Page 422 - Programmable Logic Controllers, Fifth Edition - Mobile version

Output instruction The term applied to any programmable logic
controller instruction capable of controlling the discrete or analog status Preset value (PRE) The number of time intervals or events to
be counted.
of an output device connected to the programmable logic controller. Pressure switch A switch activated at a specified pressure.
Output register or output word A particular word in a processor’s Process A continuous manufacturing operation.
output image table file in which numerical data are placed for
transmission to a field output device. Program A sequence of instructions to be executed by the processor
to control a machine or process.
Output scan One of three parts of the PLC scan. During the output
scan, data associated with the output status table are transferred to the Program files The area of processor memory in which the ladder
output terminals. logic programming is stored.
Programmable controller A computer that has been hardened to
Overflow Exceeding the numerical capacity of a device such as a
timer or counter. The overflow can be either a positive or negative value. work in an industrial environment and is equipped with special I/O
and a control programming language.
Overload A load greater than the one that a component or system is
designed to handle. Programmable read-only memory (PROM) A retentive memory
used to store data. This type of memory device can be programmed
Overload relay A special-purpose relay designed so that its contacts only once and cannot be altered afterward.
transfer whenever its current exceeds a predetermined value. Overload
relays are used with electric motors to prevent motor burnout due to Programming terminal A combination of keyboard and monitor
mechanical overload. used to insert, modify, and observe programs stored in a PLC.
Program scan One of three parts of the PLC scan. During the program
P scan, the CPU scans each rung of the user program.
Project file Contains all data associated with the PLC project. A
Parallel circuit A circuit in which two or more of the connected
components or contact symbols in a ladder program are connected project comprises five major pieces: help folder, controller folder, ladder
to the same pair of terminals so that current may flow through all the folder, data folder, and data base folder.
branches; contrasted with a series connection, in which the parts are Proportional-integral-derivative (PID) A mathematical formula
connected end to end so that current flow has only one path. that provides a closed-loop control of a process. Inputs and outputs are
continuously variable and typically will be analog signals.
Parallel instruction A programmable logic controller instruction
used to begin and/or end a parallel branch of instructions programmed Protocol A formal definition of criteria for receiving and transmitting
on a programming terminal. data through communications channels.
Parallel operation A type of information transfer in which all bits, Proximity switch An input device that senses the presence or absence
bytes, or words are handled simultaneously. of a target without physical contact.
Parallel transmission A computer operation in which two or more Pulse A short change in the value of a voltage or current level. A
bits of information are transmitted simultaneously. pulse has a definite rise and fall time and a finite duration.
Parity The use of a self-checking code that employs binary digits in
which the total number of 1s is always even or odd. R
PC Personal computer.
Rack A housing or framework used to hold assemblies; a plastic and/or
Peer-to-peer network A network in which nodes are given an equal metal assembly that supports input/output modules and provides a means
chance of initiating and controlling communications. of supplying power and signals to each input/output module or card.
Peripheral equipment Units that communicate with the programmable Random-access memory (RAM) A memory system that permits
logic controller but are not part of the programmable logic controller; the random accessing of any storage location for the purpose of either
example: a programming device or computer. storing (writing) or retrieving (reading) information. Random-access
PID Proportional-integral-derivative closed-loop control that lets the memory systems allow the data to be retrieved and stored at speeds
user hold a process variable at a desired set-point. independent of the storage locations being accessed.
Pilot-type device Used in a circuit as a control apparatus to carry Read The accessing of information from a memory system or data
electric signals for directing performance. This device does not carry storage device; the gathering of information from an input device or
primary current. devices or a peripheral device.
PLC processor A computer designed specifically for programmable Read-only memory (ROM) A permanent memory structure in
controllers. It supervises the action of the modules attached to it. which data are placed at time of fabrication or by the user at a speed
Polarity The directional indication of electrical flow in a circuit; the much slower than it will be read. Information entered in a read-only
indication of charge as either positive or negative, or the indication of memory is usually not changed once it is entered.
a magnetic pole as either north or south. Read/write memory Memory in which data can be stored (write
Polling A network access method where a master controller manages mode) or accessed (read mode). The write mode replaces previously
the communication process by interrogating each slave controller stored data with current data; the read mode does not alter stored data.
under it to determine whether the slave has any information to send. Real numbers Numbers that have both integer and fractional parts.
Port A connector or terminal strip used to access a system or circuit. Real-time clock (RTC) A device that continually measures time in
Generally, ports are used for the connection of peripheral equipment. a system without respect to what tasks the system is performing.
Power supply The unit that supplies the necessary voltage and current Rectifier A solid-state device that converts alternating current to
to a system’s circuitry. pulsed direct current.

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