Basic Computers — Page 304, Lesson 365

Let’s start with the definition, because everything else hangs off it. A computer is a device, or a set of devices, that can store data and a program that operates on that data. So you’ve got two things stored: the data itself, and the program — the set of instructions that tells the machine what to do with that data. A general purpose computer, and that’s an important qualifier, can be programmed to solve any reasonable problem expressed in logical and arithmetical terms. So it’s not built for one job; you can load different programs and it will tackle whatever logical or arithmetic task you give it. Now, a little history, because it grounds the concept. The first fully operational general purpose computer was electromechanical and used binary digits. It was called the Z3, built in Germany in 1941 by Konrad Zuse. So even at the very start, binary digits were already the language of computing. Basically there are two types of computer: analogue and digital. And by far the most common is the digital computer, or microprocessor, which now plays a part in most aspects of everyday life. Keep that term, microprocessor — it’s the digital computer on a chip, and it’s everywhere. Let’s take the analogue computer first, because it’s the contrast that makes digital clear. An analogue computer uses continuous physical variables, such as voltage or pressure, to represent and manipulate the measurements it handles. So instead of counting in discrete steps, it works with a smoothly varying quantity — the voltage or pressure literally stands in for the value you’re measuring. Why would you build one? Analogue computers are used as electronic models, or analogues, of mechanical or other systems, in cases where conducting experiments on the system itself would be costly, time consuming, or dangerous. Think about designing a bridge, an aircraft wing, or any structure where motion can occur. The engineer must know beforehand how it will react to various physical variables, such as wind speed and temperature. So you build an electronic model that behaves like the real structure, and you test that instead of the real thing. Now, in recent years analogue computers have become less popular, and the reason is important: it is now possible to program digital computers to simulate moving physical systems. So the digital machine can do the modelling job that used to require a dedicated analogue circuit. And that’s why the remainder of this chapter deals with digital computers and their use in aircraft. So let’s get to the digital computer. Digital computers use digital data — binary data — in their operations. This form of data has only two levels of voltage, as opposed to the analogue system’s continuous variables. So where analogue had a smooth range of voltage, digital has just two distinct levels. Those two levels correspond to ON or OFF, i.e. switching circuits. Digital circuits are two state circuits. Now here’s the key contrast with how we normally count. 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 has only two — and those two map directly onto the two voltage states of the circuit. That’s the whole foundation: two states, ON and OFF, and the binary number system built on top of them. So to tie it together: analogue uses continuous variables like voltage or pressure; digital uses two discrete voltage levels, ON and OFF, and works in binary. The digital computer is the one that dominates, and it’s the one we’ll be following through the rest of the chapter as it applies to aircraft.