A decoder/demultiplexer is a combinational circuit that asserts one of several outputs in response to a unique input code. It is a unit with n-inputs and m-outputs, where m < 2n. An output switches from Not Asserted state to an Asserted state, when the input code is switched to a specific one. When m = 2n, each one of the outputs can be associated with a Minterm of n-variables. Hence, such a decoder is known as Minterm Recogniser. Schematic of a 3-input demultiplexer is shown in figure below.
Schematic of a 3-input demultiplexer
In the demultiplexer illustrated, each one of the outputs have an AND (G) dependence on one of the input codes, while, Enable inputs can be used to disable/enable the entire functional unit.
Decoding and Demultiplexing Functions
One of the traditional uses of a demultiplexer is to use it in combination with a multiplexer to transmit a number of signals over a single line. An 8-channel multiplexer-demultiplexer combination is shown in the figure below.
An 8-channel multiplexer-demultiplexer combination
Notice that one of the Enable inputs of the demultiplexers is used as its data input. Though this illustration indicates that the address lines are tied together, in an actual signal transmission unit that uses such a MUX-DEMUX combination a different method will have to be used to change the addresses of both the units simultaneously.
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