P-Bus
Processor Bus,
inside a (RISC) processor. It is connected to the PIs and
managed by the Cache/Memory Management Units.
Parallel cable C-36 and CB-25
interfaces
Standard printer cables for parallel printers use a 36-pin
Centronics™ male-type "Champ" connector, on the one
end, that plugs into any printer with a Centronics™
parallel cable interface. On the other end of the cable is a
("D" shaped, 25-pin) DB-25 Male connector, which plugs into
the computer's female-type parallel (printer) cable
interface.
The significant wires are: Data Bits 1 through 8, one for each bit in a character byte. (Bit-1 is considered the least-significant bit and bit-8 is considered the most-significant.) Data Strobe fixes the pulse of on/off bit signals. Busy and ready-to-receive Acknowledgment, control flow of the pulse when they "handshake." The function of most of the other wires vary by communications application. 6.6 feet (20 m) is the recommended length. Cable longer than 12 feet could cause problems.
Standard TTL voltage levels are used for all signals. A signal between 2.4 and 5 volts is a HIGH or binary-logic 1. A signal between 0 and 0.8 volts is a LOW or binary-logic 0. Signals between 0.8 and 2.5 volts are considered invalid data.
Note: PC What's The Problem? includes functional troubleshooting suggestions.
Parallel Transmission
A transmission mode that separates bytes into bits so that each individual bit can be sent through a different wire within the same cable; for example, 8 wires for the 8 bits in a byte (after stripping off any stop, start and parity bits), 1 bit through each wire. A parallel cable has 8 or more wires with 8 wires designated for this purpose so that all 8 bits can arrive at the destination at-the-same-time making it easy to interpret them as a distinct byte again. (After each byte is received, stop, start and parity bits may be added to "frame" the byte for serial transmission within the destination device's circuitry.)
Parallel cables are limited in length, defects in individual wires over long distances could cause individual bits to arrive at different times making it impossible to interpret them as a distinct byte. Serial cables can be far greater in length than parallel cables; however, serial transmissions are slower because bytes, (with stop, start and parity bits added), are sent one bit at-a-time through a single wire. (Some time is saved; however, because the byte, when received, is already framed by stop, start and parity bits and can be immediately used within the destination device's circuitry.)
The primary advantage of parellel over serial transmission is in the presence of additional wires within the cable that are dedicated for carring signals back from the destination device for error checking each byte as it is received; as well as, communicating device availability and operational conditions. Bidirectional communications can make parallel more "intelligent" than serial transmissions.
Parity-Bit
An error-checking protocol. Error detection requires that both sender and receiver use the same parity setting. When the sending device selects Even-parity, it counts the number of 1s in an ASCII character-byte (a combination of seven or eight data-bits; a bit is either a 1 or a 0) as it is sent. If the total of 1s is not an even number, a 1 is used for the parity-bit to make it an even number.
Conversely, Odd-parity selection adds a 1 if the total of 1s is not an odd number&emdash;to make it an odd number. The receiving device adds the number of 1s in each character-byte. This total must be an even number if even-parity was selected, or an odd number if odd-parity was selected. Otherwise, an error has occurred. If so, a request is then made to send the character-byte again.
There are two other types of parity-bits which vary in their function: a Space-parity bit (always a 0), and a Mark-parity (always a 1).
PC
Inside a processor, a Program Counter.
PC
Personal Computer, a standard set by IBM in 1981 for microcomputers with Intel 8086 processors and later for faster, but backward-compatible processors. "PC" is used to indicate a product is compatible with all DOS-compatible computers.
PC card or PCcard (formerly "PCMCIA")
This is the Personal Computer Memory Card International Association's 68-pin (2 parallel 34-pin sets) interface set in one of the three standard slot widths:
PCcard-1 or "Type I" devices which are usually "flash" static RAM cards which slide into this 3.3mm-wide slot.
PCcard-2 or "Type II" devices are often 5.0mm-thick I/O cards and which plug into this 3.3mm-wide opening.
PCcard-3 or "Type III" devices require this slot which is wide enough, at 10.5 mm, to accomodate hard drives. Type III slots can also accomodate Type II and I devices, Type II slots can accomodate Type I devices. With 2-slot 11.6mm low-profile connectors, when a type III card is used in the lower slot, it is not possible to insert cards into the top slot. Typically, a card in the upper slot can be smoothly ejected by pressing the left eject button, the card in the lower slot by pressing the right eject button.
Another reference: CF Compact Flash (PCMCIA-ATA) card.
Note: PC What's The Problem? includes installation, test, and repair suggestions.
PCH
Phase-change drive.
PD-CD
Phase Dual-Compact Disk drives can read CD-ROM disks, but can only write to PD disks. PD disks cannot be read by CD-ROM drives even though they have the same 650 Megabyte capacities.
PDC
Power Disk Cartridge media standard assures that cartridge drives have the same physical dimentions as 3.5-inch hard drives.
PD-CD
Phase-change Dual-Compact Disk drives use erasable, rewritable 650 KB disks.
Persistence
A measure of how long the phosphor in a CRT picture tube will glow before fading out.
Photosensitive
An electrical or chemical sensor that reacts to, or changes state when exposed to light.
Phase Modulation
When the one pulse is at 90o, the other is at 0o. They are 90o out of phase. Phase-shifted signals can be interpreted as data bits. They may also be phased-shifted randomly (coded) by a modem to protect signals from eaves-dropping. The receiving modem, (usually of the same make and model) decodes these signals to make them readable again.
Another type of phase modulation occurs when one wave-pulse steps over another one, and becomes a reversed wave-pulse. Phase modulation like this is difficult to generate. Most high-speed modems use this process, called Phase Shift Keying (PSK), to increase the bits-per-second, and to detect signal "noise."
Phase Modulation
When the one pulse is at 90o, the other is at 0o. They are 90o out of phase. Phase-shifted signals can be interpreted as data bits. They may also be phased-shifted randomly (coded) by a modem to protect signals from eaves-dropping. The receiving modem, (usually of the same make and model) decodes these signals to make them readable again. Another type of phase modulation occurs when one wave-pulse steps over another one, and becomes a reversed wave-pulse. Phase modulation like this is difficult to generate.
Most high-speed modems use this process, called Phase Shift Keying (PSK), to increase the bits-per-second, and to detect signal "noise."
Phototransistors
These are referenced with all others: Transistors.
Plenum Cable
The National Electrical Code (NEC), sets fire safety requirements for cable installation. Spaces beneath raised floors, above drop-ceilings, and within walls&emdash;like heating and cooling ducts&emdash;move air throughout a building. These "air-plenum" spaces can feed oxygen to a fire and move smoke room to room, floor to floor. There is an increased hazard when cable is installed in these spaces. If a fire were to feed upon combustible materials, such as a cable's PVC insulation, the plenum-space would permit the rapid spread of heat, toxic-smoke and flames.
NEC regulations require cable, placed in air-plenum spaces, to be continuous within the space. Cable end-connectors, which extend the length of a cable by adding another length of cable, must not be placed inside the plenum. Cable, such as those with PVC insulation, must also be enclosed in a metal conduit-pipe (that also must be continuous within the space).
Plenum-grade cable, complying with NEC regulations, does not produce toxic smoke when burned; therefore, the conduit-pipe is not necessary to isolate the cable from the air in the plenum space. However, plenum cable still must be continuous, without connectors or repeaters physically within the plenum. Using plenum cable saves on the cost of the conduit, although it is more expensive than non-plenum cable. All computer and communications cables are available in plenum and non-plenum versions.
Other references: Cables and connectors, Instrument and Communications Cable, Computer Cable, and EIA/TIA Category Standard unshielded twisted-pair (UTP) cables.
Note: PC What's The Problem? includes installation, test, and repair suggestions.
PNP transistors
These are referenced with all others as:
Transistors..
PnP
Reference indicates that a device is compatible with Windows 95's Plug-and-Play standard.
Polarity Signals
Satellite signals can be broadcasted in horizontal and/or vertical polarity, clockwise and/or counterclockwise format. For example: C band broadcasts, 12 channels on vertical polarity and 12 channels on horizontal polarity totaling 24 channels.
Potentiometer or "pot"
These are referenced with all others as: Resistors.
POTS
Plain Old Telephone Service. A computer's modem dials a POTS number to access an Internet Service Provider (ISP). Typically, a user will choose an ISP with a local POTS number to reduce the cost of connection to that of a "local call," rather than paying for "long distance calls."
PP, PPI (Parallel-Port Interface)
They are used by drives intended for laptop and other small computers which do not have ST506, ESDI, IDE or SCSI interfaces built-in, or slots for interface adapters. Supports bidirectional transfer of data and commands.
PPM (Pulse-position modulation) format
For hard drives, it places data in the time between the positive peaks of the readback signal. The positive peaks correspond to the center of the written domain. Data is contained in the time between the transitions (edges) of the readback signals. Used for to store up o 650 Megabytes on each side of a Magnetic Optical disk-cartridge. Other references: PWM and PRML.
PRML (Partial Response Maximum Likelihood)
This technique encodes data before it is written onto magnetic media. in a hard drive. Technique is used in hard drives and tape drives. PRML can be used to store up o t 2.6 Gigabytes on each side of a MO disk-cartridge. This scheme, with its variants, including extended PRML (EPRML), increases the linear bit density despite the presence of linear symbol interference (which causes a degradation in the strength of a read-back signal through a linear cancellation of adjacent pulses).
prop.
Proprietary, not generally licenced to or available from other manufaturers except by licence or agreement.
Protocol
This set of communication rules, must be identical for both the sending and receiving systems. They are: a, the type of wave-like pulse "modulation," its shapes and frequency; b, process flow control, also known as "handshaking," using agreed-upon control-characters to coordinate the sending and receiving of data between systems; c, error detection and response&emdash;"send it again;" and sometimes d, data encryption.
PS/2
Used to indicate that a product is compatible with most (IBM standard) PS/2 computers.
psRAM
Pseudostatic RAM chips are a variety of DRAM often used in low-power notebook computers. For these applications, chose single memory modules, or boards with TSOP-type chips.
Pulse dialing
It utilizes breaks in DC current to designate the number being dialed.
PWM (Pulse-width modulation)
It increases the data storage capacity of drives by using laser-written marks of varying widths for a "4x" data storage density that is double the PPM recording method. (The PWM recording code is also known as RLL 1,7.) PWM cartridges can store up o t 1.3 Gigabytes on each side of a MO disk-cartridge. Other references: PPM and PRML.
QCIF
Quarter Common Intermediate Format (or "Quarter CIF"), is a 176 by 144 pixel image format for 29.97 frames per second videoconferencing. Compare with other video standards: CCIR, CIF, QSIF and SIF.
QSIF
Quarter Standard Image Format (or "Quarter SIF"), for composite color video in the USA is a 160 by 120-pixel image, as stipulated by the NTSC (National Television System Committee). In Europe it is a 192 by 144-pixel image, as stipulated by the PAL (Phase-alternation line) standard. Compare with other video standards: CCIR, CIF, QCIF and SIF.
QIC Quarter-inch magenetic cartridge
QIC mini-cartridge tape drives are also known as "floppy tape" backup subsystems for PCs. QIC tapes vary in length, the maximum recording density (how many flux transitions can be packed into a linear inch), and the type of formatting information on the tape (if preformatted when purchased).
Standard-length tapes are 205 feet long; extended-length (XL) tapes are 307.5 feet long. Data-storage capacity is determined by both the length of the tape and the compression software used; the number of tracks across the tape and the density of data within the tracks.
New techniques like PRML (Partial Response Maximum Likelihood read channels), MLR (Multichannel linear recording), and SPR (Symmetric Phase Recording) increase the number of tracks across the tape.
Read/record heads like TFMR (Thin-film Magnetoresistive) and MR (Magneto-resistive) increase the density of data within each track. The QIC 3010 standard specifies 40 tracks, 22,000 6pi linear density.
Other references: DAT, DDS and QIC-36.
QIC-36
The quarter-inch tape cassette drive 50-pin cable interface.
Quad Amplitude Modulation (QAM)
This technique combines up to four different phases with up to four different amplitudes (pulse-wave-heights) to send (16 combinations of) 4 bits per baud.
A 2400-baud modem, using QAM, can send 9600 bits per second. Increased amplitude piggybacks 2 bits into a single wave-pulse. Frequency (the distance between pulses) makes this possible. Piggybacked pulses with a "0" first digit, have less distance between them than those with a "1." Also reference: Combining Modulations.
References are samples only. Each one is presented in greater detail in the
Technical Research Assistant for 2001
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