Multiplexer & Decoder

The Multiplexer

A data selector, more commonly called a Multiplexer, shortened to "Mux" or "MPX", are combinational logic switching devices that operate like a very fast acting multiple position rotary switch. They connect or control, multiple input lines called "channels" consisting of either 2, 4, 8 or 16 individual inputs, one at a time to an output.

Then the job of a "multiplexer" is to allow multiple signals to share a single common output. For example, a single 8-channel multiplexer would connect one of its eight inputs to the single data output. Multiplexers are used as one method of reducing the number of logic gates required in a circuit or when a single data line is required to carry two or more different digital signals.

Generally, multiplexers have an even number of data inputs, usually an even power of two, n² , a number of "control" inputs that correspond with the number of data inputs and according to the binary condition of these control inputs, the appropriate data input is connected directly to the output. An example of a Multiplexer configuration is shown below.

4-to-1 Channel Multiplexer

 

Addressing		Input Selected
B	a
0	0	A
0	1	B
1	0	C
1	1	D

The Boolean expression for this 4-to-1 Multiplexer above with inputs A to D and data select lines a, b is given as:

Q = abA + abB + abC + abD

 

In this example at any one instant in time only ONE of the four analogue switches is closed, connecting only one of the input lines A to D to the single output at Q. As to which switch is closed depends upon the addressing input code on lines "a" and "b", so for this example to select input B to the output at Q, the binary input address would need to be "a" = logic "1" and "b" = logic "0". Adding more control address lines will allow the multiplexer to control more inputs but each control line configuration will connect only ONE input to the output.

Then the implementation of this Boolean expression above using individual logic gates would require the use of seven individual gates consisting of AND, OR and NOT gates as shown.

4 Channel Multiplexer using Logic Gates

The symbol used in logic diagrams to identify a multiplexer is as follows.

Multiplexer Symbol

 

Multiplexers are not limited to just switching a number of different input lines or channels to one common single output. There are also types that can switch their inputs to multiple outputs and have arrangements or 4 to 2, 8 to 3 or even 16 to 4 etc configurations and an example of a simple Dual channel 4 input multiplexer (4 to 2) is given below:

4-to-2 Channel Multiplexer

Here in this example the 4 input channels are switched to 2 individual output lines but larger arrangements are also possible. This simple 4 to 2 configuration could be used for example, to switch audio signals for stereo pre-amplifiers or mixers.

 

The Demultiplexer

The data distributor, known more commonly as a Demultiplexer or "Demux", is the exact opposite of the Multiplexer . 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

 

Addressing		Input Selected
B	A
0	0	A
0	1	B
1	0	C
1	1	D

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.

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.

 

The Demultiplexer Symbol

 

The Digital Encoder

Unlike a multiplexer that selects one individual data input line and then sends that data to a single output line or switch, a Digital Encoder more commonly called a Binary Encoder takes ALL its data inputs one at a time and then converts them into a single encoded output. So we can say that a binary encoder, is a multi-input combinational logic circuit that converts the logic level "1" data at its inputs into an equivalent binary code at its output.

Generally, digital encoders produce outputs of 2-bit, 3-bit or 4-bit codes depending upon the number of data input lines. An "n-bit" binary encoder has 2ⁿ input lines and n-bit output lines with common types that include 4-to-2, 8-to-3 and 16-to-4 line configurations. The output lines of a digital encoder generate the binary equivalent of the input line whose value is equal to "1" and are available to encode either a decimal or hexadecimal input pattern to typically a binary or B.C.D. output code.

4-to-2 Bit Binary Encoder

Priority Encoder

The Priority Encoder solves the problems mentioned above by allocating a priority level to each input. The priority encoders output corresponds to the currently active input which has the highest priority. So when an input with a higher priority is present, all other inputs with a lower priority will be ignored. The priority encoder comes in many different forms with an example of an 8-input priority encoder along with its truth table shown below.

8-to-3 Bit Priority Encoder

 

Binary Decoder

A Decoder is the exact opposite to that of an "Encoder". It is basically, a combinational type logic circuit that converts the binary code data at its input into one of a number of different output lines, one at a time producing an equivalent decimal code at its output. Binary Decoders have inputs of 2-bit, 3-bit or 4-bit codes depending upon the number of data input lines, and a n-bit decoder has 2ⁿ output lines.

Therefore, if a binary decoder receives n inputs (usually grouped as a binary or Boolean number) it activates one and only one of its 2ⁿ outputs based on that input with all other outputs deactivated. A decoders output code normally has more bits than its input code and practical "binary decoder" circuits include, 2-to-4, 3-to-8 and 4-to-16 line configurations.

 

 

 

A 2-to-4 Binary Decoders.

 

In this simple example of a 2-to-4 line binary decoder, the binary inputs A and B determine which output line from D0 to D3 is "HIGH" at logic level "1" while the remaining outputs are held "LOW" at logic "0" so only one output can be active (HIGH) at any one time.

74LS138 Binary Decoder

Some binary decoders have an additional input labelled "Enable" that controls the outputs from the device. This allows the decoders outputs to be turned "ON" or "OFF" and we can see that the logic diagram of the basic decoder is identical to that of the basic demultiplexer.

Then, we can say that a binary decoder is a demultiplexer with an additional data line that is used to enable the decoder. An alternative way of looking at the decoder circuit is to regard inputs A, B and C as address signals. Each combination of A, B or C defines a unique address which can access a location having that address.

Sometimes it is required to have a Binary Decoder with a number of outputs greater than is available, or if we only have small devices available, we can combine multiple decoders together to form larger decoder networks as shown. Here a much larger 4-to-16 line binary decoder has been implemented using two smaller 3-to-8 decoders.

A 4-to-16 Binary Decoder Configuration.

Inputs A, B, C are used to select which output on either decoder will be at logic "1" (HIGH) and input D is used with the enable input to select which encoder either the first or second will output the "1".