Multiplexers and Decoders

A multiplexer has multiple data inputs and a single data output. The Selector Inputs select which input drives the output. 

A line decoder has multiple output lines and no data input. The Selector Inputs select which output line is High, in the case of a non-inverting decoder, or Low for an inverting decoder.

A de-multiplexer has multiple data outputs and a single data input. The Selector Inputs select which output is driven by the data input.

Multiplexer (aka Data Selector)

The 74HC251 is an example of a multiplexer. It is an 8-to-1 multiplexer meaning it has 8 data inputs and 1 data output. The 3 Selector Inputs are used to select which data input is driving the output (2^3 = 8 options).

• D0-7 - 8 Data inputs
• S0-2 - 3 Selector Inputs
• OE' - Enable output when LOW. When HIGH both outputs are in the high impedance state. 
• Y - multiplexer output
• Y' - complementary multiplexer output (Y output inverted)

 A useful multiplexer is the 74HC157 Quad 2 Input Data Selector. This has 2 groups of 4 data inputs and 4 data outputs. The input group selected for output is controlled by a single selector S0. The 74HC157 is useful for selecting one of two registers as the input onto a bus.

• A0-3 - input A
• B0-3 - input B
• S0 - selector. Select input A or B
• G - output input when LOW. When HIGH all outputs are forced to LOW.

The 74HC153 is a dual 4 to 1 multiplexer with normal outputs. 
The 74HC253 is a dual 4 to 1 multiplexer similar to the 153 but with three state outputs.

Decoder / De-multiplexers

If a Decoder has a data input it is a de-multiplexer. A de-multiplexer can be used as a decoder by tieing the data input to Low or High. These devices are often referred to as 'Decoder/de-multiplexers'

The 74HC238 is an example of a decoder. It is a 3-to-8 decoder meaning it has 3 Selector Inputs and 8 outputs. The 3 Selector Inputs are used to control which of the 8 outputs is HIGH.

The 74HC238 has three output enable inputs two active LOW (G2A, G2B) and one active HIGH (G1). All three must be enabled otherwise every one of the 8 outputs will be LOW. The purpose of these output enable inputs is to allow 74HC238 devices to be cascaded together to create a larger decoder e.g. two can be combined to create a 4-to-16 decoder.

• A, B, C - inputs
• D0-7 - outputs
• G1 - output enable. Active HIGH
• G2A,B - output enable. Active LOW

The diagram below shows how two decoders can be cascaded together

A fourth input, D, is created by connecting the D input to G2A on the first 74HC238 and G1 on the second. When D is LOW all the outputs on the second 74HC2387 will be forced LOW as G1 must be HIGH to enable the output. Likewise when the D input is HIGH all the outputs on the first 74HC238 will be forced LOW as G2A must be LOW to enable the output.







74HC154 is an inverting 4-to-16 decoder/de-multiplexer.

74HC4514 is a non-inverting 4-to-16 deocder/de-multiplexer with input latches. 
74HC4515 is an inverting 4-to-16 decoder/de-multiplexer wtih input input latches.


[Note the 4514 DIP comes in wide and narrow body packages. The narrow body package is 5mm in width and in EagleCAD the library part is 74ttl-din > 744514N. The wide body package is 0.5inch (1.27cm) in width and in EagleCAD the library part is 45xx > 4514 > 4514N]