D0-D7
Inside a
processor chip, the Sequencer's data signal registers.
DAT (Digital audio tape)
A 4mm tape cartridge recording format was developed by HP
and Sony and is regarded by many as the de facto standard
for DAT. However the methods of preparing data for
recording, for indexing and reading, and for interfacing the
DAT to the computer are not standard. An updated
specification, the Data/DAT supports random- and
sequential-access modes and a data storage capacity of 2
GigaBytes (GB). Other references: DDS
and QIC.
Note: PC What's The Problem? includes installation, test, and repair suggestions.
Data-Storage Capacity
The number of concentric tracks that a hard drive's disk-platters have is set by the manufacturer and is dependent upon the drive's component mechanisms and the magnetic media on the platters. Before the drive can be used it must receive two formats, a low-level and a high-level. The low-level format sets the number of data-storage sectors per track on the disk through the process of storing sector addresses. Every sector on a disk begins with a non-data area reserved for storing its address code&emdash;the Sector ID Header. An equal number of sectors are addressed per concentric cylinder-track. Most drives have 17 sectors per track, some have more.
A sector is addressed by its cylinder, head and sector-sequence number. A low-level format (or "initialization"), by storing Sector ID Headers, reduces a disk's data-storage capacity. The DOS high-level format further reduces the data-storage capacity of a disk by creating a FAT.
File Allocation Table ( FAT ) keeps track of where all files are stored, and other hidden operating system files on the drive's disk-platters. (If it is a bootable and/or partitioned disk). Both formats plus any unusable "bad" sectors (due to defects in the media) equals the difference between a drive's manufactured capacity and its formatted capacity. This occurs only when the drive is matched with its intended controller. A disk matched with the wrong controller will either hold less data or not function.
Virtual File Allocation Table ( VFAT ) This is Windows 95's replacement FAT, adds long filenames, while maintaining compatibility with DOS and Windows 3.1. However, documents created by the 8- and 16-bit "legacy" applications are still limited to the 8�3 character filename format under VFAT.
Each sector holds 512 bytes of data in the standard DOS configuration. If you multiply this number by the number of sectors per track, and by the number of cylinder-tracks, and by the number of heads, the drive's capacity may be derived. (512 x spt x cylinders x heads = capacity.)
A software device-driver is necessary for the controller and drive to operate. Some drivers from third-party sources, may provide more efficient data-storage encoding methods for your drive. A sector can store only one file's data. A 1125K file has three data-clusters (512K + 512K + 101K) and needs three 512K sectors.
dB or Decibels
A unit of measure for the amplitude (strength) of a signal, often associated with sound.
DDR
Double Date Rate doubles the clock speed of SDRAM modules to 200MHz. Another reference: SDRAM.
DDS
Digital Data Storage 4mm format magnetic tape, an alternative also developed by HP, Sony and other companies, provides compatibility with current streaming tape drives&emdash;but is incompatible with data/DAT. The DDS (DDS-1) tape is 90 meters long, the DDS-2 tape is 120 meters long.
DDS-1 format stores up to 1.3 Gigabytes on a 60 meter tape and 2 GB on a thinner, longer 91 meter tape. On the tape the track width is 13.6 micro meters and linear density is 61 Kbpi. (A 8,10 mod channel is used.) D
DDS-2 format stores up to 4 Gigabytes on a 122 meter-long tape (a thinner and longer than DDS-2.) On the tape the track width is 9.1 micro meters and linear density is 61 Kbpi. (A 8,10 mod channel is used.) DDS-2 supports drives with a data transfer rate up to 466 Megabytes-per second.
DDS-3 format stores up to 12GB on a 125 meter-long tape. On the tape the track width is 13.6 micro meters and linear density is 122 Kbpi. (A PRML channel is used. ) DDS-3 drives have a data transfer rate as high as 1.2 GB per second.
DFE
Decision Feedback Equalization drive data-encoding method replaces the partial-response portion of the channel in many drive designs.
DHL
Dynamic head loading is usually found in slime-line hard drives designed for use in portable computers. DHL ensures that the drive has a very high non-operating and operating shock tolerance. The mechanism moves the heads off the platters and parks them on loading ramps whenever the drive is powered down or not is the active mode.
When the drive starts up, the heads remain on the ramps until flying speed is reached then the heads are launched off the ramps onto an established "air bearing," ensuring that no head/platter contact occurs.
When the drive is commanded to spin down (or sleep), the DC (direct current) spindle motor acts as a generator to power the suspensions back up the cams. The heads are lifted off the platters to a detent position, where they are prevented from slapping the disk or eachother in the event of a linear shock.
Dial Network
A network that can be dialed in via POTS (plain old telephone service) connection and shared among many users.
diff, Differential (SCSI-diff )
A parallel signal transceiver that supports SCSI cable length to 25 meters.
Digital-signal transmission
They have an advantage over analog-audio-signal transmissions because digital-pulses determine contents by modulation pattern and not by power strength and/or phase transitions (as for analog-audio-signals), digital signals are relatively immune to noise.
DIMM
Dual In-line Memory Module packaging, groups of chips are soldered onto both the front and back sides of the small board&emdash;as if two SIMMs were placed back to back. A 168-pin DIMM has two parallel rows of 64 connectors. Typically, DIMMs support Fast Page Mode (FPM) for either Extended Data Output (EDO) or Synchronous DRAM (SDRAM).
Tree adapters, such as the DimmSaver from Technology, Inc., can convert two 32-bit 72-pin SIMMs (placed back to back) into one 64-bit, 168-pin DIMM. Note: PC What's The Problem? includes identification, installation and testing suggestions.
Diodes
An electrical component. Generally, there are four families of diodes: rectifiers, zeners, SCRs, and triacs. They are used for regulating AC current and demodulating signals because diodes&emdash; except for the triacs&emdash;allow current to flow in only one direction with little loss. Also reference: Rectifier diode and Zener rectifier diode.
Rectifier diodes have two contact-terminals, an anode and a cathode. Most diodes allow the flow into a "cold" anode and out a "hot" cathode. Conduction is forward-biased and will not occur if a positive voltage is applied to the cathode. On circuitboards you may find many diodes; however there is a special arrangement found inside powersupplies&emdash;a rectifier. It consists of either a series of four input rectifier diodes or a four-pin bridge rectifier chip.
Zener rectifier diodes, the exception, permits current to flow in the opposite direction. Conduction is reverse-biased. When the voltage into the cathode exceeds the diode's specific (3, 3.3, 5, 6, 9, 12, or 24 Vdc) "breakdown" (or "zener") voltage, current flows through the diode. The transition to this state is sharp. While in this state, the diode keeps the voltage constant. Many Zener diodes are connected to grounds or will drop excess voltage across an adjacent current-limiting resistor.
SCRs (Silicon-controlled rectifiers) are a type of thyristor, a semiconductor used to switch heavy currents in response to low-voltage signals. In computer systems, they may be found in the monitor's (or TV's) horizontal-output circuit and/or in its switching or"chopper" regulator circuits.
SRCs have three terminals: an anode (A), a cathode (C) and a gate (G) which controls the SCR. An SCR is activated whenever it is both forward biased and when its gate-to-cathode voltage exceeds a specified threshold voltage or "trigger" (Vt) voltage. (Ordinary diodes are activated when they are forward biased.)
After the positive voltage is sent trough the gate terminal, an SCR continues to conduct current as long as it is flowing through it, even if there is no longer voltage at the gate terminal. It is "latched" into a this conductive mode until the current flow from anode to cathode is reduced below the device's holding value (Ih) by another device in series with the SRC. Note: SCRs can also be used to control the flow of AC current. The SCR is switched on during a specific phase of the AC cycle. Depending on the polarity of the device (+ or -) it will only conduct during one half of the AC cycle. At the zero-crossing point of each cycle, the holding current will be interrupted turning off the SCR.
Triacs are esentially two diodes connected in inverse parallel. Triacs have two anodes (A1 and A2) for conducting current in both directions. (Other types of diodes are monodirectional.) They also have two gates (G1 and G2). G1 for triggering the A1; G2 for the A2. A gate's trigger voltage must be the same polarity (+ or-) as the voltage through the triac. If the voltage from A1 to A2 is positive, the trigger voltage must be positive; if A1 to A2 is negative, then the trigger must be negative. Once triggered, a triac continues to conduct current as long as it is flowing through it, even if there is nolonger voltage at a gate terminal.
DIP
In Dual In-line Packaging, chips, which have pin-leads extending from two opposite sides, are installed individually. Their pin-leads are either inserted through holes in the circuit board and soldered on the reverse side, or plugged into a chip carrier's sockets. (The chip carrier is soldered onto the circuit board.) Socketed chips are easy to remove, (see Directory); soldered chips could be damaged by the difficult desoldering process.
Direct Connection
A permanent connection between your computer system and the Internet. This is sometimes referred to as a leased-line connection because the line is leased from the telephone company.
Disk array / Disc array
Two or more drives operating redundantly to provide fault protection for data recovery and which may also increase capacity and data transfer speed. Referencd RAID.
DLT
Digital Linear Tapes store digital data in a dense linear pattern. The DLT tape is stored in mini cassettes.
DPI ( Dots Per Inch )
DPI refers to the pixel density of something. It is most commonly used when dealing with image resolutions. People will often say things like "scan this image at 800 DPI" or "I can only see 72 DPI on the screen" and also "this printer can only produce output at 600 DPI". In all of these instances the people have been refering to image resolution. They are stating the desired image density which they are seeking.
The greater the DPI the more information content you are dealing with. An 8-1/2" X 11" 24-bit color image scanned at 800 DPI will have a size of 171.2MB. That same image scanned at 1200 DPI will be 385.2MB in size. As you can see, when the DPI level increases the content level goes up as does the resulting image size.
DRAM
Dynamic Asynchronous RAM Memory chip is made up of many cells, each one a very small solid-state capacitor managed by its own MOS transistor. To store a logical 1 bit, a cell retains an electrical charge. A no-charge state indicates that the cell is storing a logical 0. After a few milliseconds a cell will lose its charge. Therefore, cells inside a DRAM, storing logical 1 bits must be electrically refreshed before their charges are lost. The first Power Macs used DRAMs for main memory. Also reference: SDRAM.
. Note: PC What's The Problem? includes identification, installation and testing suggestions.
DOS
Disk Operating System was originally defined to manage the storage of data on the magnetic surface of a "floppy" disk; when hard drives were invented, DOS evolved to include the increased capacity. This mangement included the format preparation of the disk for data storage, retrieval and deletion by recording on the disk a sequence of sector addresses to identify the location of stored data. In subsequent versions of DOS, the management of other other system hardware resources were added. For software application support, DOS is a one-at-a-time manager allocating 640KB of its 1MB management capacity.
Often "DOS" is used to indicate that a product is compatible with all computers that use this operating system. Also reference: FAT and VFAT, HFS and RAID data storage methods.
ds
Double-sided disk, cartridge or CD.
DS0, DS1, DS2, DS3, and DS4 Transmission lines
Telephone companies matches their functional capabilities to categories of business services: T-1, T-2, T-3, and T-4 for leasing to businesses.
DSSS
Direct Sequence Spread Spectrum radio is used for 2.4GHz wireless local-area network communications.
DSU
The abbreviation for Digital Services Unit, DSU replaces the modem in synchronous connections to the Internet.
DTU
A processor chip's Data Transfer Unit which transfers data between WM and Cache Memory.
DTDS and DTDS+
Disaster Tolerant Disk Systems , reference: RAB.
DVD
Originally termed the "Digital Video Disc," then the "Digital Versatile Disc" format, but now it is just plain "DVD." These drives use the Micro UDF (Universal Disc Format) for data storage, based on the ISO 13346 standard, for storing files. DVD drives are backwards compatible with Video CD, but with an 18 GB capacity, DVDs will not work in CD drives. DVD supports MPEG-2..
DVD-ROM, DVD-R
This is a write-once read-only (WORM) format like that of a CD-ROM. It is used to reproduce versions based on a master CD.
DVD-R
Write-once (WORM) format, similar to CD-ROM's more do-it-yourself option.
DVD-RAM
Recordable and erasable, like the CD-E format. Note: PC What's The Problem includes identification, test, repair and replacement suggestions.
References are samples only. Each one is presented in greater detail in the
Technical Research Assistant for 2001
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