What is a De-Multiplexer

A De-Multiplexer or a De-Mux is a digital combinational circuit used to distribute one input line over a multiple number of output lines. This is achieved by changing the logic levels on the selection lines of the circuit that determines the output line on which the input will appear.

The following truth table illustrates this distribution of input in relation with the select lines inputs for a 1:4 De-Mux:-

Inputs

Outputs

Enable

I (I/P line)

S0

S1

D0

D1

D2

D3

0

X

X

X

0

0

0

0

1

0

X

X

0

0

0

0

1

1

0

0

1

0

0

0

1

1

0

1

0

1

0

0

1

1

1

0

0

0

1

0

1

1

1

1

0

0

0

1

 

Here S0 and S1 are the two select lines while D0, D1, D2 and D3 are four output lines. If there are m output lines and n select lines in a de-multiplexer, then m=2n as a general rule of thumb. From the above truth table, the following logical equations can easily be deducted for a 1:4 De-Mux.

D0 = Enable.I.S0.S1’

D1=Enable.I.S0.S1

D2=Enable.I.S0.S1

D3=Enable.I.S0.S1

Thus the above circuit can easily be synthesized by using four 4-input AND gates and four inverters. The output from the AND gates will be the output lines of the de-multiplexer. This circuit can also be used as a binary to decimal decoder by giving the binary input on the select lines and obtaining the logic level 1 on the output line representing the corresponding decimal number. The input line I to the decoder should be kept at level 1 at all times in this case.

De-Multiplexers with higher number of output lines can be easily synthesized by using de-multiplexers with small number of output lines in cascade with each other. For this purpose a logic is needed to be devised that will control the enable bit inputs to the various de-multiplexers in accordance with the required output.

The de-multiplexer circuitry is also widely used in Time Division De-Multiplexing. In this application, the TDM signal is fed to the input line of the De-Mux and the select lines are connected to the output of a counter whose clock frequency is same as that of the frequency at which the signal was multiplexed on the transmitter side. This makes this circuitry very useful in the field of communications.

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