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In electronics, a multiplexer or mux is a device that combines several electrical signals into a single signal. There are different types of multiplexer for analogue and digital circuits.

In digital signal processing, a multiplexer (often abreviated to "mux" or "muldex") is a device for taking several separate digital data streams and combining them together into one data stream of a higher data rate. This allows multiple data streams to be carried from one place to another over one physical link, which saves cost.

At the receiving end of the data link a complementary "demultiplexer" or "demux" is required to break the high data rate stream back down into the original lower rate streams.

It is usual to combine a multiplexer and a demultiplexer together into one piece of equipment and simply refer to the whole thing as a "multiplexer". Both pieces of equipment are needed at both ends of a transmission link because most communications systems transmit in both direction.

See also inverse multiplexer

In digital circuit design a two-input multiplexer is a simple connection of logic gates whose output Y is either input A or input B depending on the value of a third input C. Its boolean equation is:

 Y = (A and C) or (B and not C)
which can be expressed as the truth table:
 A B C | Y
 0 0 0 | 0
 0 0 1 | 0
 0 1 0 | 1
 0 1 1 | 0
 1 0 0 | 0
 1 0 1 | 1
 1 1 0 | 1
 1 1 1 | 1
or as the Karnaugh map:
 Y        C
       | 0 1
 AB 00 | 0 0
    01 | 1 0
    11 | 1 1
    10 | 0 1

Demultiplexers are sometimes convenient for designing general purpose logic. Basically, the inputs to the set of logic functions must be routed to the selection bits. Any function of the selection bits can be constructed by logically OR-ing the correct set of outputs.

In analogue circuit design, a multiplexer is a special type of analogue switch that connects one signal selected from several inputs to a single output.

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