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 Line Encoder

A 4 to 2 line 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.

4 to 2 line encoder

Truth table of 4 to 2 line encoder is,

Input Output
Y3 Y2 Y1 Y0 A1 A0
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 A0 and A1 as following,
{A_1=Y3+Y2}
{A_0=Y3+Y1}

The circuit diagram for a 4 to 2 line encoder is shown below,

4 2 encoder circuit

Octal to Binary Encoder

An 8 to 3 line encoder or octal to binary encoder consists of 8 input lines and 3 output lines. When any of the input lines becomes 1, we get corresponding binary at the output lines.

8 to 3 line encoder

Truth table of 8 to 3 line encoder is,

Inputs Output
Y7 Y6 Y5 Y4 Y3 Y2 Y1 Y0 A2 A1 A0
0 0 0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 1 0 0 0 1
0 0 0 0 0 1 0 0 0 1 0
0 0 0 0 1 0 0 0 0 1 1
0 0 0 1 0 0 0 0 1 0 0
0 0 1 0 0 0 0 0 1 0 1
0 1 0 0 0 0 0 0 1 1 0
1 0 0 0 0 0 0 0 1 1 1

From the truth table, we can find logic expressions for A0, A1 and A2 as following,
{A_0=Y1+Y3+Y5+Y7}
{A_1=Y2+Y3+Y6+Y7}
{A_2=Y4+Y5+Y6+Y7}

The circuit diagram for a 8 to 3 line encoder is shown below,

8 to 3 line encoder circuit

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