Test: Combinational Logic - Electronics and Communication Engineering (ECE) MCQ

1st output of AND gate is = A’B’
2nd AND gate’s output is = AB and,
OR gate’s output is = (A’B’)+(AB) = AB + A’B’.

After solving the circuit we get (A’B’)+AB as output, which is XNOR operation. Thus, it will produce 1 when inputs are even number of 1s or all 0s, and produce 0 when input is odd number of 1s.

For decoding any number output must be high for that code and this is possible in One 4-input NAND gate, one inverter option only. A decoder is a combinational circuit that converts binary data to n-coded data upto 2n outputs.

In the given diagram, S0 and S1 are selection bits. So,
I/P S0 S1 O/P
D = 0 0 0 Y0
D = 0 0 1 Y1
D = 0 1 0 Y2
D = 0 1 1 Y3
Hence, inputs are S0 and S1 are Low means 0, so output is Y0 and rest all are HIGH.

Full Adder is a combinational circuit with 3 input bits and 2 output bits CARRY and SUM. Three bits full adder requires 23 = 8 combinational circuits.

The given diagram is demultiplexer, because it takes single input & gives many outputs. A demultiplexer is a combinational circuit that takes a single output and latches it to multiple outputs depending on the select lines.

When both inputs are same then the o/p is high for a XNOR gate.
i.e., A B O/P
0 0 1
0 1 0
1 0 0
1 1 1.
Thus, it will produce 1 when inputs are even number of 1s or all 0s, and produce 0 when input is odd number of 1s.

Combinational Logic circuits are circuits for which the present output depends only on the present input, i.e. there is no memory element to store the past output.

A combinational circuit can have ‘n’ number of inputs and ‘m’ number of outputs as shown:

Code conversion circuits are used to convert a digital code from one format to another. For example, a binary code can be converted to a Gray code, or a BCD (binary-coded decimal) code can be converted to a binary code. These circuits typically use a combination of logic gates, such as AND gates, OR gates, and XOR gates, to perform the necessary conversions.

However, the most commonly used type of circuit for code conversion is the AND-OR gate circuit. This circuit is based on a combination of AND gates and OR gates, and it can be used to implement various types of code conversion, such as binary to Gray code conversion and BCD to binary conversion. The AND-OR gate circuit is preferred for code conversion because it is simple, efficient, and provides a high level of flexibility.

Designing a combinational circuit typically involves 4 steps:

1. Problem specification: This involves identifying the problem that the circuit needs to solve, and defining the input and output requirements.

2. Truth table creation: This involves creating a truth table that shows the desired output for each possible combination of input values.

3. Boolean expression derivation: This involves deriving a Boolean expression or set of expressions that represent the desired output in terms of the input variables.

4. Logic diagram implementation: This involves implementing the Boolean expressions using logic gates, such as AND, OR, NOT, XOR, etc.

These 4 steps provide a systematic approach to designing combinational circuits that can be used for a wide range of problems. However, the number of steps required may vary depending on the complexity of the problem and the circuit being designed.