What is encoder : Digital Electronics and Digital System

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What is Encoder (Digital Electronics)

An encoder is a combinational logic circuit that converts multiple input lines into a smaller number of output lines. It generates a binary code corresponding to the active input line. Encoders are widely used in digital systems such as keyboards, processors, and communication devices.

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Basic Concept of Encoder

• An encoder has 2ⁿ input lines and n output lines.
• Only one input should be HIGH at a time.
• Output represents the binary value of the active input.
• Encoder performs the reverse operation of a decoder.

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4-to-2 Encoder

A 4-to-2 encoder has four inputs (D₀–D₃) and two outputs (Y₀, Y₁). When one input becomes HIGH, the output gives its corresponding binary code.

Truth Table of 4-to-2 Encoder

D3	D2	D1	D0	Y1	Y0
0	0	0	1	0	0
0	0	1	0	0	1
0	1	0	0	1	0
1	0	0	0	1	1

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Logic Expressions (LaTeX Form)

• Output Y₀: \( Y_0 = D_1 + D_3 \)
• Output Y₁: \( Y_1 = D_2 + D_3 \)

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Limitations of Simple Encoder

• Only one input must be HIGH at a time.
• Multiple HIGH inputs give incorrect output.
• No priority mechanism.
• Not suitable for practical interrupt systems.

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Decimal to BCD Encoder

A Decimal to BCD encoder is a combinational logic circuit that converts a decimal digit (0–9) into its equivalent Binary Coded Decimal (BCD) representation. Only one decimal input is active (HIGH) at a time.

BCD uses 4-bit binary code to represent decimal digits from 0 to 9.

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Inputs and Outputs

• Inputs: D₀ to D₉ (decimal inputs)
• Outputs: A, B, C, D (BCD outputs)
• Only one decimal input should be HIGH at a time

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Truth Table of Decimal to BCD Encoder

Decimal Input	D	C	B	A
D₀	0	0	0	0
D₁	0	0	0	1
D₂	0	0	1	0
D₃	0	0	1	1
D₄	0	1	0	0
D₅	0	1	0	1
D₆	0	1	1	0
D₇	0	1	1	1
D₈	1	0	0	0
D₉	1	0	0	1

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Logic Expressions (BCD Encoder)

• A (LSB): \( A = D_1 + D_3 + D_5 + D_7 + D_9 \)
• B: \( B = D_2 + D_3 + D_6 + D_7 \)
• C: \( C = D_4 + D_5 + D_6 + D_7 \)
• D (MSB): \( D = D_8 + D_9 \)

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Working of Decimal to BCD Encoder

• When a decimal input line is HIGH, the encoder generates its BCD equivalent.
• Each output bit represents a weighted binary value (8, 4, 2, 1).
• Only one decimal input must be active to get correct output.
• If more than one input is HIGH, the output becomes invalid.

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Limitations of Decimal to BCD Encoder

• Only one input should be active at a time.
• Cannot handle multiple active inputs.
• No priority mechanism.
• Requires a priority encoder for practical systems.

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Applications of Decimal to BCD Encoder

• Digital calculators
• Digital clocks
• Keyboards and keypads
• Display systems (7-segment interface)
• Digital counters

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📺 Watch: Encoder Explained (Video)

Watch this video to understand encoder working, truth table, and priority encoder with examples.

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• Encoder definition & types
• 4-to-2 encoder explanation
• Priority encoder
• Truth tables
• Logic expressions

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