The Demultiplexer |
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The Demultiplexer
The data distributor, known more commonly as a Demultiplexer or "Demux", is the
exact opposite of the Multiplexer
we saw in the previous tutorial. The demultiplexer takes one single input data line and then switches it to any one
of a number of individual output lines one at a time. The demultiplexer converts a serial data
signal at the input to a parallel data at its output lines as shown below.
1-to-4 Channel De-multiplexer
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| Addressing | Input
Selected |
| b | a |
| 0 | 0 | A |
| 0 | 1 | B |
| 1 | 0 | C |
| 1 | 1 | D |
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The Boolean expression for this 1-to-4 Demultiplexer above with outputs
A to D and data select lines a, b
is given as:
F = ab
A + abB +
abC + abD
The function of the Demultiplexer is to switch one common data input line
to any one of the 4 output data lines A to D in our example
above. As with the multiplexer the individual solid state switches are selected by the binary input address code
on the output select pins "a" and "b" and by adding more address
line inputs it is possible to switch more outputs giving a 1-to-2n data line outputs. Some standard
demultiplexer ICīs also have an "enable output" input pin which disables or prevents the input from being passed
to the selected output. Also some have latches built into their outputs to maintain the output logic level after
the address inputs have been changed. However, in standard decoder type circuits the address input will determine
which single data output will have the same value as the data input with all other data outputs having the value
of logic "0".
The implementation of the Boolean expression above using individual logic gates would require
the use of six individual gates consisting of AND and NOT
gates as shown.
4 Channel Demultiplexer using Logic Gates
The symbol used in logic diagrams to identify a demultiplexer is as follows.
Demultiplexer Symbol
Standard Demultiplexer IC packages available are the TTL 74LS138 1 to 8-output
demultiplexer, the TTL 74LS139 Dual 1-to-4 output demultiplexer or the CMOS CD4514 1-to-16 output demultiplexer.
Another type of demultiplexer is the 24-pin, 74LS154 which is a 4-bit to 16-line demultiplexer/decoder. Here the
individual output positions are selected using a 4-bit binary coded input. Like multiplexers, demultiplexers can
also be cascaded together to form higher order demultiplexers.
Unlike multiplexers which convert data from a single data line to multiple lines and demultiplexers
which convert multiple lines to a single data line, there are devices available which convert data to and from multiple
lines and in the next tutorial about combinational logic devices, we will look at
Encoders which convert multiple
input lines into multiple output lines, converting the data from one form to another.
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