Logic elements and their electrical counterparts. Scheme, description

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Logic elements and their electrical counterparts. Radio - for beginners

Radio - for beginners

Logic elements operating as independent digital microcircuits with a low degree of integration and as components of microcircuits with a higher degree of integration can be counted by several dozen. But here we will talk only about four of them - about the logical elements AND, OR, NOT, AND-NOT. The elements AND, OR and NOT are basic, and AND-NOT is a combination of AND and NOT elements.

What are these "bricks" of digital technology, what is the logic of their action? Let's clarify right away: the voltage from 0 to 0,4 V, i.e., corresponding to the level of logical 0, we will call the voltage of the low level, and the voltage of more than 2,4 V, corresponding to the level of logical 1, will be called the voltage of the high level. It is these levels of voltage at the input and output of logic elements and other microcircuits of the K155 series that are commonly used to characterize their logical states and operation.

The conditional graphic designation of the logical element AND is shown in fig. 1a. Its conditional symbol is the "&" sign inside the rectangle; this sign replaces the union "and" in English. On the left - two (maybe more) logical inputs - X1 and X2, on the right - one output Y. The logic of the element is as follows: a high-level voltage appears at the output only when signals of the same level are applied to all its inputs

Logic elements and their electrical counterparts
Fig 1. Logic element

To understand the logic of the operation of the logical element AND will help its electrical analogue (Fig. 1, b), composed of a series-connected power source GB1 (for example, a 3336 battery), pushbutton switches SB1, SB2 of any design and an incandescent lamp HL1 (MNZ, 5-0,26 ,1). The switches simulate the electrical signals at the analog input, and the lamp filament indicates the signal level at the output. The open state of the switch contacts corresponds to a low level voltage, the closed state corresponds to a high level voltage. While the contacts of the buttons are not closed (at both inputs of the element, the voltage is low), electrical; the analog circuit is open and the lamp, of course, does not shine. It is easy to draw another conclusion: the incandescent lamp at the output of the AND element turns on only after the contacts of both buttons SB2 and SBXNUMX are closed. This is the logical connection between the input and output signals of the AND element.

Now take a look at fig. 1, in. It shows timing diagrams of electrical processes that give a reliable idea of the operation of the logic element AND. At the input Xi, the signal appears first. As soon as the same signal is at input X3, a signal immediately appears at output Y, which exists as long as there are signals corresponding to a high level voltage at both inputs.

The so-called state table (Fig. 1, d), resembling a multiplication table, gives an idea of the state and logical connection between the input and output signals of the AND element. Looking at it, we can say that a high-level signal will be at the output of the element only when signals of the same level appear at both of its inputs. In all other cases, the output of the element will have a low-level voltage, i.e. corresponding to logical 0. The conditional symbol of the logical element OR is the number 1 inside the rectangle (Fig. 2, a). This element, like the AND element, can have two or more inputs. The signal at output Y, corresponding to a high level voltage, appears when the same signal is applied to input X1, or to input X2, or both inputs simultaneously. To verify this action of the OR element, conduct an experiment with its electrical counterpart (Fig. 2, b).

Logic elements and their electrical counterparts
Rice. 2,3 OR gate

The HL1 incandescent lamp at the analog output will turn on whenever the contacts or the SB1 or SB2 buttons, or both (all) buttons at the same time, are closed. states (Fig. 2d), which determines the logical connection between the input and output signals.

The conditional symbol of the NOT logical element is also the number 1 in the rectangle (Fig. 3, a). But he has one entrance and one. exit. A small circle, which begins the output signal line, symbolizes the logical negation at the output of the element. In the language of digital technology, it does NOT mean that this element is an inverter-electronic device, the output signal of which is opposite to the input. In other words, while a low-level signal is NOT active at the input of the element, a high-level signal will be at its output, and vice versa.

The electrical analogue of the NOT element can be assembled according to the circuit shown in fig. 3b. The electromagnetic relay K1, which operates when the battery GB1 is energized, must be selected with a group of closed contacts. While the contacts of the SB1 button are open, the relay winding is de-energized, its contacts K1-1 remain closed and, therefore, the HL1 lamp shines. When the button is pressed, its contacts are closed, simulating the appearance of a high level input signal, as a result of which the relay is activated. Its contacts, opening, break the power supply circuit of the lamp HL1-extinguishing, it symbolizes the appearance of a low-level signal at the output. Try to draw your own time diagrams of the operation of the NOT element and compile its state table - they should turn out the same as those shown in Fig. 3, in, g.

As we have already said, the AND-NOT gate is a combination of AND and NOT gates. Therefore, on its graphic designation (Fig. 4, a) there is an "&" sign and a circle on the output signal line, symbolizing logical negation. There is only one exit, but two or more entrances.

Logic elements and their electrical counterparts
Rice. 4 NAND gate

To understand the principle of operation of such a logical element of digital technology, you will be helped by its electrical counterpart, assembled according to the diagram in Fig. 4b. The electromagnetic relay K1, the battery GB1 and the incandescent lamp HL1 are the same as in the analogue of the NOT element. In series with the relay winding, turn on two buttons (SB1 and SB2), the contacts of which will simulate the input signals. In the initial state, when the contacts of the buttons are open, the lamp shines, symbolizing a high-level signal at the output. Click on one of the buttons in the entrance strand.

How does the indicator light react to this? She continues to shine. What if you press both buttons? In this case, the electrical circuit formed by the battery supply, the relay winding and the button contacts is closed, the relay is activated and its contacts K1.1, opening, break the second analog circuit - the lamp goes out.

These experiments allow us to conclude: with a low-level signal at one or at all inputs of the AND-NOT element (when the contacts of the analog input buttons are open), a high-level signal acts at the output, which changes to a low-level signal when the same signals appear at all inputs of the element (contacts of analog buttons are closed). This conclusion is confirmed by the operation diagrams and the state table shown in fig. 4, c, d.

Let's pay attention to the following fact: if the inputs of the AND-NOT element are connected together and a high-level signal is applied to them, the output of the element will be a low-level signal. Conversely, when a low-level signal is applied to the combined input, the element will output a high-level signal. In this case, the NAND element, as you probably already guessed, becomes an inverter, i.e., a NOT logical element. This property of the NAND element is very widely used in instruments and devices of digital technology.

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