Lec 18 - Input Devices

1. INPUT DEVICES
Disk units, tape units, and drum units are all input and output (I/O) devices and they share a common use for mass storage. In this section, we look at a few devices that are used exclusively for input of data. We start with one of the most prevalent devices – the keyboard.

1.1. KEYBOARD
By far the most common input device is the keyboard. It is standard equipment on virtually every personal computer. The essential elements of a keyboard are a switch matrix and a keyboard interface encoder chip. The keyboard encoder is a microprocessor with an in-built ROM. The switch matrix consists of a number of horizontal and vertical lines crossing each other. At each crossing, a small switch operated by a spring –loaded key is located. When a key is pressed, the switch closes and relevant horizontal and vertical lines are shorted. This is then detected by the keyboard encoder, which generates a special 8-bit code known as the scan code. Each key is allocated its own unique scan code. The scan code is entered into internal buffer for transmission to the computer via especial serial cable on the computer side, a keyboard controller. Also known as the keyboard bios chip receives the serial data stream and converts it into an appropriate ASCII code. The keyboard controller is another dedicated processor with an in-built few kilobytes of ROM and hundred bytes Of RAM. The characters you type are stored until the CPU is ready to accept them.
Keyboard interface connector: On the computer side, a 5-pin DIN socket is made available for the keyboard serial cable on line, pin2, is used for serial data, and second line, pin1, is used for the synchronizing clock. The keyboard receives its 5V.D.C. power from the motherboard on pin 5 with pin 4 used for signal ground.



1.2. MOUSE
Since the keyboard is likely to remain the dominate device for entering text. But many operations on personal computers involve pointing selecting or removing items already on the screen, often you can perform these tasks more quickly with pointing devices, such as the touch screen and the mouse.
The function of a mouse is to convert a small movement of the mouse into digital signals, which may be interpreted into corresponding movement at pointer on the screen.
There are two types of mouse: 1-the mechanical, and 2- the optical. The mouse is connected to one of the available serial ports, normally com1.
The basic components of mouse:
The main components of mechanical mouse are: the rubber ball rests against two rollers, X and Y, placed to detect the horizontal and vertical movements of the ball.
The two rollers are connected to two small wheels or disks with a fine grill. Each wheel is placed in the light path of opto-couplers. The opto-couplers detect the movement of the wheel and produce a stream of pulses as the light beam in interrupted by the grill. These pulses are fed into the mouse interface chip which converts them to a coded serial signal for transmission to the computer. Each wheel and its associated opto-couplers are designed in such a way as to be able to detect the direction of the movement of each roller, left/right for x and up/down for y.

An optical mouse replaces the ball with a light emitting diode (LED) and uses a special reflective mousepad that consists of alternating reflective and absorptive horizontal and vertical stripes. Motion is sensed through transitions between reflective and absorptive areas. The optical mouse does not accumulate dirt as readily as the ball mouse, and can be used in a vertical position or even in a weightless environment. The natural rotation of the wrist and elbow, however, do not match the straight horizontal and vertical stripes of the optical mousepad, and so some familiarity is required by the user in order to use the device effectively.

1.3. TOUCH SCREEN
Most people would say that their finger is the most nature pointing device. The touch screen uses infrared sensors to tell where a finger or pencil touches the surface of the screen.
A touchscreen comes in two forms, photonic and electrical. An illustration of the photonic version is shown in Figure 1.






A matrix of beams covers the screen in the horizontal and vertical dimensions. If the beams are interrupted (by a finger for example) then the position is determined by the interrupted beams. In an alternative version of the touchscreen, the display is covered with a touch sensitive surface. The user must make contact with the screen in order to register a selection.