Passive Matrix Liquid Crystal Displays

Liquid crystals have been used for digital calculators and watches for years, but the size required had made them impractical for computer use. Recent improvements in LCD technology reduced the size of the LCD pixel to compare with the size of a CRT pixel. Liquid crystal displays operate on the principle of scattering the light from an outside source to provide the desired pattern. The display from a liquid crystal is usually gray or black, but color can be achieved through the use of filters or dyes. They require low power and low voltage, making them ideal for laptop and notebook computers. In manufacturing LCDs, a clear, conductive material is deposited on the inside surfaces of two sheets of glass. This material acts as one electrode. The liquid crystal material is then deposited on the glass in the desired pattern. This pattern can be segmented (watches and calculators), dot matrix (graphic and computer screens), or a custom layout for special purposes. A terminal conductor is connected to an external terminal to control each liquid crystal. The two sheets of glass are then hermetically sealed at the edges. Passive Matrix Liquid Crystal Displays Passive matrix liquid crystal displays are used in most monochrome and color laptop computers today. The LCDs are arranged in a dot matrix pattern. Resolution of 640 columns by 480 rows is not uncommon. Characters are formed by addressing each row and column. Color passive matrix LCDs use three layers of crystals each separated by a color filter. Color is achieved by energizing one, two, or all three LCDs for each pixel. Passive matrix LCDs have some distinct disadvantages. They have low contrast. This lack of contrast has required the addition of a backlight to aid the user in viewing the screen. The response time to turn the pixels on and off is too slow for full-motion video and can produce a ghosting effect when changing full-screen displays. Color passive matrix LCDs are limited to displaying 16 colors simultaneously, even though the VGA adapter can have a palette of 262,144 colors. Active Matrix Liquid Crystal Displays Active matrix liquid crystal displays closely emulate the capabilities of the full-color CRT. The perfection of the thin film transistor (TFT) is largely responsible for the development of the active matrix LCD. Active matrix LCDs offer a brighter screen, provide response times fast enough to accommodate full-motion video, and can display 256 colors simultaneously. In manufacturing an active matrix display panel, each pixel consists of three crystals, one each for red, green, and blue. Three TFTs control each pixel, one for each color. The TFT technology allows for entire logic circuits, driver circuits, and even microprocessors to be deposited transparently on the glass plates, increasing the brightness, speed, and color quality of the display. INPUT DEVICES The displays discussed in this chapter are output devices. They display information from the computer for the user. To allow the user to act on the information being displayed, some type of input device is required. The most common input device is a keyboard. Increasing in popularity are cursor pointing devices such as the mouse or trackball. KEYBOARD The keyboard is the basic input device for personal computers. There are several styles of keyboards available, but the most common one today is the 101-key enhanced keyboard. Keyboard Layout The 101-key enhanced keyboard made several improvements over the 84-key keyboard. Two new function keys, F11 and F12, were added. The function keys were moved from the left side of the keyboard to the top of the keyboard. A group of dedicated cursor and screen control keys were added 2-4