Combinational logic circuits produce outputs solely based on current inputs. They are made up of basic logic gates like NAND, NOR, and NOT connected together. A half adder adds two binary digits and produces a sum and carry output. A full adder adds three binary digits and produces a two-bit sum and carry output. A half subtractor subtracts one bit from another and produces a difference and borrow output, while a full subtractor subtracts three bits. Parallel adders use cascaded full adders to add multiple bits simultaneously, while serial adders add bits sequentially with the carry from the previous addition. BCD to 7-segment decoders take a 4-bit BCD number and output the correct segments to display

1. Unit-4
 Combinational Logic Circuits design:-
The outputs of Combinational Logic Circuits are only determined by the logical
function of their current input state, logic “0” or logic “1”, at any given instant in
time. In other words, in a Combinational Logic Circuit, the output is dependant
at all times on the combination of its inputs. Combinational Logic Circuits are
made up from basic logic NAND, NOR or NOT gates that are “combined” or
connected together to produce more complicated switching circuits. These logic
gates are the building blocks of combinational logic circuits.
 Half Adder and Full Adder Circuit:-
Half adder:- The half adder adds two binary digits called as augend and addend
and produces two outputs as sum and carry; XOR is applied to both inputs to
producesum and AND gate is applied to both inputs to producecarry.

2. Half Adder Truth Table:
Full Adder:- The full adder adds 3 one bit numbers, where two can be referred
to as operands and one can be referred to as bit carried in. And produces 2-bit
output, and these can be referred to as output carry and sum. The first two inputs
are A and B and the third input is an input carry as C-IN.
Full Adder Truth Table:

3.  Half Subtractor And Full Subtractor:-
Half Subtractor : Half Subtractor is used for subtracting one single bit binary digit
from another single bit binary digit.The truth table of Half Subtractor is
shownbelow.
Difference = A'B+AB'=A B
Borrow=A'B
Full Subtractor : A logic Circuit Which is used for Subtracting Three Single bit
Binary digit is known as Full Subtractor.The Truth Table of Full Subtractor is
Shown Below.

4.  Parallel and serial adder:-
Parallel Adder:- Adders are the combinatorial circuits which are used to add
two binary numbers. Parallel adder is nothing but a cascade of several full adders.
The number of full adders used will depend on the number of bits in the binary
digits which require to be added.
first, FA1 adds A1 with B1 to generate S1 (the first bit of sum output) and Co1. Next,
FA2 uses this Co1 as its carry in bit and adds it with its input bits A2 and B2 to
generate the second bit of the sum output S2 and Co2. Next, this Co2 is considered
as an input by FA3 which adds it with the bits A3 and B3.
Serial adder:- serial adders are digital logic circuits which add two N bit number.
The addition takes place bit by bit starting from LSB . subsequently carry is
generated at each step .After each bit addition the addition of the next higher bit
takes place along with carry from previous bit. The classic way to implement an N-
bit adder is to use N 1-bit full-adders in parallel. For instance, for a 4-bit adder four
1-bit full-adders are needed.
The adder adds the two inputs A and B in parallel producing the sum S. Assuming
that A and B are stored in two input registers and S is stored in an output register,
the adder can perform the addition is a single clock cycle.

5. DECODER:-
BCD to 7-Segment Display Decoders:-
A binary coded decimal (BCD) to 7-segment display decoder such as the TTL
74LS47 or 74LS48, have 4 BCD inputs and 7 output lines, one for each LED
segment. This allows a smaller 4-bit binary number (half a byte) to be used to
display all the denary numbers from 0 to 9 and by adding two displays together, a
full range of numbers from 00 to 99can be displayed with just a single byte of 8
data bits.