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.

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.

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,

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

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.

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,

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

Further Reading

• Encoder | Tutorials Point
• Encoder | Geeks for Geeks