Traditional computer monitors have been based on the CRT. While CRTs have evolved to meet the demands of improved resolution, superior color quality, and faster vertical refresh rates (less flicker), they are still limited by their unwieldy size, weight, and heavy power consumption - certainly undesirable factors for mobile computing. Flat-panel displays (based on liquid crystal technology) offer the small size, light weight, reasonably low power consumption, and general ruggedness that are ideal for notebook, palmtop, and pen systems. Concerns about the environment and monitor emissions have even pushed flat-panel displays into use as desktop monitors. Given the growing importance of mobile computers, liquid crystal displays (or LCDs) will become even more important in the coming years. This chapter will show you the technology behind LCDs, and present a series of display troubleshooting procedures.

Before jumping right into detailed discussions of flat-panel display technologies, it would be helpful for you to have a clear understanding of an LCD’s major characteristics. This part of the chapter also presents a set of cautions to ensure safe display handling. Even if you are already familiar with flat-panel specifications, take a moment to review their handling precautions.

As with CRT-based displays, the images formed on a flat-panel display are NOT solid images. Instead, images are formed as an array of individual picture elements (or pixels). Pixels are arranged into a matrix of columns (top-to-bottom) and rows (left-to-right) as illustrated in Fig. 27-1. Each pixel corresponds to a location in video RAM (not core memory which holds programs and data). As data is written into video RAM, pixels in the array will turn on and off. The on/off patterns that appear in the array form letters and graphics.

The resolution of a flat-panel display is little more than the total number of pixels that can be displayed. More pixels allow the display to present finer, higher-quality images. Resolution is expressed as the number of columns times the number of rows. The total number of pixels is simply the result of that multiplication. For example, the newest 12.1" and 13.3" LCDs provide 1024x768 resolution (786432 pixels). Here’s a comparison of LCD resolutions:

• Old ~10" monochrome LCDs: 640 x 200 or 128000 pixels (or less)
• Newer 10.4" mono/color LCDs: 640 x 480 or 307200 pixels
• Current 10.4" color LCDs: 1024 x 600 or 614400 pixels
• Current 11.3" and 12.1" color LCDs: 800 x 600 or 480000 pixels
• Latest 12.1" and 13.3" LCDs: 1024 x 768 or 786432 pixels
• Latest 14.1" LCDs 800 x 600 or 480000 pixels

The aspect ratio is basically the "squareness" of each pixel - and indirectly, the "squareness" of the display. For example, a display with perfectly square pixels has an aspect ratio of 1:1. A rectangle box 100 pixels wide and 100 pixels high would appear as an even square. However, pixel aspect ratios are not always 1:1. Typical pixels are somewhat higher than they are wide. For a display with 320 x 200 resolution, a pixel width of 0.34mm and height of 0.48mm (1:1.41) is not uncommon. Higher resolution displays use smaller dots to fit more pixels into roughly the same viewing area. As a result, smaller pixels tend to approach 1:1 aspect ratios. Keep in mind that aspect ratio may not be shown as an individual display specification. Figure 27-2 illustrates the concept of aspect ratio.

Figure 27-2 also presents the concept of LCD dot pitch. For an LCD, dot pitch is the horizontal and vertical distance between an adjacent pixel (usually expressed as "width x height" millimeters or "mm"). For a 10.4" display, dot pitch averages about 0.33mm (W) x 0.33mm (H). Dot pitch can be important because it effects the "granularity" of an image. For example, an 800x600 image displayed on a 14.1" LCD will tend to look a bit more "pixelated" than the same 800x600 image displayed on a 12.1" LCD.

Every display has a particular viewing angle. It is the angle through which a display can be viewed "clearly" (at reasonable contrast) as shown in Fig. 27-3. Viewing angle is rarely a concern for bright, crisp displays such as CRTs and gas plasma flat-panels. Such displays generate light, so they can be seen up to a very wide angle (usually up to 70 degrees from center). For liquid crystal displays, however, the viewing angle is a critical specification. LCDs do not generate their own light, so the display contrast tends to degrade quickly as you leave a direct line of sight.

Position yourself or your display so that you look directly at the display (a perpendicular orientation). Tilt the screen up and away from you. Do you see how the contrast and brightness of the display decreases as you tilt the screen? The angle the display is at when the picture just becomes indiscernible is the positive vertical limit (+q ). Return the screen to a direct line of sight, then slowly tilt the screen down toward you. Once again, you will see the display degrade. The angle the display is at when the display becomes indiscernible is the negative vertical limit (-q ). Ideally, both vertical limits should be the same. Return the display to a direct line of sight, then repeat this test in the horizontal orientation. Swing the display right or left until the image becomes indiscernible. These are the negative and positive horizontal limits respectively (-f ) and (+f ). Ideally, both horizontal limits should be the same. As a general rule, the larger a viewing angle is, the easier the display is to look at.

The contrast of an image is loosely defined as the difference in luminous intensity between pixels that are fully ON and pixels that are fully OFF. The greater this difference is, the higher the contrast is, and the sharper an image appears. Many graphic flat-panel LCDs offer contrast ranging from a low of 10:1 (for old monochrome LCDs) to 60:1 and higher (often found in current active color LCDs). For the purposes of this book, contrast is a unit-less number. Since luminous intensity is strongly dependent on a display's particular viewing angle, a reference angle is typically added to the contrast specification - an angle of "6 o’clock" is usually a straight-on view. Remember that contrast is a comparison of black versus white. It is desirable (especially with monochrome displays) to simulate 16, 32, 64, or more gray levels that are somewhere between black and white. Any gray level other than pure black provides a lower contrast versus white - do not confuse gray scale levels with poor contrast.

The response time of a display is the time required for a display pixel to reach its ON or OFF condition after the pixel has been addressed by the corresponding driver circuitry. Such on/off transitions do not occur instantaneously. Depending on the vintage and quality of the display, pixel response times can vary anywhere from 40mS to 200mS. For example, the Sharp LQ10D311 10.4" display panel offers a response time of 80mS. Active-matrix LCDs (AMLCDs) typically offer the shortest response times, while older passive-matrix LCDs provide the slowest performance. You will see much more about the various display types and techniques a bit later in this chapter.

Next to magnetic hard drives, flat-panel displays are some of the most sophisticated and delicate assemblies in the computer industry. While most displays can easily withstand the rigors of everyday use, there are serious physical, environmental, and handling precautions that you should be aware of before attempting any sort of LCD system service:

• Work GENTLY - you must be extremely careful with all liquid crystal assemblies. Liquid crystal material is sandwiched between two layers of fragile glass. The glass can easily be fractured by abuse or careless handling.
• Clean up LC spills quickly - if a fracture should occur and liquid crystal material happens to leak out, use rubber gloves and wipe up the spill with soap and water. Immediately wash off any LC material that comes in contact with your skin. Do NOT, under ANY circumstances, ingest or inhale LC material.
• Careful with the polarizer - avoid applying pressure to the surface of a liquid crystal display. You risk scratching the delicate polarizer layer covering the display's face. If the polarizer is scratched or damaged in any way, it will have to be replaced (it cannot be buffed or compounded out). Excessive pressure or bending forces can fracture the delicate connections within the display.
• Careful with water and solvents - lightly wet a soft (lint-free), clean cloth with fresh isopropyl alcohol or ethyl alcohol, then gently wipe away the stain(s). You may prefer to use photography-grade lens wipes instead of a cloth. NEVER use water or harsh solvents to clean a display. Water drops can accumulate as condensation, and high-humidity environments can corrode a display's electrodes. Chemicals can also damage the display polarizer.
• Keep the display at room temperature - NEVER store a display assembly in a cold vehicle, room, or other cold environment. Liquid crystal material coagulates (becomes firm) at low temperatures (below 0 deg. C or 32 deg. F). Exposure to low temperatures can cause black or white bubbles to form in the material.
• Use ALL anti-static precautions - flat-panel display circuitry is very susceptible to damage from electrostatic discharge (ESD). Make certain to use anti-static bags or boxes to hold the display. Use an anti-static wrist strap to remove any charges from your body before handling a display. Use a grounded soldering iron and tools. Work (if possible) on an anti-static workbench mat. Try to avoid working in cool, dry environments where static charges can accumulate easily. If you use a vacuum cleaner around your display, make sure that the vacuum is static-safe.

Liquid crystal is an unusual organic material that has been known by science for many years. While it is liquid in form and appearance, LC exhibits a crystalline molecular structure that resembles a solid. If you were to look at a sample of LC material under a microscope, you would see a vast array of rod-shaped molecules. In its normal state, LC is virtually clear - light would pass right through a container of LC. When LC material is assembled into a flat panel, the molecules have a tendency to twist. Quite by accident, it was discovered that a voltage applied across a volume of liquid crystal forces the molecules between the active electrodes to straighten. When the voltage is removed, the straightened LC molecules return to their normal twisted orientation.

It would have been a simple matter to have dismissed the liquid crystal effect as little more than a scientific curiosity, but further experiments revealed an interesting phenomenon when light polarizing materials (a polarizer is a thin film which allows light to pass in only one orientation) are placed on both sides of the liquid crystal layer, and light is passed through; areas of the LC material that are excited by an external voltage became dark and visible. When voltage was removed, the area became clear and invisible again. This is because light is twisted by the unexcited LC material and passes from the rear polarizer to the front polarizer without interference. When an area of the LC material is excited by an external voltage, the molecular twist "straightens", and light entering from the rear polarizer is blocked by the front polarizer - making that area appear dark.

By using electrodes with different patterns, limitless images can be formed. Notebook LCDs use arrays of pixel electrodes etched onto both sides of the LC glass shell. By energizing the pizel electrodes as required, the LC material straightens at those points, causing the dark spots we see as pixels. The earliest developments in commercial liquid crystal displays were simply referred to as twisted nematic (TN) displays. A basic LCD assembly is illustrated in Fig. 27-4. Notice that an array of transparent electrodes are printed and sealed on the inside of each glass layer.

There are four major varieties of liquid crystal assemblies that you should be familiar with; twisted nematic (TN), super twisted nematic (STN), neutralized super twisted nematic (NTN or NSTN), and film-compensated super twisted nematic (FTN or FSTN). Each of these variations handles light somewhat differently, and offers unique display characteristics that a technician should be familiar with.

The twisted nematic (TN) display is illustrated in Fig. 27-5. Light can originate from many different sources and strike the front polarizer, but the vertically oriented polarizer only allows light waves in the vertical orientation to pass through into the LC cell. As vertically oriented light waves enter the LC assembly, its orientation twists 90 degrees following the molecular twist in the LC material. As light leaves the LC cell, its orientation is now horizontal. Since the rear polarizer is aligned horizontally, light passes through and the LC display appears transparent.

When a pixel is activated, the LC material being energized straightens its alignment - twist will become 0 degrees, and light will not change its polarization in the LC cell. Vertically polarized light is blocked by the horizontally oriented rear polarizer. This makes the activated pixel appear dark. TN technology is appealing for its low cost, simple construction, and good response time, but it is limited by poor viewing angle and low contrast in high-resolution displays. Today, TN displays have been largely replaced by any one of the three following technologies.

A super twisted nematic (STN) approach is shown in Fig. 27-6. Initially, the STN approach appears identical to the TN technique, but there are two major differences. First, a "super" twisted LC material is used which provides more than 200 degrees of twist instead of only 90 degrees with the TN formulation. The rear polarizer angle must also be changed to match the twist of the LC material. For example, if the LC material has a twist of 220 deg, the rear polarizer must be aligned to that same orientation.

In STN operation, the vertically oriented light passing through the front polarizer enters the LC cell. As light passes through the LC cell, its orientation changes following the formulation's particular twist. The twist may be as little as 200 degrees, or as much as 270 degrees. Light leaving the LC cell should then pass through the customized rear polarizer and make the display appear transparent. If a pixel is activated, the LC material at that point will straighten completely. Light no longer twists to match the rear polarizer, so the pixel appears dark. STN displays offer much better contrast and viewing angle than TN versions because of the additional twist. STN technology also performs very well at high resolutions (up to 1024 x 768 pixels). However, STN displays cost more than regular TN displays, and the response time to activate each pixel is somewhat slow because of the extra twist. This can blur fast-changing images.

The neutralized super twisted nematic (NTN or NSTN) display is shown in Fig. 27-7. Light is vertically oriented by the front polarizer before being admitted to the first LC cell. Light entering the first LC cell is twisted more than 270 degrees. A second LC cell (known as a compensator cell) adds extra twist to light polarization resulting in a horizontally oriented light output. Light that passes through the second LC cell also passes through the rear polarizer and results in a clear (transparent) display. Keep in mind that only the first LC cell offers active pixels. The compensator cell only adds twist.

When a pixel is activated in the first LC cell, the LC molecules align so light at that point is not twisted. The untwisted light is not twisted enough by the compensator cell, so that point is blocked by the rear polarizer and appears dark. Light passing through an idle pixel is twisted, then twisted again by the compensator cell. With this additional twist, light passes through the rear polarizer and the deactivated points appear transparent. NTN displays produce some of the finest, high-contrast, high viewing angle images available, but NTN displays are also much heavier, thicker, and costlier than other types of displays. It is also difficult to backlight such a configuration of LC cells. For most mobile computer applications, FTN displays are preferred over NTN models.

Figure 27-8 illustrates the basic structure of a film-compensated super twist nematic (FTN or FSTN) display. As you may see, the FTN display is very similar to the NTN display shown in Fig. 27-7. However, an FTN display uses a layer of optically compensated film instead of a second LC cell to achieve horizontal light polarization. Vertically oriented light passes through the front polarizer, than is twisted more than 200 degrees by the LC cell. When light emerges from the LC cell, it passes through a compensator film. Assuming that light is oriented properly from the LC cell, the compensator layer changes light polarization to a horizontal orientation. Light then passes through the horizontal polarizer causing a clear (transparent) display.

When a pixel is activated, the LC material at that point straightens and light polarization does not occur. As unaltered light passes through the compensation film, it does not twist enough to pass through the rear polarizer, so the pixel appears dark. FTN LCDs are much lighter, thinner, and less expensive than their NTN counterparts. The FTN display does not have nearly as much optical loss as NTN versions, so FTN displays are easy to backlight. The only major disadvantage to an FTN display is that its contrast and viewing angle are slightly reduced because of the compensating film.

It is important for you to realize that light plays an absolutely critical role in the formation of liquid crystal images. The path that light takes through the LC assembly and your eye can have a serious impact on the display's image quality, as well as the display's utility in various environments. There are three classical viewing "modes" that you should understand; reflective LCD, transflective LCD, and transmissive LCD. Figure 27-9 illustrates the action of each mode.

In the reflective viewing mode, only available light is used to illuminate the display. A metallized reflector is mounted behind the display's rear polarizer. Light from the outside environment that penetrates the LC assembly is reflected back to your eye resulting in a clear (transparent) image. Light that is blocked due to an activated pixel appears dark. Reflective displays work best when used in an outdoor or well-lit environments, and are typically found in hand-held equipment like digital multimeters. If light is blocked from the display, the image will virtually disappear. However, since no backlighting is used, the display consumes very little power.

The transflective viewing mode uses a partial reflector behind the LC cell's rear polarizer. This partial reflector will reflect light provided by the outside environment, and pass any illumination provided from behind the assembly (the backlight). Transflective operation allows the display to be operated in direct light with the backlight turned off. The backlight can then be activated in low-light conditions.

A transmissive LCD uses a transparent rear polarizer with no reflector at all. Light entering the LCD assembly from the outside environment is not reflected back to your eye. Instead, a backlight is required to make the image visible. When pixels are off, backlight illumination passes directly through the display to your eye resulting in clear (transparent) pixels. Activated pixels block the backlight and result in dark points. The backlight can be overpowered by bright light or sunlight, so the transmissive display can appear pale or "washed out" when used outdoors or in other bright environments.

Backlighting is the process of adding a known light source behind a liquid crystal display in order to improve the display's visibility in low-light situations. Mobile computers use one of two approaches to backlighting: electroluminescent (EL) panels or cold-cathode fluorescent tubes (CCFTs). EL panels are thin, light-weight, and produce a very even light output across their surface area. EL panels are available in a several colors, but white is preferred for computer displays. The EL panel is usually mounted directly behind the display's rear polarizer (or transflector if used) as shown in Fig. 27-10. EL panels are reasonably rugged and reliable, but they require a substantial AC excitation voltage in order to operate. An AC inverter supply is used to convert low-voltage DC into a high-voltage AC level of 100 Vac or more. Not only do EL panels require substantial power, but EL panels also suffer a relatively short working life (2,000 to 3,000 hours) before a serious loss of backlight intensity occurs. This makes EL panels better suited for backlighting LCDs in plugged-in office equipment like photocopiers and fax machines.

Cold-cathode fluorescent tubes (CCFTs) offer an inexpensive source of very bright white light which consumes reasonably little power (usually 4-5 watts per tube). CCFTs also enjoy a long life (10,000 to 15,000 hours) without serious degradation. Such characteristics have made CCFTs very popular in a great many notebook and pen-computer displays. Figure 27-11 illustrates the two common methods of mounting CCFTs: edgelighting, and backlighting.

As you may imagine, edgelighting is favored in thin or low-profile displays. A layer of translucent material referred to as the diffuser distributes the lamp's light evenly behind the LC cell. To create an even brighter display, a second CCFT can be added on the opposite edge of the diffuser. If a smaller, thicker display assembly is preferred, one or two CCFTs can be mounted in a cavity directly behind the LC cell. A diffuser is still used to spread light evenly behind the LC cell. CCFTs need a high-voltage AC source in order to operate, so an inverter supply is employed to provide the high voltages that most CCFTs need.

Now that you understand what liquid crystal displays are and how they are constructed, you must understand how each pixel in a display is controlled (or addressed). There are two classical methods of LCD addressing; passive addressing and active addressing. You will probably encounter both types of addressing at one time or another.

A passive-matrix LCD is illustrated in Fig. 27-12. Each layer of glass in an LC cell contains transparent electrodes deposited on the inside of the glass sheet. The upper (or front) glass contains column electrodes, and the lower (or rear) glass is printed with row electrodes. When both sheets of glass are fitted together as shown, a "matrix" is formed. Every point where a row electrode and a column electrode intersect is a potential pixel. To light a pixel, the appropriate row and column electrodes must be energized. Wherever an energized row and column intersect, a visible pixel will appear. In order to excite a pixel, a voltage potential must be applied across the LC material. For the example of Fig. 27-12, if a voltage is applied to column 638 and row 1 is connected to ground, pixel (638,1) should appear.

A small transistor is used to switch power to each electrode. These driver transistors are operated by digital control signals generated in a display control IC which is usually located on the LCD panel itself. When a row electrode is selected, multiple column electrodes can be addressed along that row. In this way, a complete display can be developed an entire row at a time instead of a pixel at a time. The passive-matrix display is updated continually by scanning rows in sequence and activating each column necessary to display all of the pixels in the selected row. Most passive-matrix displays can update row data several times per second (refreshing the entire image several times per second).

While passive-matrix displays are simple and straightforward to design and build, the inherent need to scan the display slows down its overall operation. It is difficult to display computer animation or fast graphics on many passive monochrome displays. Even a mouse cursor may disappear while moving around a passive-matrix LCD.

To overcome the limitations of classical passive-matrix LCDs, the active-matrix display was developed. As shown in Fig. 27-13, each pixel is handled directly by a dedicated electrode instead of using common row and column electrodes. Individual electrodes are driven by their own transistors, so there is one transistor driver for every pixel in a monochrome display. Driver transistors are deposited onto the rear glass substrate (foundation) in much the same way that integrated circuits are fabricated. For a display with 640x480 resolution, a total of [640 x 480] 307,200 thin film transistors (TFTs) must be fabricated onto the rear glass. A single, huge, common electrode is deposited onto the front glass. To excite the desired pixel, it is only necessary to activate the corresponding driver transistor. The ICs that manage operation of the driver transistor array are generally included in the display panel.

When a driver transistor is fired, a potential is applied to the corresponding electrode. This potential establishes an electric field between the electrode and the common electrode on the front panel. For the example of Fig. 27-13, you will see that the pixel in row 2 and column 0 is activated simply by applying a control signal to its driver transistor. Since each pixel in an active-matrix LCD can be addressed individually, there is no need to continually "sweep" the display as with passive displays. Active-matrix addressing is much faster than passive matrix addressing. As a result, active-matrix displays offer impressive response time (allowing high frame rates) with extremely good contrast. Unfortunately, active-matrix LCDs are also some of the most expensive parts of your laptop or notebook computer.

The desire for high-quality flat-panel color displays continues to be somewhat of a quest for display designers. While two very effective color LCD techniques are well-established, both types of color displays offer their own particular drawbacks. Passive-matrix FSTN and active-matrix TFT color displays are the two dominant color LCD technologies available. This section of the chapter describes today's color technologies.

Passive-matrix color LCD technology is based on the operation of film-compensated super twisted nematic (FSTN or FTN) LCDs. FSTN principles were presented earlier in this chapter. The most striking difference between color and monochrome LCDs is that the color LCD uses three times as many electrodes as the monochrome display. This is necessary because three primary colors (red, green, and blue) are needed to form the color of each "dot" that your eye perceives. As shown in Fig. 27-14, each colored dot is made up of three tiny pixels.

Pixels do not actually generate the colors that you see. It is the white light passing through each pixel which is filtered to form the intended color. The front glass is coated with color filter material in front of each "red", "green", and "blue" dot. For example, if the dot at row 0 column 0 is supposed to be red, the green and blue dots turn on at that point to block white light through all but the "red" filter. White light travels through the red filter on the front glass where it emerges as red. When the red, green, and blue dots are all on, ALL light is blocked and the pixel appears black. If all three dots are off, all light passes through and the pixel appears white. With only three primary colors to work with, a color display can only produce 8 "native" colors. But with the use of pixel grouping and color dithering, a passive color display can be made to simulate up to 256 colors - sometimes more depending on the speed and quality of the display.

The red, green, and blue (RGB) column electrodes for each pixel are deposited onto the front glass, while a single row electrode for each dot is fabricated onto the rear glass. As you might imagine, tripling the number of column electrodes complicates the manufacture of passive color displays. Not only is electrode deposition more difficult because electrodes are close together, but three times the number of column driver transistors and IC driver signals are needed. Like monochrome LCDs, the color display is updated by scanning each row sequentially, then manipulating the RGB elements for each column. Typical color LCD data can be updated (or "refreshed") several times per second.

FSTN color displays suffer from many of the disadvantages inherent in monochrome passive matrix displays. First, response time is slow (about 200-250ms). This means that no matter how fast data is delivered to the display, the image you see will only change about four times per second. Such slow update times make passive displays poor choices for fast graphic operations or animation. Their contrast ratio is a poor 10:1 or so - which generally results in washed out or hazy displays. Viewing angles for color passive-matrix LCDs are also poor at around 45 degrees. Your clearest view of the display will be to look at it straight on. Still, constant advances in materials and fabrication techniques are improving the speed and quality of passive color displays.

Active-matrix color LCD technology takes the contents of monochrome active panels one step further by using three electrodes for every dot. Each electrode is completely independent and is driven by its own thin-film transistor (TFT). The three elements control the "red", "green", and "blue" light source for each pixel that your eye perceives. Figure 27-15 illustrates the structure of a TFT active matrix color LCD. Every electrode driver transistor and all interconnecting wiring is fabricated onto the rear glass plate. With three transistors per dot, a 640 column x 480 row color display requires [640 x 480 x 3] 921,600 individual transistors. A 1024x786 display would require [1024 x 768 x 3] 2359296 transistors. Essentially, the rear plate of a TFT color display is one huge integrated circuit. The front glass plate is fabricated with a single, large common electrode that every screen element can reference to.

As with passive-matrix displays, the LC material used in active-matrix displays do not actually generate color. The individual elements simply turn white light on or off. White light that is permitted through the display is filtered by colored material applied to corresponding locations on the front glass. When the red, green, and blue elements are all off, white light shines through the three elements, and the pixel appears white. If the red, green, and blue elements are all on, all light is blocked, and the pixel appears black. With only three primary colors to work with, a color display can only produce 8 "native" colors. But with the use of pixel grouping and color dithering, a fast active-matrix color display can be made to simulate up to "true color" mode (16.8 million colors).

Active-matrix color displays do away with many of the limitations found in passive-matrix displays. Response time is very fast - on the order of 20-40ms or better. Such fast response times allow the screen image to change up to 30 times per second. This provides excellent performance for graphics or animation applications. The control afforded by active-matrix screens provides a brilliant contrast ratio of 60:1 with a comfortable viewing angle of 80 deg or so. Color active-matrix displays will most likely reflect the state-of-the-art in mobile computer technology for quite some time.

Cost has been a major constraint for active-matrix displays. The addition of an active-matrix color display can raise the cost of a notebook or sub-notebook computer by $1000 (US) or more. Designers have sought to improve the speed of flat-panel displays without incurring the cost of active-matrix fabrication. The dual-scan technique (sometimes called DSTN) is a recent improvement for passive-matrix displays. Instead of scanning the entire display in one pass, the display area is broken up into upper and lower halves. Each half of the display is scanned independently - as if each half of the panel were a different display. This allows any one row in the display to be updated at least twice as fast. Passive-matrix performance is improved without a substantial increase in display complexity or cost. The disadvantage to dual-scan displays is that a faint horizontal bar becomes visible along the center of the display (where the upper and lower scanning areas meet). Some users may find this objectionable.

A complete flat-panel display assembly for a notebook computer is shown in Fig. 27-16. The plastic outer housings (marked K1 and K22) form the cosmetic shell of the display panel. An LC cell with its driver ICs and transistors (marked E6) is mounted to the front housing. A number of insulators may be added to protect the LC cell from accidental short circuits. Note that the front polarizer layer is built into the LC cell. A rear polarizer (marked K15) is placed directly behind the LC cell, followed by the translucent backlight diffuser panel (marked K16). The backlight mechanism is a long, thin CCFT (marked K13) located along the bottom of the diffuser. Several spacers/brackets are used to secure the diffuser panel and tie the entire assembly together. Finally, a rear housing is snapped into place to cover the display. A cable (marked K19) connects the display assembly to the motherboard.

Liquid crystal displays are delicate and sophisticated sub-assemblies which are very sensitive to poor manufacturing tolerances, ambient temperature, and physical stress. In order to deal most effectively with LCD problems, you should first understand the various reasons why LCDs fail in the first place:

• Glass damage - this is normally caused by end-user abuse, or by stress when the display is being installed into the notebook assembly. Glass damage can be seen as cracks in the display, or as geometric patterns in the display. These failures are not repairable, and the entire LCD must be replaced.
• Scratches on the polarizers - scratches normally occur because of poor handling on the part of the computer manufacturer, or because of end-user abuse (i.e. using a pen or pencil as a pointer on the display). Fortunately, polarizers can often be replaced.
• Display controller failures - the IC(s) which drive the display can fail (one symptom of this type of failure is a blank display, or a display that misses every other line, or every fourth line). Similar symptoms can also result from video-cable failure between the display and computer’s main board, and/or cold solder joints at the controller cable.
• Bond (TAB) failures - the electrical patterns which connect the "pixels" to the driver ICs can fail, resulting in vertical or horizontal lines or bars in the display. Bond failures occur during all phases of the LCD life cycle because of poor bonds made during initial manufacturing, cracked traces caused by impact, and bad solder connections. Defective bonds can also disable the whole display.
• Broken backlight lamps - as notebook computers drop in size, their structural strength and rigidity decreases, which results in more flexing of the display panel. The CCFTs themselves are also being made smaller and thinner to accommodate thinner display panels. These factors increase the chance that a CCFT will break, resulting in a severe dimming of the display. If you see one side of the display significantly dimmer than the other, the CCFT on that side of the display may have failed.
• Dimming backlight - the luminance of CCFTs naturally declines with age. This causes the display to eventually loose brightness. You’ll need to replace aged CCFTs.
• Quality problems due to cost reductions - as manufacturers reduce the prices of their displays, materials have become poorer, and the manufacturing process less precise. For example, in one widely used display (which shall remain nameless), the manufacturing process left some sensitive conductors exposed to the environment. Over time, these conductors degrade and fail.
• Bad pixels - these are a number of pixels which are always on or always off. This is a manufacturing defect which occurs during the manufacturing process. The specifications set by display manufacturers DOES allow a small number of bad pixels (often 10). But additional pixels can fail during the display’s working life.
• Deteriorating bonds (TABs) - the connection between the driver bonds (TABs) and the display glass start to fail. You’ll usually see this as an intermittent or total failure of the affected lines. Such bond failures can be caused by mishandling/abuse of the display, shear forces between the bonds and the glass that loosen the anisotropic bonding tape, or natural aging of the tape that causes it to become brittle.

Even when a liquid crystal display is running perfectly, the display image may appear marginal (or not appear at all). There are a myriad of possible issues which can impair LCD image quality, but some of the most common problems are outlined in this part of the chapter.

Environmental distortion - remember that liquid crystal works because the presence of an electrical field through the material causes a change in the LC molecular alignment. Stray electrical interference from other nearby devices (i.e. televisions, radios, microwave ovens, and so on) can effect these alignments causing the display to "jump" or "shake". Electrical field interference can also make the display appear blurry or fuzzy.

If the system is close to a commercial fluorescent light, turn the light off, or move the light away from the display to see if the video clears up. If the system is plugged into a surge protector, unplug the system and reconnect it to the wall outlet to see if the video improves. If you notice that the display acts up at regular intervals, see if there is a major appliance (such as a freezer or air conditioner) on the same electrical circuit. If so, move the computer to a new location. If possible, move the system to another location and see if the distortion clears up.

Incorrect Windows 95 monitor/display adapter type - if the monitor or video type is not correctly identified in Windows 95, the display may appear distorted, or you may not be able to enter Windows 95 at all (however, DOS applications will be uneffected). If you are able to enter Windows 95, you can check the current video adapter and monitor selections:

1. Right-click an open area on the Windows 95 desktop, then click Properties in the menu.
1. Select the tab marked Settings, then click Change Display Type. The "Adapter Type" and "Monitor Type" are displayed here.
1. In the Monitor Type section, click the Change button. In the lower left hand corner of the Select Device window, click the circle beside "Show all devices".
1. In the Manufacturers window, scroll to the top of the window to select "Standard monitor types".
1. In the Models window, choose "Laptop Display Panel (640x480)" or other suitable resolution.
1. Click the OK button to return to the Change Display Type window.
1. Click the Close button to return to the Display Properties window.
1. Click OK to return to the Windows 95 desktop.
1. Restart the computer system - Windows 95 should now appear normally.

If you cannot enter Windows 95, reboot the computer. When it finishes counting the memory and displays, "Starting Windows 95", immediately press <F5> on the keyboard. This starts Windows 95 in the Safe Mode. Follow the nine steps above to check the video and monitor settings.

An excessive boarder is present in the display - if there is a one inch border around the image on an LCD, the mobile PC possibly has an 800x600 LCD display (confirm this with your laptop documents). To change the screen size to fit an 800x600 display:

1. Click on an open area on the Windows 95 desktop, then click Properties in the menu.
1. Select the tab marked Settings, then click Change Display Type.
1. In the Change Display Type window, click on the Change button in the Monitor Type section.
1. In the Select Type window, click on "Show all devices".
1. Under Manufacturers, scroll to the top and choose "Standard monitor types".
1. Under Models, choose "Laptop Display Panel (800x600)", and click on OK.
1. Back at the Change Display Type screen, click on Close.
1. Select the Desktop area in the middle of the Display Type screen.
1. Click and move the pointer to the right until the desktop area says "800x600" pixels.
1. Click on OK, and select "Yes" to restart the computer.

NOTE: DOS runs in 640x480, and the black border will exist until the computer is booted back into Windows 95. There are no video drivers for DOS, and unless a DOS application has its own video drivers, it will not run full screen.

Screen savers, background wallpaper, and color schemes - Screen savers, background wallpapers, and color schemes can occasionally make Windows 95 appear fuzzy, blurry, or distort the colors appearing on the screen. To disable the current screen saver, background wallpaper, and color scheme:

1. Click an open area on the Windows 95 desktop, click Properties in the menu, and click the Screen Saver tab.
1. In the center of the window, click the down arrow beside the current screen saver selection. From the list, click the word (None) to select it.
1. Now click the Appearance tab at the top of the window.
1. In the center of the window, click the down arrow beside the current scheme selection.
1. Scroll through the choices and select Windows Standard from the list.
1. Click the Background tab at the top of the window.
1. In the Wallpaper window, click the down arrow to scroll through the settings and select (None), then click the Apply button.
1. If your screen now appears normal, you can experiment with different settings.

Graphics acceleration can have an effect on LCD performance - adjusting the Advanced Graphics Settings slider can correct video problems ranging from error messages to erratic mouse movement. To get to the Advanced Graphics Settings slider indicator:

1. Right-click the My Computer icon in the Windows 95 desktop, then click Properties in the menu.
1. Click the Performance tab, then click the Graphics button.
1. Adjust the settings as desired:

• Full allows full utilization of the acceleration features available to the display driver.
• The 2/3 mark corrects some mouse pointer display problems.
• The 1/3 mark corrects some display errors.
• "None" corrects problems if the system stops responding to input or other severe problems.

4. Once the adjustments are made, click the OK button, click Close button in the System Properties window.
5. Click the "Yes" button to restart Windows 95.

In spite of the complexity found in most LCDs, they are remarkably modular devices. The flat-panel display contains all of the driver circuitry needed to turn each pixel or dot element on and off. The backlight assembly produces the light that makes each dot visible. The display controller, VRAM, system controller, and other circuitry needed to convert raw data into video signals is typically contained on the mobile computer’s motherboard. The display panel and motherboard are interconnected by a cable (usually a ribbon cable). Chances are very good that regardless of the particular symptoms that you see, the error can be tracked to one of these three areas. To aid you in referencing LCD parts, Table 27-1 lists display panels for many popular laptop models, while Table 27-1 provides an index of many displays based on part numbers.

NOTE: LCDs are extremely delicate and expensive assemblies. Use all precautions to prevent accidental damage while handling or replacing an LCD. Keep ample anti-static containers on-hand to hold new or used LCDs while not installed in a system.

Symptom 27-1. A portion of the display drops out or flickers intermittently. For example, the entire top 25% of the display cuts out or displays garbage, while the remainder of the display continues working fine. More or less of the display may be defective, but the problem typically extends all the way across the image. This problem also tends to be intermittent - flexing the flat-panel slightly or tilting the panel toward or away from you may stop the problem. Generally speaking, an intermittent connection has developed in either the flat-panel driver circuitry (on the rear of the display itself), or the display cable that connects the LCD and motherboard. Unfortunately, since the problem is intermittent, it may be extremely difficult to pinpoint.

Open the housing covering the LCD and check the interconnecting signal cable. Make sure that the cable is inserted into the LCD completely. You may also take a peek and see that the cable is inserted properly at the motherboard connector as well. Gently prod and flex the signal cable. If the intermittent appears, you may have a faulty signal cable. Try a new signal cable. If the cable appears steady, the problem is likely in the flat-panel assembly. In that event, the best course is to replace the LCD assembly outright - it would be virtually impossible to fix intermittent circuitry on the back of an LCD - the assembly is simply too delicate. Given the expense of most LCDs, you may want to check the price and availability of a new display panel, and contact your customer for their approval prior to actually ordering a new display.

There may be another option when it comes to repairing faulty LCDs. While the procedure is too involved for the purposes of this book, there are some companies that specialize in LCD panel repairs specifically (such as Man & Machine, Inc. listed at the end of this chapter). You may be able to send the defective LCD out for repair or exchange for a much lower price than purchasing one new.

Symptom 27-2. After the backlight times out, you notice that the display appears rather dim after the backlight kicks in again. After several minutes, display contrast improves, and the image looks fine. This is not necessarily a problem. The "even-ness" of a backlight source is somewhat dependent on heat which is produced as a CCFT warms a diffuser. Contrast adjustments are typically optimized for a "warm" backlight. If the backlight shuts off to conserve battery power, the CCFT and diffuser cool. Once mouse or keyboard activity start the backlight again, contrast may appear dull for several minutes as the backlight warms up again. Also check the contrast control to be sure that it is set at an optimum level after the backlight assembly is warm.

If it takes a relatively long time to restore contrast, or contrast is chronically dull (the image may also appear somewhat dark), it may indicate that the CCFT or EL element is failing - both enjoy only limited working lives. If the mobile PC is over several years old, or has logged an unusually large number of working hours, it is probably time to change the backlight element.

Symptom 27-3. One or more pixels is defective. The defective pixel may be black (opaque), white (clear), or fixed at some color. Before beginning any repair procedure, turn off your computer and initialize it from a cold start. A cold start ensures that your pixels are not being locked up due to any possible software glitch. If the questionable pixels disappear, there may be a bug in your application software - not in your hardware.

Run a diagnostic and test video memory thoroughly. If an error in VRAM is detected, one or more VRAM address locations may be defective, resulting in faulty video data being provided to the display. It is unusual for a problem in VRAM to manifest itself in this way, but you should be prepared for this possibility. If you do not have the proper tools or inclination to replace defective surface-mount ICs, you may prefer to replace the entire mobile PC motherboard.

Faulty pixels are a symptom that commonly occurs with active-matrix display panels. Since each screen dot is driven by either one or three individual driver transistors, the loss of one or more drivers will ruin a pixel. For monochrome displays, the single driver transistor may be open (pixel will not turn off) or shorted (pixel will not turn on). For color displays, damage to a driver transistor may cause a certain color to appear and remain fixed on the screen as long as the computer is running. Unfortunately, there is no way to readily repair a failed screen driver transistor. Active-matrix flat-panel circuitry is fabricated much the same way as any integrated circuit, so when any part of the IC fails, the entire IC must be replaced. Your best course is simply to replace the suspect LCD assembly. An outside LCD repair organization may be able to repair or exchange the LCD for less than it would cost to purchase a new one. It is important for you to remember that active-matrix LCD manufacturers may allow up to 10 bad pixels in a new assembly, so replacing the LCD may still leave you with a few bad pixels.

Symptom 27-4. There is poor visibility in the LCD. The image is easily washed out in direct or ambient light. The vast majority of current mobile computer LCDs operate in the transmissive mode - the light which your eyes see is generated from a backlight system. As a result, the strength and quality of the backlight directly effects the display's visibility. If the computer is older and has accumulated a great deal of running time, the EL backlight panel or CCFT(s) may be worn out or failing. The high-voltage power supply that is operating the backlight may also be faulty. Check your LCD's contrast control. Contrast adjusts the amount of driver voltage that is used to light the CCFT or EL panel. Less driver voltage means less light. This results in less contrast.

Disassemble your flat-panel assembly to expose the backlight unit (either an EL panel or a CCFT assembly) as well as the inverter power supply. GENTLY brush away any dust or debris that may have accumulated on the EL panel, CCFT(s), or diffuser. If the light source has been badly fouled by dust, retry the system with the cleaned light source. Use your multimeter to measure the AC output voltage from your inverter power supply. A working inverter should output 150 to 200 Vac for an EL panel, or 200 to 1000 Vac for CCFTs. Remember to use extreme caution when measuring high voltages. If your inverter output is low or non-existent, troubleshoot or replace your faulty backlight power supply. If the inverter's output voltage appears normal, replace the failing CCFT(s) or EL panel assembly.

Symptom 27-5. The display is completely dark. There is no apparent display activity. This symptom assumes that your computer has plenty of power and attempts to boot up with all normal disk activity. You simply have no display. If there are no active LEDs to indicate power or disk activity, there may be a more serious problem with your system. Begin by removing all power from your system. Remove the outer housings of your computer and inspect all connectors and wiring between the motherboard and display. Tighten any loose connectors and re-attach any loose or broken wiring. Defective connections can easily disable your display.

Check to see if the backlight illuminates. If there is no backlight, you will not see the display. Chances are that the backlight inverter power supply has failed. Measure the inverter output with your multimeter. Depending on the type of backlight being used, you may see anywhere from 200 to 1000 Vac. Remember to use extreme caution when measuring high voltages. If this voltage is absent, the inverter is probably defective and should be replaced. If this voltage is present, the CCFT or EL panel are probably defective and should be replaced.

If the backlight fires, but there is no display, the fault is likely in the display control circuit. If your mobile PC has a connector for an external analog monitor, attach one and see if a display is being generated. If an external monitor works, check the display cable leading from the motherboard to the LCD. Try a new cable. Run a diagnostic to check the video controller IC. If the video controller checks bad, it should be replaced (you may have to replace the entire motherboard). If the video controller checks OK, the LCD assembly may be defective. Try a new LCD.

Symptom 27-6. The display appears erratic. It displays disassociated characters and garbage. This is another symptom that assumes your mobile computer has plenty of power and attempts to boot up with normal disk activity. Your display is simply acting erratically. If no power indicators or disk activity LEDs are lit, there may be a much more serious problem in your system. Remove all power from your system and remove the outer housings to expose the motherboard and display assembly. Inspect all cables and connectors between the motherboard and display assembly. Tighten any loose connectors and secure any loose or broken wiring. Defective connectors or wiring can easily interfere with normal display operation. You may also wish to check the voltage levels powering your display.

Run a diagnostic and inspect the video system. Chances are that the video controller or VRAM has failed. If either part of the video system is identified as faulty, you should replace the motherboard outright. If your diagnostic(s) report the video system to be working properly, check or replace the video cable between the motherboard and LCD. Signal interruptions can easily interfere with the LCD. If another cable does not correct the problem, try a new LCD assembly.

Symptom 27-7. There is no video after closing the display cover. If the computer appears to restart after you lift the display cover, but there is no video, the problem could be with the small plunger switch beneath one of the hinges (often the right hinge). This is a known problem with some Micron notebook PCs. This switch turns the screen off when the display panel is shut. If the plunger switch gets stuck in the down position, it can cause this type of problem. Make sure no dust or dirt is causing the switch to stick in the down position. Compressed air can be used to blow away dust and dirt that has collected around the switch.

Symptom 27-8. A thin, faint horizontal line appears across the middle of a display. Otherwise, the display (and overall system) are running perfectly. This is really not a problem - that thin horizontal line going across the screen is normal for notebook computers with DSTN (Dual-Scan Twisted Nematic) displays. The DSTN screen is actually composed of two panels scanning simultaneously. By dividing the screen in two and refreshing both sides at the same time, you get a sharper, more responsive image than conventional passive-matrix displays. The thin line you see is the dividing line between the two panels. This line is most noticeable on screens with a white background, and least noticeable on screens with a colored background. The best way to deal with this issue is to experiment with different desktop color schemes.

Symptom 27-9. You note that there are unusual lines or spots on the LCD. The image looks fine when viewed from an external monitor. You may notice that every other line, or every forth line, or even an entire section of the image has cut out. This is almost certainly a fault of one or more bonds (TABs), and the entire LCD assembly should be replaced. It may be possible to send the display out to a service center which specializes in LCD repair.

Symptom 27-10. After the display goes blank, the system must be rebooted. This is often a mobile power management problem under Windows 95. Make sure you’re using the latest version of your laptop’s power management software. For example, Micron laptops require version 1.60 or later of the Phoenix Power Panel software (http://www.micronpc.com/support/file_lib/bbs/notebook.html). Until you get the latest version of the power management software, you can disable some of the power management features in Windows 95:

1. Click Start, then select Settings, then double-click the Control Panel icon.
1. In Control Panel, double-click the Display icon.
1. Click the Screen Saver tab.
1. Click the check boxes in front of the lines "Low power standby" and "Shut off monitor" so that both the check boxes are empty, and then click the Apply button.
1. Click the OK button to close the Display Properties window.

With these power management features disabled, the screen should not go blank, and the system should not lock up.

That’s it for Chapter 27. Be sure to review the glossary and chapter questions on the accompanying CD. If you have access to the Internet, take a look at some of the display panel resources listed below:

Laptop Computer Displays: http://www.buylcds.com/prod03.htm

Man & Machine, Inc.: http://www.man-machine.com/

Sharp Microelectronics: http://www.sharpmeg.com

Cobra Displays: http://www.cobradisplays.com/faq.htm

VTG Computer Monitor Tester Kit: http://www.datasynceng.com/vtgdoc.htm

Matrix LCD Repair List: http://www.matrixintl.com/lrc_co.htm