4). 

Now, for table-2 take minterms from successive groups(simultaneous group only) which have an only a 1-bit difference in their representation and form their pair by merging them and making a group of the pairs which are from the same groups that are merged (for example 0 is from group 0 and 1 is from group 1 so it is added to the group 0. 0 belongs to group 0 in table 1 and 2 belongs to group 1 in table 1 so its kept in the same group in table 2. Similarly, make all the possible pairs with the help of the above table and mark - where there is a bit difference. 

For table 3 repeat the same step by taking pairs of successive groups merging them where there is only a 1-bit difference and keeping them in groups according to the groups from where they are merged and placed - in bit difference.

After table 3 the process is stopped as there is no 1-bit difference in the remaining group minterms in the simultaneous groups of table 3.

Now, the remaining quads present in table 3 represent the prime implicants for the given Boolean function. So, we construct prime implicants table which contains the obtained prime implicants as rows and the given minterms as columns. Place 1 in the corresponding place which the minterm can represent. Add the minterm to the simplified Boolean expression if the given minterm is only covered by this prime implicant.  

B'C' is in simplified function as minterm 1 is only covered by B'C'. Similarly, minterms 2,4,6 are only covered by A'D' and minterms 11,13,15 are only covered by AD.

Example: Simplify using tabulation method : F(A,B,C,D,E,F,G) = ∑m(20,28,52,60)

Solution: Convert the given minterms in their binary representation and arrange them according to number of one's present in the binary representation. 

As 20 has 2 1s in its representation it is kept in one group(0). Similarly, 28 and 52 contain 3 1s in their representation so it is kept in the next group(1). 60 in the next group(2).

Now, for table-2 take minterms from successive groups(simultaneous group only) which have an only a 1-bit difference in their representation and form their pair by merging them and making a group of the pairs which are from the same groups that are merged (for example 20 is from group 0 and 28 is from group 1 so it is added to the group 0. 20 belongs to group 0 in table 1 and 52 belongs to group 1 in table 1 so its kept in the same group in table 2. Similarly, make all the possible pairs with the help of the above table and mark - where it is a bit different. 

For table 3 repeat the same step by taking pairs of successive groups merging them where there is only a 1-bit difference and keeping them in groups according to the groups from where they are merged and placed - in bit difference.

After table 3 the process is stopped as there is no 1-bit difference in the remaining group minterms in the simultaneous groups of table 3.

Now, the remaining quads present in table 3 represent the prime implicants for the given Boolean function. So, we construct prime implicants table which contains the obtained prime implicants as rows and the given minterms as columns. Place 1 in the corresponding place which the minterm can represent. Add the minterm to the simplified Boolean expression if the given minterm is only covered by this prime implicant. 

A'CEF'G' is obtained from table 3 as A, F, G contains 0 so A'F'G', C, and E contain 1 so CE. 

A'CEF'G' is in simplified function as it is the only prime implicant that covers all minterms.

Advantages of Quine McCluskey Method

• This method is suitable for a large number of inputs(n>4) for which K-map construction is a tedious task.
• It does not require pattern recognition.
• It provides a systematic and tabular approach.
• It guarantees the minimal Sum of Products (SOP) or Product of Sums (POS) form.

Disadvantages of Quine McCluskey Method

• The computational complexity of this method is high.
• It becomes impractical for functions with more than 6-7 variables due to large tables.
• The method is less intuitive and harder to use manually compared to Karnaugh Maps for smaller functions