SOP and POS, Maxterm and Minterm, Karnaugh Graph

SOP and POS Notes (GATE – Digital Logic)

1. Boolean Expression Forms

A Boolean function can be represented mainly in:

SOP (Sum of Products)
POS (Product of Sums)

These are most used in:

simplification
K-map
logic circuit design

2. SOP (Sum of Products)

Definition:

SOP is an OR ( + ) of multiple product terms.
Each product term is an AND ( · ) of literals.

General Form:

$F = P_1 + P_2 + P_3 + ...$ where each $P\_i$ is a product term.

Example :

$F = A'B + ABC' + B'C$

Circuit Meaning :

AND gates for product terms
then OR gate at output

Types of SOP

(a) Standard SOP

each product term contains all variables
Example: $F = A'BC + AB'C + ABC$

(b) Non-standard SOP

product terms may miss some variables
Example: $F = AB + C$

(c) Canonical SOP

SOP written as sum of minterms
notation: Σm
Example: $F(A,B,C)=\Sigma m(1,3,5,7)$

3. POS (Product of Sums)

Definition:

POS is an AND ( · ) of multiple sum terms.
Each sum term is an OR ( + ) of literals.

General Form:

$F = S_1 \cdot S_2 \cdot S_3 ...$ where each $S\_i$ is a sum term.

Example :

$F = (A + B')(B + C)(A' + C')$

Circuit Meaning:

OR gates for sum terms
then AND gate at output

Types of POS

(a) Standard POS

each sum term contains all variables
Example: $F = (A + B + C')(A' + B + C)$

(b) Non-standard POS

sum terms may miss some variables
Example: $F = (A + B)(C')$

(c) Canonical POS

POS written as product of maxterms
notation: ΠM
Example: $F(A,B,C)=\Pi M(0,2,4,6)$

4. SOP vs POS (Quick Difference)

Feature	SOP	POS
Form	OR of AND terms	AND of OR terms
Based on	minterms	maxterms
K-map grouping	group 1s	group 0s
Canonical notation	Σm	ΠM

5. Conversion (Important)

SOP can be converted to POS and vice versa using:
- Boolean algebra
- DeMorgan’s theorem
- K-map method (best for GATE)

Minterm and Maxterm Notes (GATE – Digital Logic)

1. Basic Idea

Boolean function can be represented using:
- Minterms (Σm) → for SOP form
- Maxterms (ΠM) → for POS form

2. Minterm

Definition:

A minterm is a product (AND) term that contains all variables exactly once (either complemented or uncomplemented).
A minterm evaluates to 1 for exactly one input combination.

Notation:

$m\_i$
Function form: $F = \Sigma m(\dots)$

Example (2 variables A,B):

$m\_0 = A'B'$ (00)
$m\_1 = A'B$ (01)
$m\_2 = AB'$ (10)
$m\_3 = AB$ (11)

Key Point:

If for an input:

bit = 0 → variable is complemented
bit = 1 → variable is uncomplemented

Example:

input 101 (A,B,C) $m\_5 = AB'C$

3. Maxterm

Definition:

A maxterm is a sum (OR) term that contains all variables exactly once (either complemented or uncomplemented).
A maxterm evaluates to 0 for exactly one input combination.

Notation:

$M\_i$
Function form: $F = \Pi M(\dots)$

Example (2 variables A,B)

$M\_0 = (A + B)$ (00)
$M\_1 = (A + B')$ (01)
$M\_2 = (A' + B)$ (10)
$M\_3 = (A' + B')$ (11)

Key Point:

If for an input:

bit = 0 → variable is uncomplemented
bit = 1 → variable is complemented

Example:

input 101 (A,B,C) $M\_5 = (A' + B + C')$

4. Relation between Minterm and Maxterm

For same index i:

$m\_i$ is complement of $M\_i$
$m\_i = (M\_i)'$
$M\_i = (m\_i)'$

5. Canonical Forms

Canonical SOP (Sum of Minterms):

OR of minterms
Uses Σm
Example: $F(A,B,C)=\Sigma m(1,3,5,7)$

Canonical POS (Product of Maxterms):

AND of maxterms
Uses ΠM
Example: $F(A,B,C)=\Pi M(0,2,4,6)$

6. Conversion Rule (Very Important)

If total variables = n, then:

$F = \Sigma m(S)$
$F = \Pi M(\text{all indices except }S)$

Example (3 variables → indices 0 to 7):

If $F=\Sigma m(1,2,6)$
then $F=\Pi M(0,3,4,5,7)$

Similarly:

If $F=\Pi M(0,3,4)$
then $F=\Sigma m(1,2,5,6,7)$

7. Finding Minterm/Maxterm Index

For minterm index:

take binary input as number Example:
$ABCD = 1011$ → index = 11 so minterm = $m\_{11}$

For maxterm index:

same indexing rule (binary → decimal) Example:
$ABCD = 1011$ → maxterm = $M\_{11}$

8. K-Map Connection

In K-map:
- place 1s for minterms in Σm
- place 0s for maxterms in ΠM

9. Shortcut Table (Must Remember)

Form	Represents	Use in K-map	Expression Type
Σm	where F=1	mark 1	SOP
ΠM	where F=0	mark 0	POS

10. Common GATE Traps

Confusing complement rule in maxterm.
Mixing Σ and Π.
Forgetting: maxterm corresponds to 0 output, minterm corresponds to 1 output.
Wrong conversion between Σm and ΠM.

If you want, I can also give 1 full solved example showing conversion between truth table → Σm → ΠM → K-map in GATE pattern.

K-Map (Karnaugh Map) Notes (GATE Ready)

1. What is K-Map?

Karnaugh Map (K-Map) is a graphical method to simplify Boolean functions.
It gives minimum literals expression (best for GATE).
Used for:
- SOP minimization (group 1s)
- POS minimization (group 0s)

2. Why Gray Code?

K-map uses Gray code ordering so that adjacent cells differ in only 1 variable. Gray order for 2 bits:

00, 01, 11, 10

This is the reason why grouping works.

3. K-Map Tables (Must Remember)

(A) 2-Variable K-Map (A,B)

A\B	0	1
0	m0	m1
1	m2	m3

$m\_0=A'B'$ , $m\_1=A'B$ , $m\_2=AB'$ , $m\_3=AB$

(B) 3-Variable K-Map (A,B,C)

Rows = A (0,1) Cols = BC in Gray order (00,01,11,10)

A\BC	00	01	11	10
0	m0	m1	m3	m2
1	m4	m5	m7	m6

(C) 4-Variable K-Map (A,B,C,D) (Most Important)

Rows = AB (00,01,11,10) Cols = CD (00,01,11,10)

AB\CD	00	01	11	10
00	m0	m1	m3	m2
01	m4	m5	m7	m6
11	m12	m13	m15	m14
10	m8	m9	m11	m10

4. Adjacency in K-Map (GATE Trap)

Adjacent means 1-bit difference only.

Adjacency allowed:

left-right
up-down
wrap around
- first column adjacent to last column
- first row adjacent to last row
corners are adjacent via wrap-around

Not allowed:

diagonal adjacency

5. Grouping Rules (SOP / POS)

Groups must have size $2^n$ only:

1,2,4,8,16…

Rules:

group must be rectangle
overlapping allowed
group can wrap around
choose largest groups first
cover all required cells

6. SOP using K-Map (group 1s)

SOP Steps:

Put 1 at given minterms.
Use X (don’t care) as 1 if it helps.
Make groups of 1s.
Write product term for each group.
OR all terms.

Term Formation Rule (SOP):

Inside a group:

variable constant 1 → keep as normal
variable constant 0 → keep as complement
variable changing → remove

7. POS using K-Map (group 0s)

POS Steps:

Put 0 at given maxterms.
Use X as 0 if it helps.
Make groups of 0s.
Write sum term for each group.
AND all terms.

Term Formation Rule (POS):

Inside a group:

variable constant 0 → keep as normal
variable constant 1 → keep as complement
variable changing → remove

8. Shortcut: Group Size → Remaining Variables

If variables = n and group size = $2^k$ :

eliminated variables = k
remaining variables = n − k

Example:

4-variable K-map
group of 8 (= $2^3$ )
remaining vars = 4−3 = 1 literal

9. Prime Implicant and Essential Prime Implicant

Prime Implicant (PI):

group that cannot be expanded further.

Essential Prime Implicant (EPI):

PI covering a 1 that no other PI covers.
EPIs are compulsory in answer.

10. Example (GATE Level SOP)

Given:

$F(A,B,C) = \Sigma m(1,3,5,7)$

Step 1: Put 1s in 3-variable K-map

3-variable K-map table:

A\BC	00	01	11	10
0	0	1	1	0
1	0	1	1	0

( 1s at m1, m3, m5, m7 )

Step 2: Grouping

All 4 ones form one group of size 4.

Step 3: Write term

In this group:

B changes? No (always 1)
C changes? Yes? Actually columns 01 and 11 → C=1 always
A changes (0 and 1) → eliminate A

So remaining: C

Final Answer:

$F = C$

11. Example (GATE Level POS)

Given:

$F(A,B,C) = \Pi M(0,2,4,6)$

This means:

F=0 at indices 0,2,4,6

Step 1: Put 0s in K-map

A\BC	00	01	11	10
0	0	1	1	0
1	0	1	1	0

(0s at m0, m2, m4, m6)

Step 2: Group 0s

All 4 zeros form one group of size 4.

Step 3: Write POS term

In this group:

C is constant 0 (columns 00 and 10 → C=0 always)
A changes → eliminate
B changes → eliminate

So sum term = (C)

Final Answer:

$F = C$

(Here POS gives same simplified result.)

12. Don’t Care (X) Notes

X can be used as 1 in SOP or 0 in POS.
Use X only if it makes bigger groups.
Bigger groups = smaller expression.

13. Common GATE Mistakes

wrong Gray order in table
ignoring wrap-around
grouping diagonal cells
group size not power of 2
writing wrong literal (complement error)
not taking largest groups