Encoders
An encoder can also be described as a combinational circuit that performs the inverse operation of a decoder. An encoder has a maximum of 2^n (or less) input lines and n output lines.
In an Encoder, the output lines generate the binary code corresponding to the input value.
The following image shows the block diagram of a 4 * 2 encoder with four input and two output lines.
The truth table for a 4-to-2 line encoder can be represented as:
| A3 | A2 | A1 | A0 | D1 | D0 |
|---|---|---|---|---|---|
| 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 write the Boolean function for each output as:
D1 = A3 + A2 D0 = A3 + A1
The circuit diagram for a 4-to-2 line encoder can be represented by using two input OR gates.
The most common application of an encoder is the Octal-to-Binary encoder. Octal to binary encoder takes eight input lines and generates three output lines.
The following image shows the block diagram of an 8 * 3 line encoder.
The truth table for an 8 * 3 line encoder can be represented as:
| D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 | x | y | z |
|---|---|---|---|---|---|---|---|---|---|---|
| 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 write the Boolean function for each output as:
x = D4 + D5 + D6 + D7 y = D2 + D3 + D6 + D7 z = D1 + D3 + D5 + D7
The circuit diagram for an 8 * 3 line encoder can be represented by using two input OR gates.
