The alternative to generating accurate (expensive) circuits is digital computing, where we only allow electronic hardware to represent the binary numbers zero and one. Digital circuits allow us to carry out both arithmetic and a limited amount of human reasoning (tautology). The reasoning component is achieved by using zero to represent "false" or "off" and one to represent "true" or "on". The basis of digital circuits, and hence digital computing, is that voltage accuracy must not be permitted to dominate circuit design. As long as we can achieve voltages within certain ranges, then we can represent the only two numbers that we need to handle in digital computing.

The major design effort in the fabrication of digital semiconductor devices is therefore oriented towards achieving:

• High circuit speeds
• Small component sizes / High Component Density
• Low power dissipation
• Low circuit fabrication costs.

Figure (a) shows two, cascaded digital circuits, each with two inputs and one output. Figure (b) shows the allowable output and input voltage ranges for each of the circuits. An error margin exists between the output of one circuit and the input of the subsequent circuit, so that minor voltage variations that may occur do not affect the meaning of the information represented.





(a) Cascaded Digital Circuits.
(b) Allowable Input and Output Voltage Ranges for Transistor to Transistor Logic (TTL) Digital Circuits.

The actual digital voltage ranges shown in Figure (b) correspond to a family of digital circuits known as Transistor to Transistor Logic or TTL. This is the oldest of the widely used digital circuit technologies (dating back to the 1960s) and is important because most modern digital circuits are still designed with the same allowable input and output voltage ranges.

One of the major objectives of this blog is to help you to come to terms with the design of modern digital computer systems, so that you can understand why it is so difficult to interface them to the outside world. The strategy chosen in order to achieve this is to begin with an overview of the role of computers in the industrial arena, so that you may gain some insight into the range of performance attributes required by modern computers. The next stage in the process is to introduce (or rekindle) the basics of modern electronic circuits, including transistors, thyristors and so on. This serves two purposes. Firstly, it will help you to understand how both analog and digital circuits are designed and the limitations of those circuits. Secondly, it will introduce you to the basic elements used in interfacing computers to the outside world.