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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Logical elements and truth tables. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Absolutely all digital microcircuits consist of the same logical elements - the "bricks" of any digital node. That's what we'll talk about now. Logic element This is a circuit that has multiple inputs and one output. Each state of the signals at the inputs corresponds to a certain signal at the output. So what are the elements? Element "AND" (AND) Otherwise, it is called "conjunctor". In order to understand how it works, you need to draw a table that lists the output states for any combination of input signals. Such a table is calledtruth table". Truth tables are widely used in digital technology to describe the operation of logic circuits. This is how the "AND" element and its truth table look like:
Since you will have to communicate with both Russian and bourgeois tech. documentation, I will give conditional graphic symbols (UGO) of elements both according to our and not our standards. We look at the truth table, and clarify the principle in the brain. It is not difficult to understand it: a unit at the output of the "AND" element occurs only when units are applied to both inputs. This explains the name of the element: units must be AND on one AND on the other input. If you look a little differently, then you can say this: the output of the "AND" element will be zero if at least one of its inputs is zero. We remember. Go ahead. OR element (OR) In other words, his name is "disjunctor". We admire:
Again, the name speaks for itself. A unit appears at the output when a unit is applied to one OR to another OR to both inputs at once. This element can also be called the "AND" element for negative logic: its output is zero only if zeros are applied to one and the second input. Let's go further. Next we have a very simple, but very necessary element. Element "NOT" (NOT) More often, it is called "inverter".
Need to say something about his work? Well then let's move on. The next two elements are obtained by setting the inverter to the output of the "AND" and "OR" elements. NAND element (NAND)
The NAND element works exactly the same as the AND, only the output signal is completely opposite. Where the "AND" element should have "0" at the output, the "AND-NOT" element should have one. And vice versa. This is easy to understand by the equivalent circuit of the element:
OR-NOT (NOR) element
The same story - the "OR" element with an inverter at the output. The next comrade is somewhat more cunning: Exclusive OR (XOR) element He is like this:
The operation it performs is often referred to as "modulo 2 addition". In fact, digital adders are built on these elements. Let's look at the truth table. When is the output of the unit? That's right: when the inputs are different signals. On one - 1, on the other - 0. That's how cunning he is. The equivalent circuit is something like this:
It is not necessary to memorize it. Actually, these are the main logical elements. Absolutely any digital microcircuits are built on their basis. Even your favorite Pentium 4. Next, we'll tinker about how to synthesize a digital circuit given its truth table. This is not difficult at all, but you need to know, because it will come in handy (even how useful) to us in the future.
And finally - a few microcircuits, which contain digital elements. Near the pins of the elements, the numbers of the corresponding legs of the microcircuit are indicated. All ICs listed here have 14 legs. Power is supplied to pins 7 (-) and 14 (+). Supply voltage - see the table in the previous paragraph. Publication: radiokot.ru
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