Basic Computers — Page 304, Lesson 365

Let’s start with the definition that anchors everything in this chapter. A computer is a device, or a set of devices, that can store data and a program that operates on that data. So it’s not just a box that holds numbers — it holds both the information and the instructions that act on that information. And a general purpose computer is one that can be programmed to solve any reasonable problem expressed in logical and arithmetical terms. That means if you can state the problem in logic or arithmetic, you can write a program for it. Now, a nice piece of history. The first fully operational general purpose computer was the Z3, built in Germany in 1941 by Konrad Zuse. It was electromechanical, and it used binary digits — that’s the ON/OFF, two-state system we’ll get to in a moment. So even the very first working general purpose machine was already binary. There are basically two types of computer: analogue and digital. By far the most common today is the digital computer, or microprocessor, which now plays a part in most aspects of everyday life. But let’s look at both, because the contrast is important. An analogue computer uses continuous physical variables — things like voltage or pressure — to represent and manipulate the measurements it handles. So instead of counting discrete steps, it works with a smoothly varying quantity. Analogue computers are used as electronic models, or analogues, of mechanical or other systems. Why? Because conducting experiments on the real system itself could be costly, time consuming, or dangerous. Think of designing a bridge, an aircraft wing, or any structure where motion can occur. The engineer needs to know beforehand how that structure will react to various physical variables, such as wind speed and temperature. So you build an electronic model that behaves like the real thing, and you test that instead. In recent years, analogue computers have become less popular. Why? Because it’s now possible to program digital computers to simulate moving physical systems. So the digital machine can do the modelling job that used to require an analogue one. The rest of this chapter deals with digital computers and their use in aircraft. Now the digital computer. Digital computers use digital data — that is, binary data — in their operations. This form of data has only two levels of voltage, as opposed to the analogue system’s continuous variables. Those two levels correspond to ON or OFF — in other words, switching circuits. Digital circuits are two state circuits. Here’s the key contrast. Normally, when we work on paper, we count from zero to nine — that’s the decimal number system. But when the digital computer works, it has to use the ON–OFF, two state, or BINARY number system. So instead of ten digits, it only has two states to work with, and all its arithmetic is built on that. Let me tie that together. The analogue machine represents a measurement as a continuous voltage or pressure — infinitely variable. The digital machine represents everything as a two-level voltage — either ON or OFF, high or low. That’s why we say digital circuits are two state circuits, and why the binary number system is fundamental to how a digital computer operates. Everything the aircraft’s digital systems do — every calculation, every signal — ultimately reduces to those two states. That’s the foundation. We’ve got the definition, the two types, the analogue model idea, and the crucial switch to binary for digital machines. That binary idea is going to carry us through the rest of the chapter.