Logic Gates

A logic gate is a simple abstract device that receives binary inputs and generates a binary output. (binary meaning ones or zeros, on or off) There are a few types, each of which react differently to their inputs. Logic gates can be physically implemented to automate these logic operations. This can be done in many ways, thanks to their simple nature. The most common way is by connecting transistors and representing binary signals with voltage. A single logic gate alone cannot accomplish much, but when connected together, very complex systems can be created. Computers today rely on these components to operate, but it is easy to forget that watching a YouTube video is actually using millions of logic gates, in the form of transistors.

This page will decribe the behaviour of the fundamental logic gates. The names summarize their operation, and are pronounced as words (not an acronym). Also, my interpretation of the symbols is given. Each gate has one or two inputs and one output.

The OR Gatethe symbol for an OR logic gate

This gate will turn on its output when one OR the other of its inputs are on. In other words, it will turn on its output when at least one of its inputs are on. I think of the symbol as being like a funnel, with stuff coming out if anything is put into it.

The XOR Gatethe symbol for an XOR logic gate

The X here stands for "exclusive". This gate is similar to the OR gate, except that it behaves different when both inputs are on. It will turn on its output only when one OR the other of its inputs are on. But not both. It is the most complex gate. The XOR symbol has an additional line to its curved body, which has some meaning. To actually build a phyical, transistor-based XOR gate, usually several other gates are connected together. And so this extra line symbolizes the hidden complexity inside.

The AND Gatethe symbol for an AND logic gate

This gate will turn on its output only in one case: When one AND the other of its inputs are on. In other words, when both of the inputs are on. The symbol for this gate is filled out more than an OR gate, but is somewhat arbitrary.

The Bufferthe symbol for a buffer logic gate

The simplest gate, hardly even considered a gate at all. This little gate only accepts one input and merely passes it directly through to the output without change. In other words, it outputs the input. You'll see why this is useful next. The shape of the symbol is meant to be just an arrow, as the signal passes right through.

The NOT Gatethe symbol for a NOT logic gate

This gate simply outputs the opposite of its input. Its output is NOT its input. A common use for this gate is to connect it to the output of another gate so that its output is reversed. When representing this symbolically, the NOT gate is drawn as a small circle, and cannot stand alone. The NOT gate doesn't have its own symbol, and must always be drawn with another gate. But then, what if we have a NOT gate on its own? How do we represent it? Well, we attach it to a buffer, since the buffer doesn't affect the signal. So, a NOT gate can be thought of as a buffer with its output inverted. And that's exactly how it's drawn.

Inverted Gates

When inverting the output of another gate by appending the NOT gate, an "N" is added to its name. The inverted gates then, are: NOR, XNOR, NAND, and NOT.



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