Multiplexers and demultiplexers belong to the class of combinational devices that are designed to switch data streams in communication lines at given addresses. Most of the data in digital systems transmitted directly through wires and conductors 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 specified address, are called demultiplexers. Parallel data of one of digital devices using a multiplexer can be converted into serial information signals that are transmitted over one wire. At the outputs of the demultiplexer, these serial input signals can be grouped back into parallel data.
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 regrouped 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 may have a different time base (that is, 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 the multiplexer is based on the properties of a memory buffer - information is written to it from one clock frequency, but is read at a different, 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)
A multiplexer is a device that samples one of several inputs and connects it to its output. The multiplexer has several information inputs (D 0, D 1, ...), address inputs (A 0 A 1, ...), an input for supplying a strobe signal C and one output Q. In Fig. 1,f shows a symbolic image of a multiplexer with four information inputs.
Each information input of the multiplexer is assigned a number called an address. When a strobe signal is applied to input C, the multiplexer selects one of the inputs, the address of which is specified by a binary code at the address inputs, and connects it to the output.
Thus, by supplying the addresses of various information inputs to the address inputs, it is possible to transmit digital signals from these inputs to the output Q. Obviously, the number of information inputs n inf and the number of address inputs n adr are related by the relation n inf = 2 nadr.
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Table 1 |
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Addressinputs |
Strobesignal |
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The operation of the multiplexer is determined by table. 1. In the absence of a strobe signal (C = 0), there is no connection between the information inputs and the output (Q = 0). When a strobe signal (C = l) is applied, the logical level of that of the information inputs D i whose number i in binary form is specified at the address inputs is transmitted to the output. So, when setting the address A l A 0 = ll 2 = 3 10, an information input signal with address 3 10, i.e. D 3, will be transmitted to output Q.
Using this table, we can write the following logical expression for output Q:
The circuit diagram of a multiplexer constructed using this expression is shown in Fig. 1, b.
In cases where it is necessary to transmit multi-bit input data to the outputs in parallel form, parallel connection of multiplexers is used according to the number of bits of the transmitted data.
Using multiplexers for the synthesis of combinational devices.
Multiplexers can be used to synthesize logic functions. In this case, the number of elements (integrated circuit packages) used in the circuit can be significantly reduced.
The multiplexer's Boolean expression contains members with all combinations of address variables. Consequently, if it is necessary to synthesize a function of three variables f(x 1, x 2, x 3), then two of these variables (for example, x 1, x 2) can be supplied to the address inputs A 1, and A 0, and the third x 3 - to the information input.
For example, suppose you need to synthesize a function given in table. 2. Logical function expression
Considering the variables x l, x 2 as address variables, we obtain table. 3, from which it can be seen that the multiplexer at output Q implements a given logical function. Schematic diagram shown in Fig. 2.
Obviously, any function of three variables can be synthesized on four-input multiplexers, any function of four variables on eight-input multiplexers, etc.
When synthesizing combinational circuits, multiplexers can be used in conjunction with elements of a certain basis. Let the total number variable functions n. Then, if the multiplexer has n addresses of address inputs, then n addresses of variables are supplied to them, and to its information inputs functions n-n variable address.
For example, suppose you want to synthesize a four-variable logic function using a four-input multiplexer. If the address variables are x 1, x 2, then the functions of the variables x 3 and x 4, defined as shown in the table, must be supplied to the information inputs of the multiplexer. 5 areas of the Veitch table. Within each area of the Veitch table outlined for information inputs, minimization is carried out using conventional methods, after which circuits are constructed that form the functions supplied to the information inputs of the multiplexer.
Let us demonstrate this technique using the implementation of the function given in Table. 6.
When the variables x 1 and x 2 are supplied to the address inputs of the multiplexer, D 0 = 1 must be supplied to its information inputs; D 1 = 0; D2 = x3. 4, D 3 = 4. The circuit that implements the given function is shown in Fig. 3.
It should be borne in mind that when synthesizing a logic device using a multiplexer, it is also necessary to construct a version of the circuit without using a multiplexer. Then, by comparing the resulting options, determine which option is the best in terms of the number of integrated circuit packages used in the circuit.
A multiplexer is a switch that connects multiple inputs to a single output according to a given digital code. In fact, there are two types of multiplexers: analog and digital, analog are built on field effect transistors and pass the signal in both directions, while digital ones from the selected input duplicate the signal to the output. Next we will talk about the analog multiplexer.
The channel selection, as described above, is carried out according to the specified digital code, as shown in the picture below.
Let's imagine the following situation: we have an ADC and several analog sensors, information from which it must process. Since there is only one ADC, and there are many sensors, he can only serve them one by one, and a multiplexer will help him with this.
Using a conventional voltage divider and multiplexer, you can attenuate the signal by required quantity once.
And by adding a multiplexer and several resistors in feedback An amplifier built on an op-amp can amplify the signal the required number of times.
In the pictures above, the multiplexer was depicted schematically for better perception, but in the diagram it is depicted like this.
Now that we know where a multiplexer is used, let's look at how it differs from a switch.
First, modern multiplexers are built using CMOS technology and, as a result, the open channel has some resistance, the value of this resistance can be less than 1 Ohm and depends on the value of the supply voltage. The channel resistance can be found in the datasheet; it is designated Ron.
Second, the voltage that the multiplexer can switch, as well as the voltage at the control inputs, should not exceed the supply voltage. The maximum switching current of modern multiplexers can reach 400mA. Again, the maximum current can be found out from the datasheet; it is indicated differently in different datasheets.
Third, since the multiplexer is built using CMOS technology, its structure contains capacitances that worsen its characteristics. The equivalent circuit of a two-channel multiplexer is as follows.
- The picture shows that between the channels there is some capacitance Css and Cdd, through which a signal from one channel can penetrate into another.
- The presence of capacitance Cds leads to the fact that at high frequencies the signal passes through an open switch.
- Resistance Ron, together with capacitance Cd, forms a low-pass filter that limits the passband.
The equivalent circuit also shows current sources that reflect leakage current, which in turn can be a source of error.
Multiplexers are devices that allow you to connect multiple inputs to one output. are called devices that allow you to connect one input to several outputs. In the simplest case, such switching can be done using keys:
Figure 1. Switch (multiplexer) assembled using keys
Such a switch will work equally well with both analog and digital signals. However, the speed of operation of mechanical keys leaves much to be desired, and the keys often have to be controlled automatically using some kind of circuit.
Digital circuits require switches to be controlled using logic levels. That is, you need to select a device that could perform the functions of an electronic key with electronic control of a digital signal.
Features of constructing multiplexers on TTL elements
Let's try to make the ones we already know work as an electronic key. Let's consider the truth table of the logical element "AND". In this case, one of the inputs of the “AND” logical element will be considered as an information input of the electronic key, and the other input as a control one. Since both inputs of the AND gate are equivalent, it does not matter which of them will be the control input.
Let input X be the control one, and Y be the information one. For simplicity of reasoning, we divide the truth table into two parts depending on the level of the logical signal at the control input X.
The truth table clearly shows that while a zero logical level is applied to the control input X, the signal applied to the Y input does not pass to the Out output. When a logical one is applied to the control input X, the signal received at the Y input appears at the Out output.
This means that the AND gate can be used as an electronic key. In this case, it does not matter which of the inputs of the “AND” element will be used as a control input, and which one will be used as an information input. All that remains is to combine the outputs of the “AND” logic elements into one output. This is done using the OR gate in the same way as with . The resulting switch with logical level control is shown in Figure 2.
Figure 2. Schematic diagram of a digital multiplexer made on logical elements
In the circuits shown in Figures 1 and 2, you can simultaneously connect several inputs to one output. However, this usually leads to unpredictable consequences. In addition, to control such a switch, many inputs are required, so the multiplexer usually includes a binary one, as shown in Figure 3. We obtained this decoder earlier using. This allows you to control the switching of information inputs using binary codes supplied to the control inputs. The number of information inputs in such circuits is chosen to be a multiple of the power of two.
Figure 3. Schematic diagram of a binary code controlled multiplexer
The graphical designation of a four-input multiplexer with binary control is shown in Figure 4. Inputs A0 and A1 are the control inputs of the microcircuit in question, determining the address of the input signal that will be connected to output Y. The input signals themselves are designated as X0, X1, X2 and X3.
Figure 4. Conventionally graphic designation of a four-input multiplexer
In the conventional graphic designation, the names of information inputs A, B, C and D are replaced by the names X0, X1, X2 and X3, and the name of the output Out is replaced by the name Y. This name for inputs and outputs is more common in domestic literature. The address inputs are designated A0 and A1.
Features of constructing multiplexers based on CMOS elements
When working with an electronic switch, it is very easy to obtain one or two MOS transistors, therefore, in CMOS circuits, the “AND” logic element is not used as an electronic key. The circuit of an electronic switch made on complementary MOS transistors is shown in Figure 5.
Figure 5. Circuit diagram of an electronic key made on CMOS transistors
Such a key can switch both digital and analog signals. The resistance of open transistors is tens of ohms, and the resistance of closed transistors exceeds tens of megohms. There are both advantages and disadvantages to this. The fact that the switch assembled on a MOS transistor is not ordinary logical element, allows you to combine the outputs of electronic keys in exact accordance with the diagram shown in Figure 1. This clearly simplifies the device circuit.
In addition, a CMOS multiplexer can be used to switch analog signals. However, one should not forget that the circuit cannot withstand negative voltages. This means that for analog signals it is necessary to use a bias circuit so that the values analog signal were in the range from the potential of the common wire of the circuit to the supply voltage of the multiplexer.
At the same time, when working with a multiplexer assembled on CMOS switches, you have to install logic elements at its input and output. Only in this case digital circuit will generally function correctly. It should be noted that in most cases this condition is satisfied automatically.
Now remember that in a multiplexer you only need to connect one of the input signals to the output. Exactly the same as for control electronic keys A decoder is introduced into the multiplexer using a binary code. The diagram of such a multiplexer is shown in Figure 6.
Figure 6. CMOS multiplexer circuit
The graphical designation of multiplexers does not depend on the manufacturing technology of the microcircuits, that is, a CMOS multiplexer is designated exactly the same as shown in Figure 4.
In domestic microcircuits, multiplexers are designated by the letters KP, immediately following the series number of the microcircuits. For example, the K1533KP2 chip is a dual four-channel multiplexer made using TTL technology, and the K1561KP1 chip is a dual four-channel multiplexer made using CMOS technology.
Literature:
Along with the article "Multiplexers" read:
The laws of logical algebra allow you to transform logical functions. Logic functions are transformed in order to simplify them, and this leads to a simplification of the digital circuit...
http://site/digital/AlgLog.php
Any logic circuit without memory is completely described by a truth table... To implement a truth table, it is enough to consider only those rows...
http://site/digital/SintSxem.php
Decoders (decoders) allow you to convert some types of binary codes into others. For example...
http://site/digital/DC.php
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http://site/digital/Coder.php
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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 device being designed requires more, in this case, 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 to generate digital streams (primary) at a speed of 2048 kbit/s from (speech), as well as data digital interfaces cross-switching of electronic channels at a speed of 64 kbit/s, transmission of digital streams 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 you to use broadcast connections, that is, send 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.
