The digital world

At the heart of today's computer-based electronic systems lies the elementary transistor on/off switch. The simplicity and reliability of semiconductor switching have led circuit design into the digital world where the signals represent numbers, and circuit functions are expressed by logic and arithmetic. Even applications involving analogue inputs and outputs, such as audio recording, are rapidly becoming digital, by using analogue to digital and digital to analogue converters to convert signal voltages into numbers and back from numbers to voltages again, thereby achieving greater fidelity in the recording process.

We shall see as this chapter unfolds that the binary numbering system, working to a base of 2 instead of the base of 10 used in the familiar decimal system, is ideally suited to electronic implementation. The simple ‘ons’ and ‘offs’ of electronic switches represent the noughts and ones of binary numbers. Digital electronics is therefore free from many of the critical features of analogue circuits such as distortion and drift, the basic gate circuits being essentially simple. This simplicity has the advantage that several million such circuits can be packed onto one ‘chip’ in an integrated circuit microcomputer.

These final two chapters lead from the electronic fundamentals so far described into the world of microcomputers and the vital software which controls them. We begin by examining the way that logical operations can be carried out using ordinary switches and then see how simple transistor circuits can expand into arithmetical adders, memories, counters, timers and finally the computer itself.