Multiplexer

A multiplexer (abbreviated MUX) is a selection circuit at the analog and digital electronics, it can be one selected from a plurality of input signals, and is switched through to the output with the. Multiplexer are similar to rotary switches, which are made by hand, but with electronic signals. The difference to the relay is that the connections are not mechanically, but (now ) come with integrated semiconductor circuits about.

With cyclic run parallel data streams can be converted into a serial multiplexer. Also can be implemented with a multiplexer or a switching function to each possible switch state. For signal transmission with optical fibers there are optical multiplexer and demultiplexer that work with optical switches and also for wavelength -division multiplexing with wavelength-selective elements. The counterpart to the multiplexer, demultiplexer, with the summarized data channels are separated again. Analog multiplexer operate bidirectionally, that is, they can be used also as a demultiplexer.

In addition to several inputs and one output, a multiplexer has one or more control signals is determined by the which input is selected. It is one input to the output is turned on, which has the identifier that is applied as a control signal in the form of a binary number. A parallel of driven multiplexer with the name key n- MUX control signals, for example, n, 2n inputs and one output. The inputs are usually numbered with the numbers 0 to 2n -1.

Designations

In satellite technology MUX called a multiplexer or demultiplexer. IMUX (input multiplexer ) is at the entrance behind a receiving antenna technically a demultiplexer, according to a OMUX at the output before transmitting antenna, a multiplexer. For video formats, a multiplexer ( mux-er ) is used to merge video tracks, audio tracks, subtitles and menu structures in a data stream.

In communications engineering, a multiplexer means a device, the data and / or voice channels summarizes and transmits on a shared line. Since the data both sent, as will also receive is usually also required a demultiplexer ( PCM30 ). The signals to be multiplexed can be analog or digital words themselves, but the control is performed always by digital signals, which act as further inputs.

Multiplexer for digital signals, logic gates,

Simply multiplexer

The simplest case is the two -input multiplexer ( also single multiplexer short " 1 MUX ", see Figure 1), which has a control signal s0, e0 and e1 2 inputs and one output a. If the control signal s0 a 1, so the output provides a signal that is present at the input e1, otherwise the input of e0.

Instead of the more general term called MUX X / Y is used for a transcoder in data sheets usually. In addition, however, the term MUX maintained as it is unique.

Dual - and m- multiplexer

Figure 2a shows the recursive construction of a dual multiplexer (short: "2- MUX " ) from 1- muxing. Analog MUXs you can build with even more control signals and correspondingly more inputs. In this case, one needs 2m-1 MUXs each m control signals for the design of a M- MUX. The number of inputs and the cost of a multiplexer that is increasing exponentially with the number of its control signals.

Multiplexer having many control signals have a high number of gate stages, resulting in high running time.

The switching function of this 2- MUX is:

The switching function of this 2- MUX is:

Example

Given a switching function f ( s3, s2, s1, s0 ) which is exactly 1 if the binary number [ s3s2s1s0 ] 2 is a prime number. It must about F ( 0, 0, 1, 1 ) may be = 1, since the binary number 0011 the decimal 3 corresponds to, and 3 is a prime number (as the 1 is not a prime, should the logic for 0 0 0 1 at the output a a 0 will follow).

The function f satisfies the following truth table:

This switching function is to be implemented with a 4- MUX. The voltages on the inputs of the 4- MUX bits you can read this, from the Result column of the truth table of f. Thus, the 4- MUX must be connected as follows:

But it is also possible to implement the same function with a 3- MUX. The problem is that the function f has four parameters, but only three control signals are available. Dissolving it by expressing the function value of a function of S3.

Thus, the following truth table is created:

The 3- MUX is so connected as follows:

In CMOS technology, multiplexers are in this case designed with both digital logic gates, as well as with analog switches ( transmission gate ). Those type is also called Analog-Multiplexer/Demultiplexer.

When using transmission gates, the selected logic level is not copied to the output, but actually created a direct conductive connection between the signal input and output. This has the advantage that the multiplexer can also be passed through analog signals. In addition, the signal flow direction is not specified, the multiplexer always acts simultaneously as a demultiplexer. Third, there is no Durchleitverzögerung of the signal as it would be inevitable when passing through a logic path. A disadvantage has to be considered that the signal is not the same amplified by the circuit when the logic blocks, but (usually 50 ohms) is even alleviated by the longitudinal resistance of the analog switch.

The OR operation on the output can also be realized by a wired- OR operation. Will you thereby can prevent the long rise time at the output you also tri- state gates connecting to the output. This solution is, however, not be used in integrated circuits other than in bus systems in which the signal sources are spatially separated.

Multiplexer modules

Multiplexers are commercially available as a pre- IC components. The main types are summarized in the following table: