Digital logic | Variable entrant map (VEM)



Prerequisite – Karnaugh Map (K-map)
K-map is the best manual technique to solve Boolean equations, but it becomes difficult to manage when number of variables exceed 5 or 6. So, a technique called Variable Entrant Map (VEM) is used to increase the effective size of k-map. It allows a smaller map to handle large number of variables. This is done by writing output in terms of input.

Example – A 3-variable function can be defined as a function of 2-variables if the output is written in terms of third variable.

Consider a function F(A,B,C) = (0,1,2,5)

If we define F in terms of ‘C’, then this function can be written as:

And the VEM for this is:



Advantages of using VEM –

  • A VEM can be used to plot more than ‘n’ variables using an ‘n’ variable K-map.
  • It is commonly used to solve problems involving multiplexers.

Minimization procedure for VEM – Now, let’s see how to find SOP expression if a VEM is given.

  1. Write all the variables(original and complimented forms are treated as two different variables) in the map as 0, leave 0’s, minterms and don’t cares as it is and obtain the SOP expression.
  2. (a) Select one variable and make all occurrences of that variable as 1, write minterms (1’s) as don’t cares, leave 0’s and don’t cares as it is. Now, obtain the SOP expression.
    (b) Multiply the obtained SOP expression with the concerned variable.
  3. Repeat step 2 for all the variables in the k-map.
  4. SOP of VEM is obtained by ORing all the obtained SOP expressions.

Let’s apply the above procedure on a sample VEM (X is used to represent don’t care):

Step 1: Write all the variables as 0 (D and D’ are considered as two different variables), leave minterms, 0’s and don’t cares as it is and obtain the SOP expression.

 SOP obtained: A'C

Step 2:
(a) Replace all occurances of D with 1, all occurrences of D’ with 0 and all 1’s with don’t care. Leave 0’s and don’t cares as it is.

(b) Multiply the obtained SOP with the concerned variable.

SOP obtained: AC'D

Step 3: Repeat step 2 for D’

(a) Replace all occurrences of D’ with 1, all occurrences of D with 0 and all 1’s with don’t care. Leave 0’s and don’t cares as it is.

(b) Multiply the obtained SOP with the concerned variable.

SOP obtained: CD' 

Step 4: SOP of VEM is obtained by ORing all the obtained SOP expressions. Therefore, the SOP expression for the given VEM is:

A'C + AC'D + CD' 


My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.




Article Tags :

Be the First to upvote.


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.