Positive and Negative Logic (1)

 Part a of the figure shows a type of package that the chip is provided in, called a dual-inline package (DIP). Part b illustrates the 7404 chip, which comprises six NOT gates. The chip’s external connections are called pins or leads. Two pins are used to connect to VDD and Gnd, and other pins provide connections to the NOT gates.

As an example of how a logic circuit can be implemented using 7400-series chips, consider the function f = x₁x₂ + x₂’x₃. A NOT gate is required to produce x2, as well as 2 two-input AND gates and a two-input OR gate.

An implementation of  f = x₁x₂ + x₂’x₃

The  figure shows three 7400-series chips that can be used to implement the function. We assume that the three input signals x₁, x₂, and x₃ are produced as outputs by some other circuitry that can be connected by wires to the three chips.

Because of their low logic capacity, the standard chips are seldom used in practice today, with one exception. Many modern products include standard chips that contain buffers. Buffers are logic gates that are usually used to improve the speed of circuits. An example of a buffer chip is depicted here:

It is the 74244 chip, which comprises eight tri-state buffers. We describe how tri-state buffers work later. Rather than showing how the buffers are arranged inside the chip package, as we did for the NOT, we show only the pin numbers of the package pins that are connected to the buffers. The package has 20 pins, and they are numbered in the same manner as shown for NOT-gates chip; Gnd and VDD connections are provided on pins 10 and 20, respectively. Many other buffer chips also exist. For example, the 162244 chip has 16 tri-state buffers. It is part of a family of devices that are similar to the 7400-series chips but with twice as many gates in each chip. These chips are available in multiple types of   packages, with the most popular being a small-outline integrated circuit (SOIC) package. An SOIC package has a similar shape to a DIP, but the SOIC is considerably smaller in physical size. As integrated circuit technology has improved over time, a system of classifying chips according to their size has evolved. The earliest chips produced, such as the 7400-series chips, comprise only a few logic gates. The technology used to produce these chips is referred to as small-scale integration (SSI). Chips that include slightly more logic circuitry, typically about 10 to 100 gates, represent medium-scale integration (MSI). Until the mid-1980s chips that were too large to qualify as MSI were classified large-scale integration (LSI). In recent years the concept of classifying circuits according to their size has become of little practical use. Most integrated circuits today contain many thousands or millions of transistors. Regardless of their exact size, these large chips are said to be made with very large scale integration (VLSI) technology. The trend in digital hardware products is to integrate as much circuitry as possible onto a single chip. Thus most of the chips used  today are built with VLSI technology, and the older types of chips are used rarely.