Implementation of combinational logic functions. (Decoders)

Decoders

• In digital electronics, a decoder can take the form of multi-input, multi-output logic circuits that converts coded input into coded output.
• The output of AND gate is "HIGH" only when all inputs are "HIGH". So output are called "active high output."
• If NAND gete is connected then the output will be "LOW" only when all its inputs are "HIGH" such output is called "active low output."

Example of decoder;

n to 2ⁿ  decoder.

2:4 Decoder

Block diagram of 2:4 decoder

In this figure of 2:4 decoder A1 and A0 are two inputs and Y0, Y1, Y2 and Y3 are four outputs.

E is enable line,
when E = 0, no output, disable line.
          E = 1, output, enable line.

Truth table and Logic Diagram of 2:4 Decoder

3:8 Decoder

Block diagram of  3:8 Decoder

There are 3 inputs and 8 outputs.







Truth table and logic diagram of 3:8 Decoder.

Truth table of 3:8 Decoder

Logic diagram of 3:8 Decoder.

4:16 Decoder

There are 4 inputs and 16 outputs.

Block diagram of 4:16 Decoder.

Truth table of 4:16 Decoder.