Portable computers have a flat, LCD screen that is about .5 inch thick. The display is typically the most expensive component in a portable system. Often it is more economical to replace the entire computer than to replace the screen. An LCD display is designed to operate at a specific resolution because the size of the pixels on an LCD panel cannot be changed. On a desktop system, by contrast, the signal output from the video adapter can change the resolution on the monitor, thereby changing the number of pixels on the screen. An LCD panel should be thought of as a grid ruled to a specific resolution. Transistors control the color that is displayed by each pixel. The two major types of LCD displays used in portable systems today (dual-scan and active-matrix) are defined by their arrangement of transistors.
The dual-scan display (also known as a passive matrix display) consists of transistors running down the x and y axis of the screen. The number of transistors determines the screen's resolution. Each pixel on the screen is controlled by the two transistors that intersect on the x and y axis.
If a transistor fails, the entire line of pixels is disabled, leaving a black line across the screen. There is no way to repair this problem except to replace the display. The term "dual-scan" is derived from the fact that the processor redraws half of the screen at a time, which speeds up the refresh rate a little.
Dual-scan displays are considered inferior to active-matrix screens because they tend to be dimmer. They work by modifying the properties of reflected light rather than generating their own light. They are also more prone to ghost images, and make it difficult for two people to see the screen at the same time, because these displays can't be viewed well from an angle. The standard size for this type of screen is 10.5 inch (measured diagonally) with a resolution of 640 by 480. New systems are available with 12.1 inch displays that have a resolution of 800 by 600.
Active-matrix displays are also known as thin film transistors (TFTs). They differ from dual-scan screens because they have a transistor for every pixel on the screen rather than just at the edges. Voltages are applied by electrodes at the perimeter of the grid to address each pixel individually.
Because each pixel is powered individually, generating its own light and the appropriate color, a much brighter and more vivid picture results. Creating light instead of altering reflection provides a wider viewing angle, which allows more than one viewer to see the screen at a time. The refreshes are faster and lack the fuzziness associated with the dual-scan systems.
Naturally, the cost of having 480,000 transistors instead of merely 1,400 (on an 800 by 600 screen) makes the active-matrix screen more expensive. Another drawback is that it also requires a lot more power and drains batteries faster. Failure of a transistor causes individual "dead pixels," but this is far less noticeable than the black line caused by a transistor failure of the dual-scan screen.
The 12.1-inch screen has become the standard on high-end laptops with resolutions running at 800 by 600, or even 1,024 by 768. Many portable systems today also include PCI bus video adapters. These screens come very close to the quality of a desktop display.
An LCD display's resolution is determined as much by the screen hardware as by the drivers and amount of installed video memory. Some portables can use a "virtual screen" to achieve resolutions of 800 by 600 (and even more) on a 640 by 480 pixel screen. The larger display is held in video memory while the actual screen displays the portion that fits into a 640 by 480 window. The cursor can be used to "pan" the image so that the 640 by 480 window is moved around within the 800 by 600 display. Some manufacturers advertise an 800 by 600 display while using this method, which is a little misleading.
Like a desktop system, color depth is affected by video memory. To operate any LCD display in 16-bit or 24-bit color mode, you must have sufficient video memory available. Portables usually have the video adapter hardware permanently installed on the motherboard, which makes an upgrade virtually impossible. A few PC Card video adapters, however, allow you to connect to an external monitor and increase your video capabilities.
LCD technology has progressed to the point that large, flat-panel LCD-type displays are now available for desktop computers, although they're quite expensive.
Computer CPU manufacturers spend a great deal of time and effort on the design and creation of chips specifically for the portable market. In desktop systems, CPU heat is dissipated by cooling fans housed inside the case. There is no room for this solution in a portable system, so manufacturers have addressed this problem in the packaging of the chip itself.
Chip manufacturer, Intel's, solution to the size and heat problems is the Tape Carrier Package. This method of packaging reduces the size, power consumption, and heat generated by the chip. A Pentium mounted on a motherboard using Tape Carrier Packaging is much smaller and lighter than the pin grid array (PGA) used in desktop systems. The 49-millimeter (mm) square of the PGA is reduced to 29 mm, the thickness to approximately 1 mm, and the weight from 55 grams to under 1 gram.
The Tape Carrier Packaging processor is bonded to a piece of polyamide film (which is like photographic film) using tape automated bonding (TAB). This is the same process that is used to attach electrical connections to LCD panels. The film (called tape) is laminated with copper foil etched to form the leads that connect the processor to the motherboard. When the leads are formed, they are gold-plated to protect them against corrosion, bonded to the processor chip itself, and then the entire assembly is coated with a protective resin.
After being tested, the tape is cut to the proper size and the ends folded into a "gull wing" shape that allows the leads to be soldered to the motherboard while the processor is suspended slightly above it. A thermally conductive paste is inserted between the processor chip and the motherboard, allowing heat to be dissipated through a sink on the underside of the motherboard, while keeping it away from the soldered connections. Of course, because Tape Carrier Packaging processors are soldered to the motherboard, they usually cannot be upgraded.
Some manufacturers use standard PGA processors, sometimes accompanied by fans. As well as a greatly reduced battery life, these systems can be too hot to touch comfortably. Always check the exact model of processor that is used in a system you intend to purchase, not just the processing speed. You might not want to purchase a non-Tape Carrier Packaging processor for the aforementioned reasons.
Mobile Pentiums have operated at 3.3 volts from the days of the original 75-MHz chip, but the newer and faster models have reduced the voltage to only 2.9 volts for internal operations, while retaining the 3.3 volt interface with the motherboard. This translates into a processor that uses as little as 60 percent of the power of a desktop system.
As with desktop systems, adding memory is one of the most common upgrades performed on portable computers. Unlike desktop computers, which offer only three basic types of slots for additional RAM, there are dozens of different memory-chip configurations designed to squeeze memory upgrades into the small cases of the portable systems.
Some portables use memory cartridges that look a lot like PC Cards, but they plug into a dedicated IC memory socket. Others use extender boards like the SIMMs and DIMMs. In any case, it is strongly recommended that you only install memory modules that have been designed for your system, and only in the configurations recommended by the manufacturer. This does not necessarily limit you to products made by your system's manufacturer, however, because a number of companies manufacture upgrade modules for dozens of systems.
Portable computers use the same types of DRAM and SRAM as desktops and, thanks to advances in thermal management, today's high-end portable systems usually include SRAM cache memory.
Hard Disk Drives
Except for their size and packaging, portable hard disk drive technology is mostly similar to desktops. EIDE drives are standard in portable computers with the exception of the Macintosh computer, which uses SCSI. Internal hard drives, depending on the size of the system, are typically 12.5 mm or 19 mm tall, and use 2.5-inch platters. As with memory modules, hard drives are also mounted in the system a little differently by manufacturers. And, as with memory modules, this can cause upgrade compatibility problems.
Some manufacturers use a caddy to hold the drive and make connections to the system. This makes upgradability as simple as inserting a new hard disk drive into the caddy and then mounting it in the system. Other systems require you to purchase a specifically designed drive complete with the proper connections built into it. Replacing the hard drive can be much easier in many portable systems than in their desktop counterparts. This makes it possible for multiple users to share a single machine by simply snapping in their own hard drives. However, because laptops are specialized equipment, any servicing beyond batteries, hard drives, and memory is usually left to specialists or the manufacturer.
The support provided by the system's BIOS determines the upgradability of a system. Older systems, particularly those manufactured before 1995, might offer only limited drive-size options. BIOS chips made before EIDE hard disk drives became the standard can support a maximum hard drive size of 528 MB. A flash BIOS upgrade might be available for your system to provide additional drives. Another option for expanding hard drive space is the PC Card hard drive. This device fits into a Type III PC Card slot and can provide as much as 450 MB of additional space. External drives are also available and can be connected using a PC Card SCSI host or specialized parallel port drive interfaces-you can use any size SCSI drive you choose without being limited by your system's BIOS.
Portable systems are now equipped with other types of storage media that can provide access to large amounts of data. CD-ROM and Zip drives are now available, as well as standard floppy disk drives. Just as in their desktop counterparts, CD-ROM is becoming standard on portables.
The swappable drive bay is increasing in popularity. This product allows the user to switch one of several types of components in the unit. For example, you might not need a floppy disk drive when traveling, so you can insert an extra battery.
Portable keyboards are integrated into the one-piece unit and are therefore very difficult to repair or replace. Unfortunately, the keypad is almost always the first component to fail in a portable. The functionality and durability of the keyboard should be an important concern when purchasing a portable system.
Today's portable keyboards are approaching the size and usability of desktop systems, thanks to the larger screens found in most systems. This has created more space for manufacturers to utilize in the overall design.
Today's portable computers come with built-in pointing devices. Most of these pointing devices conform to one of three types: trackball, trackpoint, or trackpad.
This small ball (approximately .5 inch in diameter) is partially embedded in the keyboard below the spacebar. The ball is manipulated by the user's finger. These are accurate and serviceable, but they are unpopular because of their tendency to gather dirt and dust, which dramatically reduces performance.
The trackpoint was developed by IBM and many manufacturers install it in their systems. It is a small, rubberized button (approximately .25 inch in diameter) located above B and below G and H on the keyboard. The user nudges it in any direction (rather like a tiny version of a joystick) to move the cursor around the screen. It is convenient because the user's hands don't need to leave the keyboard to manipulate the trackpoint.
The trackpad is the most recent development of the three-it is an electromagnetically sensitive pad measuring about 1 by 2 inches located in the keyboard below the spacebar. It responds to the movement of a finger across its surface to move the cursor. Mouse clicks are simulated by tapping the pad (buttons are also provided). It's a truly innovative device, but does tend to be overly sensitive to accidental touches and taps. It is also sensitive to humidity, so moist fingers can cause unpredictable performance.