What is a multiplexer, circuits and operating principle. Digital multiplexer: description, purpose, types Application of multiplexers

Computer circuits use a lot of parts that individually seem useless (and in most cases, they are). But once they are collected into a logical system, adhering to the laws of physics, they can turn out to be simply irreplaceable. A good example is multiplexers and demultiplexers. They play an important role in the creation of communication systems. A multiplexer is not difficult. And you will see this for yourself by reading the article.

What is a multiplexer?

A multiplexer is a device that selects one of several inputs and then connects it to its output. It all depends on the state of the binary code. A multiplexer is used as a signal switcher that has multiple inputs and only one output. The mechanism of its operation can be described by the following table:

Similar tables can be seen when studying programming, and more specifically, when solving logical choice problems. First, about the analog multiplexer. They connect inputs and outputs directly. There is an optical multiplexer, which is more complex. They simply copy the resulting values.

What is a demultiplexer?

A demultiplexer is a device with one input and multiple outputs. What will be connected to what is determined by the binary code. To do this, it is read and the output, which has the required value, is connected to the input. As you can see, these devices do not necessarily have to operate in pairs for full operation, and got their name because of the functionality they perform.

Multiplexer circuit

Let's look at the multiplexer circuit. The largest part is the AND-OR element. It can have a different number of inputs, ranging from two and theoretically to infinity. But, as a rule, they are not made for more than 8 inputs. Each individual input is called an inverter. Those on the left are called information. In the middle there are addressable inputs. An element is usually connected to the right, which determines whether the multiplexer itself will work. This can be supplemented with an inversion input. To indicate in writing the number of inputs and to show that this is a multiplexer, entries of this type are used: “1*2”. By unit we mean the number of pins that go into a triple. Two is used to indicate output and is usually equal to 1. Depending on the number of addressable inputs, it is determined what bit the multiplexer will have, and in this case the formula is used: 2 n. Instead of n, just substitute the required value. In this case, 2 2 = 4. If for a binary or ternary multiplexer the difference in the number of inputs and outputs is two and three, respectively, then they are said to be complete. At a lower value they are incomplete. This device has a multiplexer. The diagram is additionally presented in the form of an image so that you have the most complete idea of its structure.

Demultiplexer circuit

For channel switching, demultiplexers use only “AND” logic elements. Keep in mind that CMOS chips are often built using field-effect transistor switches. Therefore, the concept of a demultiplexer does not apply to them. Is it possible to make it so that one device can change its properties to diametrically opposite ones? Yes, if you swap the information outputs and inputs, as a result of which the prefix “de-” can be added to the name “multiplexer”. In their purpose they are similar to decryptors. Despite the existing differences, both devices in domestic microcircuits are designated by the same letters - ID. Demultiplexers perform single-operand (single-input, unitary) logical functions, which have a significant number of possible response options to a signal.

Types of multiplexers

Basically, there are only two types of multiplexers:

Terminal. This type of multiplexers is located at the ends of the communication line through which some data is transmitted.
I/O. They are used as tools that are installed in a communication line gap to remove several channels of information from the general flow. In this way, the need to install terminal multiplexers, which are more expensive mechanisms, is avoided.

Cost of multiplexers

It is worth noting that multiplexers are not a cheap pleasure. The cheapest one at the moment costs more than 12 thousand rubles, the upper limit is 270,000. But even at such prices, they are still almost always more profitable than laying a new line. But such a benefit is present only if there are qualified personnel who can perform the entire scope of work properly and install the multiplexer correctly. The price may increase slightly if there is no full-time specialist. But they can always be hired in specialized companies.

Multiplexing

Multiplexing of signals is carried out due to the significant cost of the communication channels themselves, as well as due to the costs of their maintenance. Moreover, from a purely physical point of view, what is available now is not being used to its full potential. Installing a multiplexer to work in the system is more profitable in monetary terms than organizing a new channel. In addition, you have to spend less time on this process, which also implies certain material benefits.

In this article, we will get acquainted with the operating principle of frequency multiplexing. With it, a separate frequency range is specially allocated for each incoming stream in a common communication channel. And the multiplexer is tasked with transferring the spectrum of each of the incoming spectra to a different range of values. This is done to eliminate the possibility of different channels crossing. To prevent them from becoming an obstacle to one another even when going beyond the allotted limits, they use the technology of protective intervals. It consists in leaving a certain frequency between each channel, which will absorb the impact of malfunctions and will not affect the overall condition of the system. FDMA multiplexing can be used in optical and electrical communication lines.

Limited resources created the opportunity to improve the mechanism. The end result was a process called “time multiplexing.” With this mechanism, a short time period is allocated in the general high-speed stream for the transmission of one input signal. But this is not the only implementation option. It may also be that a certain part of time is allotted, which is repeated cyclically at a given interval. In general, the multiplexer in these cases is faced with the task of providing cyclic access to the data transmission medium, which must be open to incoming flows over short intervals.

Conclusion

A multiplexer is something that expands communication capabilities. The article examined devices used for data transmission, which allow significant savings on this expense item. Their schematic structure and the concept of multiplexing, its features and application were also briefly reviewed. Thus, we have reviewed the theoretical framework. You will need it to move on to practice if you want to explore multiplexers and demultiplexers.

A digital multiplexer is a logical combined device that is designed for controlled transmission of information from several data sources to an output channel. Essentially, this device is a series of digital position switches. It turns out that a digital multiplexer is a switch of input signals into one output line.

This device has three groups of inputs:

address ones, which determine which information input needs to be connected to the output;
informational;
resolving (strobe).

In manufactured digital multiplexers, there are a maximum of 16 information inputs. If the designed device requires a larger number, then the structure of the so-called multiplexer tree is built from several chips.

A digital multiplexer can be used to synthesize almost any logic device, thereby significantly reducing the number of logic elements used in circuits.

Rules for the synthesis of devices based on multiplexers:

a Karnaugh map is constructed for the output function (based on the values of the variable functions);
the order of use in the multiplexer circuit is selected;
a masking matrix is constructed, which must correspond to the order of the multiplexer used;
it is necessary to superimpose the resulting matrix on the Karnaugh map;
after this, the function is minimized separately for each area of the matrix;
Based on the minimization results, it is necessary to construct a circuit.

Now let's move from theory to practice. Let's consider where such devices are used.

Flexible multiplexers are designed for generating digital streams (primary) at a speed of 2048 kbit/s from (speech), as well as data from digital interfaces of cross-connection of electronic channels at a speed of 64 kbit/s, transmitting a digital stream over an IP/Ethernet network and for converting linear signaling and physical joints.

Using such a device, you can connect up to 60 (in some models this figure may be more) analog terminations into 1 or 2 or 128 subscriber sets for four E1 streams. Typically, the analog terminations are TC lines that have in-band signaling, or the signaling is implemented on a separate channel. Voice channel data can be compressed to 32 or 16 kbit/s per channel using ADPCM encoding.

Flexible multiplexers allow the use of broadcast connections, that is, the transmission of signals from one of the digital or analog channels to several others. Often used to supply radio broadcast programs simultaneously to several different points.

Optical multiplexers are devices designed to work with data streams using light beams that differ in amplitude or phase, as well as wavelength. The advantages of such devices include resistance to external influences, technical safety, and protection against hacking of transmitted information.

A multiplexer is a combined digital device, providing alternate transmission of several input signals to one output. It allows you to transmit (switch) a signal from the desired input to the output; in this case, the selection of the required input is realized by a certain combination of control signals. The number of multiplex inputs is usually called the number of channels, they can be from 2 to 16, and the number of outputs is called multiplexer bits, usually 1 - 4.

Based on the method of transmitting signals, multiplexers are divided into:

- analog;

- digital.

Thus, analog devices use a direct electrical connection to connect the input to the output; in this case, its resistance is on the order of several units - tens of Ohms. They are therefore called switches or keys. Digital (discrete) devices do not have a direct electrical connection between input and output; they only copy the signal – “0” or “1” - to the output.

Operating principle of the multiplexer

In general terms, the principle of operation of a multiplexer can be explained using the example of a switch that connects the inputs to the output of a device. The operation of the switch is ensured on the basis of a control circuit in which there are address and enabling inputs. Signals from the address inputs indicate which information channel is connected to the output. Permissive inputs are used to increase capabilities - increasing the bit capacity, synchronizing with the operation of other mechanisms, etc. To create a multiplexer control circuit, an address decoder is usually used.

Scope of application of the multiplexer

Multiplexers are designed for use as a universal logic element when implementing any functions, the number of which is equal to the number of address inputs. They are widely used for the purpose of switching individual buses, outgoing lines or their groups. In microprocessor systems, they are installed on remote objects to implement the possibility of transmitting information over one line from several sensors located at a remote distance from each other. Also, multiplexers in circuit design are used in frequency dividers, when creating comparison circuits, counters, code generators, etc., to transform parallel binary code into serial.

The number of multiplexer channels produced by the domestic industry today is usually 4, 6, 10 and 16. To build circuits with a larger number of inputs, the so-called cascade tree circuit is used, which allows you to create devices with an arbitrary number of input lines based on commercially produced multiplexers.

3.7. Multiplexers and demultiplexers

Multiplexer is a device that samples one of several inputs and connects it to its single output, depending on the state of the binary code. In other words, a multiplexer is a signal switch controlled by a binary code and having several inputs and one output. The input whose number corresponds to the control binary code is connected to the output.

Well, a private definition: multiplexer is a device that converts parallel code into serial code.

The structure of a multiplexer can be represented by various schemes, for example, this one:

Rice. 1 – Example of a specific multiplexer circuit

The largest element here is an AND-OR element with four inputs. Squares with ones are inverters.

Let's look at the conclusions. Those on the left, namely D0-D3, are called information inputs. They are presented with information that must be selected. Inputs A0-A1 are called address inputs. This is where the binary code is supplied, which determines which of the inputs D0-D3 will be connected to the output, designated in this diagram as Y. Input C – synchronization, operation permission.

The diagram also has address inputs with inversion. This is to make the device more versatile.

The figure shows, as it is also called, a 4X1 multiplexer. As we know, the number of different binary numbers that a code can specify is determined by the number of bits of the code as 2 n, where n is the number of bits. You need to set 4 multiplexer states, which means there should be 2 bits in the address code (2 2 = 4).

To explain the principle of operation of this circuit, let's look at its truth table:

This is how the binary code selects the desired input. For example, we have four objects, and they send signals, but we have one display device. We take a multiplexer. Depending on the binary code, a signal from the desired object is connected to the display device.

The multiplexer is designated by the microcircuit as follows:

Rice. 2 – Multiplexer like ISS

Demultiplexer- a device inverse to the multiplexer. That is, the demultiplexer has one input and many outputs. The binary code determines which output will be connected to the input.

In other words, demultiplexer is a device that samples one of its several outputs and connects it to its input, or else it is a signal switch controlled by binary code and having one input and several outputs.

The output whose number corresponds to the state of the binary code is connected to the input. And a private definition: demultiplexer is a device that converts serial code into parallel.

Typically used as a demultiplexer decryptors binary code into positional code, in which an additional gating input is introduced.

Due to the similarity of multiplexer and demultiplexer circuits, CMOS series have microcircuits that are simultaneously a multiplexer and a demultiplexer, depending on which side the signals are supplied from.

For example, K561KP1, operating as an 8x1 switch and a 1x8 switch (that is, as a multiplexer and demultiplexer with eight inputs or outputs). In addition, in CMOS microcircuits, in addition to switching digital signals (logical 0 or 1), it is possible to switch analog ones.

In other words, it is an analog signal switch controlled by a digital code. Such microcircuits are called switches. For example, using a switch you can switch the signals entering the amplifier input (input selector). Consider the input selector circuit UMZCH. Let's build it using flip-flops and a multiplexer.

Rice. 3 - Input selector

So, let's look at the work. On the triggers of the DD1 microcircuit, a ring counter button presses with 2 digits (two triggers - 2 digits). Two-digit binary code goes to the address inputs D0-D1 of the DD2 chip. The DD2 chip is a dual four-channel switch.

In accordance with the binary code to the outputs of the microcircuit A And IN inputs A0-A3 and B0-B3 are connected respectively. Elements R1, R2, C1 eliminate bouncing of the button contacts.

Differentiation chain R3C2 sets the flip-flops to zero when power is turned on, with the first input connected to the output. When you press the button, trigger DD1.1 switches to the log state. 1 and the second input is connected to the output, etc. The inputs are enumerated in a ring, starting from the first.

On the one hand it’s simple, on the other it’s a little inconvenient. Who knows how many times the button was pressed after turning it on and which input is connected to the output now. It would be nice to have an indicator for the connected input.

Let's remember the seven-segment decoder. We transfer the decoder with the indicator to the switch circuit and connect the first two inputs of the decoder (in the diagram as DD3), i.e. 1 and 2 (pins 7 and 1) to the direct outputs of the triggers DD1.1 DD1.2 (pins 1 and 13) . We connect decoder inputs 4 and 8 (pins 2 and 6) to the housing (i.e., we supply logic 0). The indicator will show the status of the ring counter, namely numbers from 0 to 3. Number 0 corresponds to the first input, 1 to the 2nd, etc.

Multiplexers and demultiplexers belong to the class of combinational devices that are designed to switch data streams in communication lines at given addresses. Most data in digital systems is transmitted directly through wires and traces on printed circuit boards. Often there is a need to transmit information binary signals (or analog in analog-to-digital systems) from the signal source to consumers. In some cases, it is necessary to transmit data over long distances over telephone lines, coaxial and optical cables. If all data were transmitted simultaneously over parallel communication lines, the total length of such cables would be too long and they would be too expensive. Instead, data is transmitted over a single wire in serial form and is grouped into parallel data at the receiving end of that single communication line. Devices used to connect one of the data sources with a given number (address) to a communication line are called multiplexers. Devices used to connect a communication line to one of the information receivers with a specified address are called demultiplexers. Parallel data from one of the digital devices can be converted into serial information signals using a multiplexer, which are transmitted over one wire. At the outputs of the demultiplexer, these serial input signals can be grouped back into parallel data.

Multiplexers

The multiplexer is used to combine digital streams from various sources into a single transport stream.– compression encoders, outputs of other multiplexers, outputs of receivers – decoders, etc. Incoming signals can have a different time base (that is, they can be generated with slightly different clock frequencies), and the task of the multiplexer is to form an asynchronous stream while maintaining the synchronizing information of each of the components.

The operating principle of a multiplexer is based on the properties of a memory buffer - information is written to it at one clock frequency and read at another, higher frequency. If we imagine a chain of sequentially connected buffers, synchronized in such a way that the output bursts of pulses do not overlap in time, this will be a multiplexer.

The main parameter of the multiplexer is the output speed of the transport stream, which for most models is 55...60 Mbit/s. There are also samples with speeds up to 100 Mbit/s. Of course, the flow rate set at the output must be at least not lower than the sum of the speeds of all combined flows. Exceeding the speed of the output stream is compensated by introducing zero packets at the output of the multiplexer.

A demultiplexer is a functional unit of a computer designed to switch (switch) the signal of a single information input D to one of n information outputs. The number of the output to which the value of the input signal is supplied at each cycle of computer time is determined by the address code A0, A1..., Am-1. Address inputs m and information outputs n are related by the relation n2m. A DC decoder can be used as a demultiplexer. In this case, the information signal is supplied to the permission input E (from the English enable - permission). A gated demultiplexer with information input D, address inputs A1, A0 and gate input C is shown in Figure 2.1. A demultiplexer performs the inverse function of a multiplexer. In relation to multiplexers and demultiplexers, the term “data selectors” is also used.

Demultiplexers are used for switching individual lines and multi-bit buses, converting serial code into parallel. Like a multiplexer, a demultiplexer includes an address decoder. The decoder signals control logic gates, allowing the transfer of information through only one of them (Fig. 1.1)