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The 2-to-4 Line Decoder/Demultiplexer

Like the multiplexer circuit, the decoder/demultiplexer is not limited to a single address line, and therefore can have more than two outputs. With two, three, or four addressing lines, this circuit can decode a two, three, or four-bit binary number, or can demultiplex up to four, eight, or sixteen time-multiplexed signals.

A 2-to-4 line decoder/demultiplexer is shown below.

2-to-4-line decoder/demultiplexer

As a decoder, this circuit takes an n-bit binary number and produces an output on one of 2n output lines. It is therefore commonly defined by the number of addressing input lines and the number of data output lines. Typical decoder/demultiplexer ICs might contain two 2-to-4 line circuits, a 3-to-8 line circuit, or a 4-to-16 line circuit. One exception to the binary nature of this circuit is the 4-to-10 line decoder/demultiplexer, which is intended to convert a BCD (Binary Coded Decimal) input to an output in the 0-9 range.

If you use this circuit as a demultiplexer, you may want to add data latches at the outputs to retain each signal while the others are being transmitted. However, this does not apply when you are using this circuit as a decoder — then you will want only a single active output to match the input code.

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