A multiplexer selects one of several input signals and sends it to one output. A demultiplexer routes one input to a selected output. The select bits choose the path; they are separate from the data being routed. A 2^n-way selector normally needs n select bits.
Worked example
A 4-to-1 multiplexer with select S1S0 = 10 sends input D2 to its output. If D2 = 1, the output is one regardless of the unselected data inputs. A 1-to-4 demultiplexer using the same select sends its input to output 2, with the others inactive under an active-high convention.
What to watch for
Data-sheet enable signals and output polarity vary. A demultiplexer does not copy the input to every output simultaneously.
Try it yourself
How many select bits are needed for an 8-to-1 multiplexer?
Answer: Three, because 2^3 = 8. Their patterns 000 through 111 select the eight input positions.
How to study this topic
Name each input and define its active level before drawing gates. Follow one data path at a time, then check the other combinations. A complete specification is the best starting point for a reliable circuit.
Explore related programmes
These recorded programmes are upcoming. This article is general educational content, not a syllabus or course sample.
Digital Electronics