De-multiplexers |
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The De-multiplexer
De-multiplexers or "De-muxes", are the exact opposite of the
Multiplexers we saw in the previous
tutorial in that they have one single input data line and then switch it to any one of their individual multiple output
lines one at a time. The De-multiplexer converts the 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 | Output
Selected |
| b | a |
| 0 | 0 | A |
| 0 | 1 | B |
| 1 | 0 | C |
| 1 | 1 | D |
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The Boolean expression for this De-multiplexer is given as:
F = ab
A + abB +
abC + abD
The function of the De-multiplexer 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 lines output. Some standard De-multiplexer 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".
Standard De-multiplexer IC packages available are the TTL 74LS138 1 to 8-output De-multiplexer,
theTTL 74LS139 Dual 1 to 4-output De-multiplexer or the CMOS CD4514 1 to 16-output De-multiplexer.
Another type of De-multiplexer is the 24-pin, 74LS154 which is a 4-bit to 16-line De-multiplexer/decoder. Here the output
positions are selected using the 4-bit binary coded input.
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