Peripherals: Input devices and The keyboard

Introduction

A peripheral is a device which is outside the central process­ing unit of a computer but controlled by it. There are four main types of peripheral, namely input devices such as keyboards, storage devices such as magnetic disks, output devices such as monitors, and communication devices such as modems.

The last chapter dealt with computers in general and covered the work of the CPU. This chapter covers the peripheral devices that might be connected to it. Modems and other telecommunications devices will be covered later in the chapter that deals with telecommunications (Chapter 8).

Input devices

A variety of input devices exist, able to convert information in any form (data, text, speech, or image) into the binary pulses recognized by computers. Some of these devices are described below.

The keyboard

The keyboard is still the primary device for inputting information to a computer, though it may ultimately be superseded by voice input devices. It operates by converting key presses to electronic signals in binary digital form.

The typical computer keyboard has the standard 'QWERTY' character keys inherited from the typewriter, together with a number of special keys described below. Alternative keyboard layouts have been designed, incorpo­ rating a more logical and natural arrangement of character keys, but these have never caught on.

The special keys vary somewhat according to the make of computer, but they usually include the following:

• Function keys, up to 12 in number, positioned either above or to the left of the character keys. These can be programmed by the user or by the software being run, so that a single key depression sends a command or string of commands to the computer.

• The CTRL and ALT keys (short for Control and Alter­ nate), which are always used in conjunction with other keys to issue commands to the computer. For example, in the WordStar word processing software, CTRL and G, pressed at the same time, delete the character at the cursor position on the screen, CTRL and T delete the word at this position, and CTRL andY delete the line.

• The backspace key, which deletes the character to the immediate left of the cursor on the screen.

• The four 'arrow' keys, which move the cursor one

character position in the direction of the arrow (up, down, left, or right).

• The PgUp and PgDn keys, which are normally pro­ grammed by your word processing or other software to move the cursor one 'page' (i.e. screen) up or down.

• The Home key, which may be programmed to move the cursor to the left of the screen, or the beginning of the document.

• The End key, which may be programmed to move the cursor to the right of the screen, or the end of the document.

• The Esc key, which is often programmed to enable you to escape from your currently selected option or task.

The mouse

The mouse is a hand-held device with a rubber or metal ball protruding from its base (see Figure 3.1). As you push the

Figure 3.1 A computer mouse

mouse over the surface of the desk, the movements of the ball are detected by the internal mechanism and converted to electrical signals. These are fed to the computer via a cable, and converted by the associated software to corre­sponding movements of the cursor across the screen. At the front of the mouse's casing are two or three buttons (or a single button in the case of the Macintosh mouse). You press these to perform tasks such as picking options dis­ played on the screen.

Similar to the mouse is the trackerball, used on some

computers. In this, the casing remains stationary while the ball, which is at the top, is rolled with the fingers.

The mouse has proved very popular, and is now utilized by a great deal of software. In particular, it is much easier to use than the keyboard for the following operations:

• Picking, i.e. selecting options from a list displayed on the screen.

• Pointing, i.e. moving rapidly from one point to another in a word-processed document, file of records, or table of data.

• Drawing, i.e. constructing lines and other shapes on the screen.

On some computer systems, such as the Apple Macintosh, the Atari ST, the IBM PS/2, and PCs running Windows, the mouse is an essential part of the hardware, as the way in which you use the software on those systems is to a large extent based upon it (see Chapter 5). With other systems, such as PCs which do not use the Windows environment, the mouse is not essential, though some software running on those systems cannot easily be used without it.