Encoders and Decoders; Multiplexers and Demultiplexers

Encoders and Decoders

Multiplexing is defined as the process of feeding several independent signals to a common load, one at a time. The device or switching circuitry used to select and connect one of these several signals to the load at any one time is known as a multiplexer.

The reverse function of multiplexing, known as demultiplexing, pertains to the process of feeding several independent loads with signals coming from a common signal source, one at a time. A device used for demultiplexing is known as a demultiplexer.

Multiplexing and demultiplexing, therefore, allow the efficient use of common circuits to feed a common load with signals from several signal sources, and to feed several loads from a single, common signal source, respectively.

In digital circuits, the term 'multiplexing' is also sometimes used to refer to the process of encoding, which is basically the generation of a digital code to indicate which of several input lines is active. An encoder or multiplexer is therefore a digital IC that outputs a digital code based on which of its several digital inputs is enabled.

On the other hand, the term 'demultiplexing' in digital electronics is also used to refer to 'decoding', which is the process of activating one of several mutually-exclusive output lines, based on the digital code present at the binary-weighted inputs of the decoding circuit, or decoder. A decoder or demultiplexer is therefore a digital IC that accepts a digital code consisting of two or more bits at its inputs, and activates or enables one of its several digital output lines depending on the value of the code.

Multiplexing and demultiplexing are used in digital electronics to allow several chips to share common signal buses. In demultiplexers, for instance, the output lines may be used to enable memory chips that share a common data bus, ensuring that only one memory chip is enabled at a time in order to prevent data clashes between the chips.

If a demultiplexer or decoder has 2^N output lines, then it has N input lines. A common example of a decoder/demultiplexer IC is the 74LS138, which is a Low-Power Schottky TTL device that has 3 input lines and 8 output lines. Of course, a decoder IC such as the 74LS138 also has chip control lines that need to be 'enabled' for the decoding function to take place.

In the case of the 74LS138, these control lines consist of one active high control line (G1, pin 6) and two active-low control lines (G2A, pin 4 and G2B, pin 5). Thus, the 74LS138 will only be in its 'decoding' mode if G1 is at logic '1' and G2A and G2B are at logic '0'. The 74LS138, whose generic product name is '3-to-8 Line Decoder/Multiplexer', obeys the truth table shown in Table 1. The outputs of the 74LS138 are 'active-low', i.e., the enabled output goes to logic '0' while all the other outputs are at logic '1'.

Table 1. Truth Table for the 74LS138, a 3-to-8 Line Decoder

G2a

G2b

If a multiplexer or encoder has N output lines, then it has 2^N input lines. A common example of a decoder/demultiplexer IC is the 74LS148, which is a Low-Power Schottky TTL device that has 8 input lines and 3 output lines. The 74LS148 is a priority encoder, which means that if more than one of its inputs are active, then the active input line with the highest binary weight will be given priority, and the output of the encoder will depend on this prioritized input. Table 2 shows the truth table for the 74LS148. Note that E0 and GS are output pins while E1 is a control pin (input).

Table 2. Truth Table for the 74LS148, an 8-to-3 Line Priority Encoder

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