Encoders

An encoder is a combinational circuit that performs reverse function of a decoder. It accepts 2n input and produces output in n output lines. This process of generating codes based on the values of input lines is called encoding.

encoder block 1

4 to 2 Encoder

A 4 to 2 bit encoder consists of 4 input lines and 2 output lines. When any of the input lines becomes 1, we get corresponding binary at the output lines.

encoder block 4 2 1

Truth table of 4 to 2 bit encoder is,

Input Output
Y3 Y2 Y1 Y0 A1 A2
0 0 0 1 0 0
0 0 1 0 0 1
0 1 0 0 1 0
1 0 0 0 1 1

From the truth table, we can find logic expressions for A1 and A2 as following,

$${A_1=Y3+Y2}$$ $${A_2=Y3+Y1}$$
4 2 encoder circuit