There are two basic designs of logical circuits

1)Combinational Circuits

2)Sequential Circuits

Our interest is only in Combinational circuits and there applications. There are following steps to design combinational circuits.

1-Find number of inputs/outputs from problem statement.

2-Assign different letters to inputs and outputs e.g X,Y,Z,a,b,c etc.

3-Write truth table of problem.

4-Write simplified Boolean’s equation from truth table for each output.

5-Implement the circuit.


is a combinational logic circuit if there are 2^n input lines they will produce n number of outputs. There are following encoder combinations

1) 4 : 2 Encoder

2) 8 : 3 Encoder

3)16 : 4 Encoder

Boolean Equation:

Q1 = D2 + D3

Q0 = D1 + D3


is totally opposite to encoder which have n number of inputs and provide us 2^n number of outputs. If we have 3 inputs D0 , D1 , D2  and there preference started from left to right when all are 0 then output is also zero but when the input is 1 then we can see output 1 showing result and then so on.

There are  following decoder combinations

1)1 : 2 Decoder

2)2 : 4 Decoder

3)3 : 8 Decoder

2*4 decoder have 2 inputs and 4 outputs. Decoders are also used in 7-segment display.


Priority Encoder

As the name stated it depends upon priority function. If the two or more inputs have input 1 at the same time, the input having highest priority will take precedence.

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