A two-input multiplexer is shown below.
  |  | The multiplexer circuit is typically used to combine two or more digital signals onto a single line, by placing them there at different times. Technically, this is known as time-division multiplexing. Input A is the addressing input, which controls which of the two data inputs, X0 or X1, will be transmitted to the output. If the A input switches back and forth at a frequency more than double the frequency of either digital signal, both signals will be accurately reproduced, and can be separated again by a demultiplexer circuit synchronized to the multiplexer. This is not as difficult as it may seem at first glance; the telephone network combines multiple audio signals onto a single pair of wires using exactly this technique, and is readily able to separate many telephone conversations so that everyone's voice goes only to the intended recipient. With the growth of the Internet and the World Wide Web, most people have heard about T1 telephone lines. A T1 line can transmit up to 24 individual telephone conversations by multiplexing them in this manner. |
A very common application for this type of circuit is found in computers, where dynamic memory uses the same address lines for both row and column addressing. A set of multiplexers is used to first select the row address to the memory, then switch to the column address. This scheme allows large amounts of memory to be incorporated into the computer while limiting the number of copper traces required to connect that memory to the rest of the computer circuitry. In such an application, this circuit is commonly called a data selector.
Multiplexers are not limited to two data inputs. If we use two addressing inputs, we can multiplex up to four data signals. With three addressing inputs, we can multiplex eight signals. If you would like to see a demonstration of a four-input multiplexer, you can follow this link. This demonstration requires 64 separate images, each approximately 4K bytes in size, so it will take a little while to load. For this reason, it is not included in the list of digital pages at the top of each page. An eight-input multiplexer would require either 2048 separate images or a rather complex implementation of dynamic HTML; therefore it will not be included on these pages.
The opposite of the multiplexer circuit, logically enough, is the demultiplexer. This circuit takes a single data input and one or more address inputs, and selects which of multiple outputs will receive the input signal. The same circuit can also be used as a decoder, by using the address inputs as a binary number and producing an output signal on the single output that matches the binary address input. In this application, the data input line functions as a circuit enabler — if the circuit is disabled, no output will show activity regardless of the binary input number.
A one-line to two-line decoder/demultiplexer is shown below.
|  | This circuit uses the same AND gates and the same addressing scheme as the two-input multiplexer circuit shown in these pages. The basic difference is that it is the inputs that are combined and the outputs that are separate. By making this change, we get a circuit that is the inverse of the two-input multiplexer. If you were to construct both circuits on a single breadboard, connect the multiplexer output to the data IN of the demultiplexer, and drive the (A)ddress inputs of both circuits with the same signal, you would find that the initial X0 input would be transmitted to OUT0 and the X1 input would reach only OUT1. The one problem with this arrangement is that one of the two outputs will be inactive while the other is active. To retain the output signal, we need to add a latch circuit that can follow the data signal while it's active, but will hold the last signal state while the other data signal is active. An excellent circuit for this is the D (or Data) Latch. By placing a latch after each output and using the Addressing input (or its inverse) to control them, we can maintain both output signals at all times. If the Address input changes much more rapidly than the data inputs, the output signals will match the inputs faithfully. |
Like multiplexers, demultiplexers are not limited to two data signals. If we use two addressing inputs, we can demultiplex up to four data signals. With three addressing inputs, we can demultiplex eight signals. The demonstration of the 2-to-4 line decoder/demultiplexer is much smaller than the demo for the four-input multiplexer, because it has fewer independent input signals. With one data input and two addressing inputs, the decoder/demultiplexer only needs 8 images for the full demonstration.
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