Page 421 - Programmable Logic Controllers, Fifth Edition - Mobile version

Multiprocessing A method of applying more than one microprocessor
to a specific function to speed up operation time and reduce the Odd parity Condition when the sum of the number of 1s in a binary
word is always odd.
possibility of system failure. Off-delay timer An electromechanical relay with contacts that
N change state a predetermined time period after power is removed from
its coil; on re-energization of the coil, the contacts return to their shelf
National Electrical Code (NEC) A set of regulations developed by state immediately; also, a programmable logic controller instruction
the National Fire Protection Association that governs the construction that emulates the operation of the electromechanical off-delay relay.
and installation of electric wiring and electric devices. The National Offline programming and/or offline editing A method of
Electrical Code is recognized by many governmental bodies, and programmable logic controller programming and/or editing in which
compliance is mandatory in much of the United States.
the operation of the processor is stopped and all output devices are
National Electrical Manufacturers Association (NEMA) An switched off. Offline programming is the safest way to develop or edit
organization of electric device and product manufacturers. The National a programmable logic controller program since the entry of instructions
Electrical Manufacturers Association issues standards relating to the does not affect operating hardware until the program can be verified
design and construction of electric devices and products. for accuracy of entry.
NEMA Type 12 enclosure A category of industrial enclosures intended On-delay timer An electromechanical relay with contacts
for indoor use and designed to provide a degree of protection against that change state a predetermined time period after the coil is
dust, falling dirt, and dripping noncorrosive liquids. They do not energized; the contacts return to their shelf state immediately on
provide protection against conditions such as internal condensation. de-energization of the coil; also, a programmable logic controller
Nested branches A branch that begins or ends within another branch. instruction that emulates the operation of the electromechanical
Network A series of stations or devices connected by some type of on-delay timer.
communications medium. One-shot A programmed technique that sets a storage bit or output
Network access control The method of accessing the network media for only one program scan.
(cable) to ensure that data are transmitted in an organized manner in Online data change Allows the user to change various data table values
order to reduce the possibilities of data corruption. using a peripheral device while the application is operating normally.
Node In hardware, a connection point on the network; in programming, Online programming and/or online editing The ability of a
the smallest possible increment in a ladder diagram. processor and programming terminal to make joint user-directed
Noise Random, unwanted electric signals, normally caused by radio additions, deletions, or changes to a user program while the processor
waves or electric or magnetic fields generated by one conductor and is actively solving and executing the commands of the existing user
picked up by another. program. Extreme care should be exercised when performing online
Noise filter or noise suppressor An electronic filter network used to programming to ensure that erroneous system operation does not result.
reduce and/or eliminate any noise that may be present on the leads to Open loop A system that has no feedback or auto correction.
an electric or electronic device.
Operand A number used in an arithmetic operation as an input.
Noise immunity A measure of insensitivity of an electronic system
to noise. Operational amplifier (op-amp) A high-gain DC amplifier used to
increase signal strength for devices such as analog input modules.
Noise spike A short burst of electric noise with more magnitude
than the background noise level. Optical coupler A device that couples signals from one circuit to
another by means of electromagnetic radiation, usually infrared or
Nonretentive output An output controlled continuously by a program visible. A typical optical coupler uses a light-emitting diode to convert
rung. Whenever the rung changes state (true or false), the output turns the electric signal of the primary circuit into light and uses a photo-
on or off; contrasted with a retentive output, which remains in its last transistor in the secondary circuit to reconvert the light back into an
state (on or off) depending on which of its two rungs, latch or unlatch, electric signal; sometimes referred to as optical isolation.
was last true.
Optical isolation Electrical separation of two circuits with the use
Nonvolatile memory A memory designed to retain its data while its of an optical coupler.
power supply is turned off.
OR A logical operation that yields a logic 1 output if one of any
NOR The logic gate that results in zero unless both inputs are zero. number of inputs is 1, and a logic 0 if all inputs are 0.
Normally closed contact (NC) A contact that is conductive when Output Information sent from the processor to a connected device
its operating coil is not energized. via some interface. The information could be in the form of control
Normally open contact (NO) A contact that is nonconductive when data that will signal some device such as a motor to switch on or off or
its operating coil is not energized. to vary the speed of a drive.
NOT A logical operation that yields a logic 1 at the output if a logic 0 Output device Any connected equipment that will receive infor-
is entered at the input, and a logic 0 at the output if a logic 1 is entered mation or instructions from the central processing unit, such as control
at the input. The NOT, also called the inverter, is normally used in con- devices (e.g., motors, solenoids, alarms) or peripheral devices
junction with the AND and OR functions. (e.g., line printers, disk drives, displays). Each type of output device
has a unique interface to the processor.
O
Output image table file A portion of a processor’s data memory
Octal number system A base eight numbering system that uses reserved for the storage of output device statuses. A 1, on, or true state
numbers 0–7, 10–17, 20–27, and so on. There are no 8s or 9s in the in an output image table file storage location is used to switch on the
octal number system. corresponding output point.

402 Glossary

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