A Guide for Online Information About:

Logic Gates

by Rick Prescott

The building blocks of a circuit are called gates. There are three fundamental logical operations, from which all other functions, no matter how complex, can be derived. These functions are named AND, OR, and NOT. Each of these has a specific symbol and a clearly-defined behavior.

The AND Gate
The AND gate implements the AND function. With the gate shown to the left, both inputs must have logic 1 signals applied to them in order for the output to be a logic 1. With either input at logic 0, the output will be held to logic 0.

There is no limit to the number of inputs that may be applied to an AND function, so there is no functional limit to the number of inputs an AND gate may have. However, for practical reasons, commercial AND gates are most commonly manufactured with two, three, or four inputs. A standard integrated circuit (IC) package contains 14 or 16 pins for practical size and handling. A standard 14-pin package can contain four 2-input gates, three 3-input gates, or two 4-input gates and still have room for two pins for power supply connections. (More)

Working Example

Truth Table: AND

RUN JAVA APPLET DIGSIM TO EXPLORE AND GATE!!!

The OR Gate
The OR gate is sort of the reverse of the AND gate. The OR function, like its verbal counterpart, allows the output to be true (logic 1) if any one or more of its inputs are true. Verbally, I might say, "If it is raining OR if I turn on the sprinkler, the lawn will be wet." Note that the lawn will still be wet if the sprinkler is on and it is also raining. This is correctly reflected by the basic OR function.

As with the AND function, the OR function can have any number of inputs. However, practical commercial OR gates are mostly limited to two, three, and four inputs, as with AND gates. (More)

Working Example

Truth Table: OR

RUN JAVA APPLET DIGSIM TO EXPLORE OR GATE!!!

The NOT Gate, or Inverter
The inverter is a little different from the AND and OR gates in that it always has exactly one input as well as one output. Whatever logical state is applied to the input, the opposite state will appear at the output. The NOT function, as it is called, is necessary in many applications and highly useful in others. A practical verbal application might be: the door is NOT locked = you may enter.

In the inverter symbol, the triangle actually denotes only an amplifier, which in digital terms means that it "cleans up" the signal but does not change its logical sense. It is the circle at the output, which denotes the logical inversion. The circle could have been placed at the input instead, and the logical meaning would still be the same. (More)

Working Example

Truth Table: NOT

RUN JAVA APPLET DIGSIM TO EXPLORE NOT GATE!!!

The NAND Gate
The NAND gate implements the NAND function, which is exactly inverted from the AND function you already examined. Both inputs must have logic 1 signals applied to them in order for the output to be a logic 0. With either input at logic 0, the output will be held to logic 1.

The circle at the output of the NAND gate denotes the logical inversion, just as it did at the output of the inverter. Also in the figure to the left, note that the overbar is a solid bar over both input values at once. This shows that it is the AND function itself that is inverted, rather than each separate input.

As with AND, there is no limit to the number of inputs that may be applied to a NAND function, so there is no functional limit to the number of inputs a NAND gate may have. However, for practical reasons, commercial NAND gates are most commonly manufactured with two, three, or four inputs, to fit in a 14- or 16-pin package. (More)

Truth Table: NAND

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