Friday, December 13, 2013



In design of large scale of digital systems,a single line required to carry two or more different digital signals. only one signal can be placed on one line. what is required is that device will allow us to select,the signal we wish to place on this common line. The circuit is what we called as a MULTIPLEXER.

Multiplexer is known as selector because its output is one of the inputs that are selected by a control.

Multiplexers are used as one method of reducing the number of integrated circuit packages required by a particular circuit design. This in turn reduces the cost of the system.

Assume that we have four lines, C0, C1, C2 and C3, which are to be multiplexed on a single line, Output (f). The four input lines are also known as the Data Inputs. Since there are four inputs, we will need two additional inputs to the multiplexer, known as the Select Inputs, to select which of the C inputs is to appear at the output. Call these select lines A and B.

The gate implementation of a 4-line to 1-line multiplexer is shown below:

Implementation of a multiplexer

Then, this is the circuit symbol for the above multiplexer:
Multiplexer circuit symbol

Truth table for Multiplexer

                                  Select Inputs
                 A                                      B
Output=input selected

If a binary 0 (A=0 and B =0) is applied to the data-select lines, the data on input C0 appear on the data output line. The table is shows of all selection input.

decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode. It is a combination circuit that converts binary information from n input lines to a maximum of 2n unique output lines.

n 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-2nbinary-coded decimal decoders. Enable inputs must be on for the decoder to function, otherwise its outputs assume a single "disabled" output code word. Decoding is necessary in applications such as data multiplexing, 7 segment display and memory address decoding.

Normally, the decoder is defined by the number of inputs and the number of outputs. For example, a decoder that has 3 inputs and 8 (2^3) outputs is named as 3-to-8 decoder. 

... operation of a decoder. it converts the decimal information to binary

Note on the truth table, the decoded output appears only when the encoded input is present and the enable line has a value of 1 whereas the other output line is 0.

A decoder that contains enable inputs is also known as a decoder-demultiplexer. Thus, we have a 4-to-16 decoder produced by adding a 4th input shared among both decoders, producing 16 outputs.

                                                                                           Published by:
                                                                  SITI NURHASTINI BINTI ROSALI (B031310320)

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