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Computer organization Notes--De-Multiplexer

A de-multiplexer performs the reverse operation of a multiplexer i.e. it receives one input and distributes it over several outputs. It has only one input, n outputs, m select input. At a time only one output line is selected by the select lines and the input is transmitted to the selected output line. Here selection lines will determine on which output line the input line will appear. The output lines which are not selected will remain at zero because at one time only one output get connected to output. Here if n is no of selection lines then there are 2n possible output lines. De-multiplexer is also known as Data Distributor. It is referred by De-MUX. A de-multiplexer is equivalent to a single pole multiple way switch.
The block diagram of a 1-input and m Control signal Multiplexer and n-output De-multiplexer can be drawn as follows:

Here n is less than or equal to 2m.
Let us consider the case of 1 x 4 Line De-Multiplexer. A 1-to-4 de-multiplexer has a single input (D), two selection lines (S1 and S0) and four outputs (Y0 to Y3). The input data goes to any one of the four outputs at a given time for a particular combination of select lines. This de-multiplexer is also called as a 2-to-4 de-multiplexer which means that two select lines and 4 output lines. The block diagram and truth table of 1:4 DEMUX is shown below. From the truth table it is clear that, when S1=0 and S0= 0, the data input is connected to output Y0 and when S1= 0 and s0=1, then the data input is connected to output Y1. Similarly, other outputs are connected to the input for other two combinations of select lines.

From the table, the output logic can be expressed as min terms and are given below.

Where D is the input data, Y0 to Y3 are output lines and S0 & S1 are select lines. From the above Boolean expressions, a 1-to-4 demultiplexer can be implemented by using four 3-input AND gates and two NOT gates as shown in figure below. The two selection lines enable the particular gate at a time. So depends on the combination of select inputs, input data is passed through the selected gate to the associated output.

Applications of De-multiplexer
Since the de-multiplexers are used to select or enable the one signal out of many, these are extensively used in microprocessor or computer control systems such as Selecting different IO devices for data transfer, Choosing different banks of memory, Depends on the address, enabling different rows of memory chips, Enabling different functional units. Other than these, de-multiplexers can be found in a wide variety of application such as
• Synchronous data transmission systems.
• Boolean function implementation.
• Data acquisition systems.
• Combinational circuit design.
• Automatic test equipment systems.
• Security monitoring systems (for selecting a particular surveillance camera at a time), etc.

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