Glossary of terms - PC Based Instrumentation and Control - page 453

Network A system which allows two or more computers to be linked via
a physical communications medium (e.g. coaxial cable) in order to exchange
information and share resources.

Nibble A group of 4 bits which make up one half of a byte. A hexadecimal
character can be represented by such a group.

Noise Any unwanted signal component which may appear superimposed on
a wanted signal.

NOR Inverse of the logical OR function.

NRZI NRZI (or Non Return to Zero Invert) is a method of encoding serial
data in which 1's and 0's are represented by opposite and alternating high- and
low- voltage states and where there is no return to the reference (zero) voltage
between encoded bits. NRZI is self-clocking and does not require a train of
separately transmitted clock pulses.

Operating system A control program which provides a low-level interface
with the system hardware. The operating system thus frees the programmer
from the need to produce hardware specific I/O routines (e.g. those associated
with disk filing). See also Disk operating system .

Option card A printed circuit board (adapter card) which complies with the
physical and electrical specification for a particular system and which provides
the system with additional functionality (e.g. asynchronous communications
facilities).
OR Logical function which is asserted (true) when any one or more of its
inputs are asserted.
Page A contiguous area of memory of defined size (often 256 bytes but can
be larger, see Expanded memory ).
Paragraph Sixteen consecutive bytes of data. The segment address can be
incremented to point to consecutive paragraphs of data.
Parallel interface (parallel port) A communications interface in which data
is transferred a byte at a time between a computer and a peripheral device, such
as a printer.
PCI The PCI (or 'Peripheral Component Interconnect') standard provides
a means of connecting 32- or 64-bit expansion cards to a motherboard. PCI
expansion slots are available in most modern PC's.

PCMCIA The PCMCIA (or simply 'PC Card') standard provides a means of
connecting a sub-miniature expansion card (such as a memory card or modem)
to a laptop or topic computer.
Peripheral An external hardware device whose activity is under the control
of the microcomputer system.
Port A general term used to describe an interface circuit which facilitates
transfer of data to and from external devices (peripherals).
Processor The processor is generally taken to mean the central processing
unit (CPU) which is invariably a single very large scale integrated circuit that