D

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data projector

A device that accepts output from a computer and projects it onto a screen or white wall. It accepts VGA output at resolutions typically up to 800x600 or 1024x768 and may also accept standard video output. In the 1980s, the first data projectors used tubes to create the image. They were large and weighed 40 pounds or more. Later, units were made as see-through LCD panels and put on an overhead projector for illumination. In the mid-1990s, projectors combined illumination and imaging in the same device and have shrunk down to just a few pounds. See LCD panel.

Device Manager

In Windows 95/98/2000, a dialog in the System control panel that shows the status of all the peripheral devices connected to your computer. See Win Device Manager.

DIMM

(Dual In-Line Memory Module) A printed circuit board that holds memory chips and plugs into a DIMM socket on the motherboard. See memory module.

DOS

(1) (Disk Operating System) Pronounced "dawss." A single-user operating system from Microsoft for the PC. It was the first OS for the PC and is the underlying control program for Windows 3.1, 95, 98 and ME. Windows NT, 2000 and XP emulate DOS in order to support existing DOS applications.

The DOS version that Microsoft developed for IBM was PC-DOS, and the version that all other vendors used was MS-DOS. However, except for DOS 6, which contains different versions of various utilities, PC-DOS and MS-DOS commands and system functions have been the same. All releases of PC-DOS and MS-DOS are generally called "DOS."

(2) (DOS, DOS/370, DOS/VS, DOS/VSE, VSE/ESA) A series of IBM mainframe "disk operating systems" for System/360, System/370 and System/390. DOS started as a variant of TOS (Tape Operating System), but soon supplanted TOS as disk storage became accepted in the late 1960s. DOS was always the "junior partner" to OS/360 and its progeny. It continues today as VSE/ESA.

(3) (Disk Operating System) A generic term for operating system. See operating system.

DOS/Windows

(1) Refers to a PC computer environment that uses DOS or Windows in contrast with Mac or UNIX.

(2) Refers specifically to a DOS and Windows 3.1 environment, in contrast with Windows 95/98.

DOS AUTOEXEC.BAT

AUTOEXEC.BAT is a special DOS batch file that is automatically executed whenever the computer is started or restarted. It must be stored in the root directory.

It is used to load TSR (Terminate and Stay Resident) programs that stay in memory and "pop up" whenever you call them. It's also used to start an application when the computer is turned on, perhaps a menu program that launches a variety of applications.

Two common commands in AUTOEXEC.BAT are:

PROMPT - Usually prompt $p$g.
PATH - The Path line contains the directories you want access to no matter which directory you're in.

To load or run a program, enter the program name on a separate line. Any DOS commands in AUTOEXEC.BAT will be executed like a normal DOS batch file.

The following example sets the prompt and path, loads the Doskey program in the DOS directory, switches to the LOTUS directory and runs the 123 program:

prompt $p$g
path c:\data;d:\words;e:\budget
c:\dos\doskey
cd \lotus 123

Manual changes to AUTOEXEC.BAT are done with a text editor such as the DOS Edit and the Windows Notepad utilities. Any word processor that can import and export ASCII files can also be used.

dot

(1) A tiny round, rectangular or square spot that is one element in a matrix, which is used to display or print a graphics or text image. See dot matrix .

(2) A period; for example, V dot 22 is the same as V.22.

(3) The dot, or period, is used as a name separator. For example, file names are separated from their extensions with a dot (CDE.ABC, CDE.NDX, etc.). It is also used to separate the components of Web addresses, such as between the "www," the registered domain name and the top level domain (TLD); for example, www.computerlanguage.com.

dot-com

Refers to the period (dot) followed by the abbreviation of the commercial domain (.com) at the end of an Internet e-mail or Web address. Since the .com domain is so widely used, the Internet has become known as the "dot-com" world, and dot-com companies are those that were formed to offer services or products on the Web. Since .com addresses are the most popular, Web browsers default to adding the .com to the end of the URL if no other domain, such as .org or .edu, is typed in. See Internet domain name.

dot matrix

A pattern of dots that forms character and graphic images on printers. Although ink jet and laser printers print in dots, and monitors display dots as well, the term generally refers to images created with dot matrix printer.

dot matrix printer

A printer that uses hammers and a ribbon to form images out of dots. The common desktop dot matrix printer, uses one or two columns of dot hammers that are moved across the paper. The more dot hammers used, the higher the resolution of the printed image. For example, nine pins produces draft quality, and 24-pin heads produce typewriter quality output. Speeds range from 200 to 400 cps, which is about 90 to 180 lpm.

A line matrix printer is a type of dot matrix printer that attains speeds up to 1,400 lpm and is used in datacenters and industrial environments (see line matrix printer).

Dot matrix printers are widely used to print multipart forms, address and diskette labels. The tractor and sprocket mechanisms used in these printers handle thick media much better than laser and ink jet printers.

Dot Matrix Mechanism Dot matrix printers print columns of dots in a serial fashion. The more dot hammers (pins), the better looking the printed results.

Dot Matrix Printer Dot matrix printers fitted with a tractor feed are commonly used for printing continuous multipart forms and mailing and diskette labels. The tractor feed contains a sprocket that grabs the perforated holes at both sides of the form and pulls it through uniformly.

DRAM

See dynamic RAM.

DS
(Digital Signal) A classification of digital circuits. The DS technically refers to the rate and format of the signal, while the T designation refers to the equipment providing the signals. In practice, "DS" and "T" are used synonymously; for example, DS1 and T1, DS3 and T3. See OC.
NORTH AMERICA, JAPAN, KOREA, ETC.
Service DS0 DS1 DS1C DS2 DS3 DS4	Voice Channels 1 24 48 96 672 4032	Speed 64 Kbps 1.544 Mbps (T1) 3.152 Mbps (T1C) 6.312 Mbps (T2) 44.736 Mbps (T3) 274.176 Mbps (T4)

EUROPE (ITU)
Service E1 E2 E3 E4 E5	Voice Channels 30 120 480 1920 7680	Speed (Mbps) 2.048 8.448 34.368 139.264 565.148

SONET CIRCUIT
Service STS-1 OC1 STS-3 OC3 STS-3c OC3c STS-12 OC12 STS-12c OC12c STS-48 OC48	Speed (Mbps) 51.84 (28 DS1s or 1 DS3) 155.52 (3 STS-1s) 155.52 (concatenated) 622.08 (12 STS-1s, 4 STS-3s) 622.08 (12 STS-1s, 4 STS-3c's) 2488.32 (48 STS-1s, 16 STS-3s)

DSL

(1) (Digital Subscriber Line) A technology that dramatically increases the digital capacity of ordinary telephone lines (the local loops) into the home or office. DSL speeds are tied to the distance between the customer and the telco central office. DSL is geared to two types of usage. Asymmetric DSL (ADSL) is for Internet access, where fast downstream is required, but slow upstream is acceptable. Symmetric DSL (SDSL, HDSL, etc.) is designed for short haul connections that require high speed in both directions.

Unlike ISDN, which is also digital but travels through the switched telephone network, DSL provides "always-on" operation. At the telco central office, DSL traffic is aggregated in a unit called the DSL Access Multiplexor (DSLAM) and forwarded to the appropriate ISP or data network.

Although DSL only arrived in the very late 1990s, there have been more versions and alphabet soup than most any other new transmission technology. The major DSL flavors are summarized here:

ADSL
Asymmetric DSL shares the same line as the telephone, because it uses higher frequencies than the voice band. However, a POTS splitter must be installed on the customer's premises to separate the line between voice and ADSL. A version of ADSL, known as G.lite, Universal ADSL, ADSL Lite and splitterless ADSL, is geared to the consumer. It eliminates the splitter and associated installation charge, but all phones on the line must plug into low-pass filters to isolate them from the higher ADSL frequencies. ADSL is available in two modulation schemes: Discrete Multitone (DMT) or Carrierless Amplitude Phase (CAP).

RADSL
Rate Adaptive DSL is a version of ADSL that adjusts speeds based on signal quality. Many ADSL technologies are actually RADSL.

VDSL (also VHDSL)
Very High Bit Rate DSL is an asymmetric version of DSL that is used as the final drop from a fiber optic junction point to nearby customers. VDSL lets an apartment or office complex obtain high-bandwidth services using existing copper wires without having to replace the infrastructure with optical fiber. Like ADSL, VDSL can share the line with the telephone.

HDSL
High Bit Rate DSL is a symmetric technology, which provides the same transmission rate in both directions. HDSL is the most mature DSL, because it has been used to provide T1 transmission over existing twisted pair without the additional provisioning typically required for setting up T1 circuits such as the removal of bridged taps and the installation of repeaters. HDSL requires two cable pairs and goes up to 12,000 feet, while HDSL-2 requires only one cable pair and supports distances up to 18,000 feet. HDSL does not allow line sharing with analog phones.

SDSL
Symmetric DSL is an HDSL variation that uses only one cable pair and is offered in a wide range of speeds from 144 Kbps to 1.5 Mbps. SDSL is a rate adaptive technology, and like HDSL, SDSL cannot share lines with analog telephones.

IDSL
ISDN DSL offers a rather low speed (144 Kbps) in both directions compared to other symmetric versions, but it does provide 16 Kbps more than standard ISDN, because the 16 Kbps "D" channel is used for data rather than call setup. It also offers the longest distance of 26,000 feet. Unlike standard ISDN, IDSL does not support analog phones, and signals are not switched through the telephone network. Since IDSL uses the same 2B1Q line coding as ISDN, ISDN customers can use their existing equipment (ISDN BRI terminal adapters and routers) when connecting to IDSL.

ADSL Transmission Using different frequencies in the line, ADSL allows data to ride over the same wires as voice conversations. The signals are combined and split apart at both sides. At the customer's site, the splitting is done either with an external device that must be installed by the telephone company, or it is built into the DSL modem.

DX2

A 486 with a clock-doubled CPU. Clock doubling doubles the internal speed of the CPU without requiring any changes in the chip's external connections. For example, the 486DX2/66 has an internal speed of 66MHz, while its external bus from the CPU to RAM runs at 33MHz. The 486DX2/50 has an internal speed of 50MHz and 25MHz externally. Intel called the DX2 a "Speed Doubler" chip.

     DX2 CPU       Internal     External
     486DX2/66     66MHz        33MHz
     486DX2/50     50MHz        25MHz
     486DX/50       50MHz        50MHz

     DX4 CPU       Internal     External
     486DX4/75     75MHz        25MHz
     486DX4/100   100MHz      33MHz

DX4

A 486 with a clock-tripled CPU. Clock tripling triples the internal speed of the CPU without requiring any changes in the chip's external connections. DX4s come in 75MHz and 100MHz versions that access RAM at 25MHz and 33MHz respectively. See DX2 and iCOMP for speed comparisons.

dye sublimation printer

A printer that produces continuous-tone images that look like photographic film. It uses a ribbon containing an equivalent panel of dye for each page to be printed. Color printers have either three (CMY) or four (CMYK) consecutive panels for each page, thus the same amount of ribbon is used to print a full-page image as it is to print a tenth of the page. Special dye-receptive paper is used, and consumables (ribbon and paper) cost more than other printer technologies.

The paper and ribbon are passed together over the printhead, which contains thousands of heating elements that can produce varying amounts of heat. The hotter the element, the more dye is released. By varying the temperature, shades of each color can be overlaid on top of each other. The dyes are transparent and blend into continuous-tone color.

Thermal wax transfer uses the same transport mechanism as dye sublimation, but uses a wax-based ink, not a transparent dye. Like other color printers, it puts down a solid dot of ink and produces shades of colors by placing color dots side by side (dithering). Wax transfer prints faster than dye sub and consumables (ribbon and paper) are less expensive, but it does not produce photorealistic quality.

Some printers allow swapping of both ribbons so that thermal wax can be used for draft quality and dye sublimation for final output. See printer.

Dye Sublimation/Thermal Wax Transfer The paper and ribbon are passed by the printhead. The ribbon is heated, and shaded dots of dye or solid dots of ink are transferred to the paper. The ribbon contains a panel equivalent in size to the page being printed, with three (CMY) or four (CMYK) panels for each color page to be printed.

Dual-Mode Printer In the mid-1990s, FARGO dramatically reduced the cost of dye sublimation printers with models starting below $2000. Using a thermal wax ribbon for draft quality and a dye sub ribbon for final output, this FARGO dual-mode printer supported media up to 12x20". (Image courtesy of FARGO Electronics, Inc.)

dynamic

Refers to operations performed "on the fly," which are based on decisions made while the program is running rather than beforehand. The expression, "buffers are dynamically created," means that space is created when actually needed, not reserved ahead of time. The expression, "data is compressed onto the disk dynamically" means that the compression algorithms are being applied when the data is being written rather than before. Contrast with static.

dynamic RAM

The most common type of computer memory, also known as D-RAM or DRAM. It usually uses one transistor and a capacitor to represent a bit. The capacitors must be energized hundreds of times per second in order to maintain the charges. Unlike firmware chips (ROMs, PROMs, etc.) both major varieties of RAM (dynamic and static) lose their content when the power is turned off. Contrast with static RAM.

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