3 to 8 line decoder circuit diagram


A decoder is a combinational circuit that converts binary information from the n coded inputs to a maximum of 2n unique outputs. If the n-bit coded information has unused bit combinations, the decoder may have less than 2n outputs.

The decoders presented in this section are called n-to-m-line decoders, where m= 2n. Their purpose is to generate the 2n (or fewer) binary combinations of the n input variables. A decoder has n inputs and m outputs and is also referred to as an n x m decoder.

3 to 8 Line Decoder:

The three data inputs, Ao, A1, and A2, are decoded into eight outputs, each output representing one of the combinations of the three binary input variables. The three inverters provide the complement of the inputs, and each of the eight AND gates generates one of the binary combinations. A particular application of this decoder is a binary-to-octal conversion. The input variables represent a binary number and the outputs represent the eight digits of the octal number system. However, a 3-to-8-line decoder can be used for decoding any 3-bit code to provide eight outputs, one for each combination of the binary code. The logic diagram of a 3-to-8-line decoder is shown below.

3 to 8 line decoder circuit diagram

3 to 8 Line Decoder Truth Table:

Commercial decoders include one or more enable inputs to control the operation of the circuit. The decoder of the figure has one enable input, E. The decoder is enabled when E is equal to 1 and disabled when E is equal to 0. The operation of the decoder clarified using the truth table listed in below table:

3 to 8 line decoder circuit diagram

When the enable input E is equal to 0, all the outputs are equal to 0 regardless of the values of the other three data inputs. The three x ‘ s in the table designate don’t-care conditions. When the enable input is equal to I, the decoder operates in a normal fashion. For each possible input combination, seven outputs are equal to 0 and only one that is equal to me. The output variable whose value is equal to 1 represents the octal number equivalent to the binary number that is available in the input data lines.