Decoder In digital electronics, a decoder can take the form of a multiple-input, multiple-output logic circuit that converts coded inputs into coded outputs, where the input and output codes are different e.g. n-to-2n , binary- coded decimal decoders. Decoding is necessary in applications such as data multiplexing, 7 segment display and memory address decoding. The example decoder circuit would be an AND gate because the output of an AND gate is High (1) only when all its inputs are High. Such output is called as active High output. If instead of AND gate, the NAND gate is connected the output will be Low (0) only when all its inputs are High. Such output is called as active low output. A slightly more complex decoder would be the n-to-2n type binary decoders. These types of decoders are combinational circuits that convert binary information from n coded inputs to a maximum of 2n unique outputs. In case the n bit coded information has unused bit combinations, the decoder may have less than 2n outputs. 2-to-4 decoder, 3-to-8 decoder or 4-to-16 decoder are other examples. The input to a decoder is parallel binary number and it is used to detect the presence of a particular binary number at the input. The output indicates presence or absence of specific number at the decoder input. Let us suppose that a logic network has 2 inputs A and B. They will give rise to 4 states A, A’, B, B’ . The truth table for this decoder is shown below: Table 1: Truth Table of 2:4 decoder Fig 1: Logic Diagram of 2:4 decoder Fig 2: Representation of 2:4 decoder For any input combination only one of the outputs is low and all others are high. The low value at the output represents the state of the input. Decoder expansion Combine two or more small decoders with enable inputs to form a larger decoder e.g. 3-to-8-line decoder constructed from two 2- to-4-line decoders. Decoder with enable input can function as demultiplexer. 3:8 decoder It uses all AND gates, and therefore, the outputs are active- high. For active- low outputs, NAND gates are used. It has 3 input lines and 8 output lines. It is also called as binary to octal decoder it takes a 3-bit binary input code and activates one of the 8(octal) outputs corresponding to that code. The truth table is as follows:
Posted on: Sun, 12 Oct 2014 11:00:15 +0000
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