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Introduction:
A logic circuit with one input and many output is known as demultiplexer. Demultiplexer is also defined as one into many. By applying proper control signal, input signal can be steered to one of the output lines. Demultiplexer is also known as DEMUX
Click on following image to go to MUX and DEMUX animation
Block Diagram of Demultiplexer:
As shown in above block diagram, DEMUX has one input and many output. It has one data input, S1,S2……Sn are select input and 0,1,2……N are output. Here N≤2 rest to Sn or n≤2 rest to m.
Logic Circuit of 1-to-2 demultiplexer:
Click on following image for more about DEMUX
1-to-16 Demultiplexer:
As shown in above circuit, it has one Data input, 4 control or select input and 16 output. IC-74154 is a 1-to-16 demultiplexer with pin diagram as follow.
Pin Diagram of IC-74154:
In this pin diagram, pin 18 is for the DATA D and pins 20 to 23 are for select input ABCD. Pin 1 to 11 and 13 to 17 are for output Yo to Y15. Pin 19 is for STROBE. Pin 24 is for Vcc and pin 12 for ground.
Truth table:
Notice the STROBE input in above truth table. To activate the IC, STROBE must be active low. When the STROBE is low, the control input determines which output lines are low when the DATA is low. When the DATA input is high, all output lines are high. And, also when STROBE is high, all output lines are high.
There is one DATA bit D under control of nibble ABCD. Also remember that DATA bit is steered to the output whose subscript is the decimal equivalent of ABCD.
Suppose input is steered at Yo for the first row of truth table, subscript of Yo is o and control input or select input of for this is ABCD=0000 and when you convert this oooo binary number into decimal it will 0 demimal. As it is checked below
Check it using following calculator for all the rows of truth table
http://www.easycalculation.com/binary-converter.php
Assignment
(1) Obtain 1-to-32 demultiplexer from two 1-to-16 demultiplexer.
(2) Go to the following page and in this animated object, examine the operation of a demultiplexer along with the data output and select lines. A brief quiz completes the activity.
Click on following image
(3) Go to the following applet of 1-to-4 demultiplexer and 1-to-8 demultiplexer select various control input for different DATA D and note your output.
