Encoders are the opposite of decoders. Instead of expanding a binary code into many lines, they compress several input lines into a shorter binary output code.
Encoders are core combinational circuits that convert multiple input lines into a compact binary code at the output. They are used whenever a digital system needs to know which one of many lines is active and represent that information as an n-bit binary number.
What is an encoder ?
An encoder is a combinational logic circuit that converts up to 2n input lines into an -bit binary output code, assuming only one input is active at a time.
In simple terms, it encodes the position of the active input line into a binary number on the outputs. For example:
A 4‑to‑2 encoder has 4 inputs (D₀–D₃) and 2 outputs (Y₁Y₀).
An 8‑to‑3 encoder has 8 inputs (D₀–D₇) and 3 outputs (Y₂Y₁Y₀).
Key properties
Typically 2n inputs and outputs in a simple binary encoder.
- Inputs usually active‑HIGH (logic 1 means “selected”).
- Output represents the index of the active input in binary.
Functions and Purpose of an Encoder
Encoders perform three main roles:
- Code conversion: Convert a one‑hot or decimal‑like input into binary code.
- Data compression: Replace many wires (parallel signals) with fewer binary lines.
- Source identification: Indicate which device, key, or sensor is active.
This is the opposite of a decoder, which takes binary and activates one of many lines.
8:3 Encoder
A 8:3 encoder has 8 inputs and 3 outputs. It accepts eight inputs and produces 3 bit output code, corresponding to the activated input. It is also known as Octal to Binary Encoder.
Working of a 8‑to‑3 Binary Encoder
An 8‑to‑3 encoder has inputs D0–D7 and outputs Y2,Y1,Y0. If only one input is HIGH, then
D₀ = 1 → 000
D₁ = 1 → 001
D₂ = 1 → 010
D₃ = 1 → 011
D₄ = 1 → 100
D₅ = 1 → 101
D₆ = 1 → 110
D₇ = 1 → 111
Logical Expressions for Y2 , Y1 , Y0 are :-
Y2 = D4 + D5 + D6 + D7
Y1 = D2 + D3 + D6 + D7
Y0 = D1 + D3 + D5 + D7
