Page 418 - Programmable Logic Controllers, Fifth Edition - Mobile version

Force function A mode of operation or instruction that allows an
operator to override the processor to control the state of a device. High-speed counter encoder module A module that enables you to
count and encode faster than you could with a regular control program
Force off function A feature that allows the user to reset an input written on a PLC in which the control program’s execution is too slow.
image table file bit or de-energize an output independently of the Histogram A graphic representation of the frequency at which an
programmable logic controller program. event occurs.
Force on function A feature that allows the user to set an image Host computer A main computer that controls other computers,
table file bit or energize an output independently of the programmable PLCs, or computer peripherals.
logic controller program. Human machine interface (HMI) Graphical display hardware in
Full duplex A mode of data communications in which data may be which machine status, alarms, messages, diagnostics, and data entry
transmitted and received simultaneously. are available to the operator in graphical display format.
Function block Rectangular block with inputs entering from the left
and outputs exiting on the right. I
Function block diagram (FBD) Graphical language where the IEC 1131 programming standard The international standard for
basic programming elements appear as blocks. programmable controller programming languages.
Function keys Keys on a personal computer, electronic operator Image table An area in programmable logic controller memory
device, or hand-held programmer keyboard that are labeled F1, F2, dedicated to input/output data. Ones and zeros (1s and 0s) represent
and so on. The operation of each of these keys is defined on many on and off conditions, respectively. During every input/output scan,
electronic operator interface devices. each input controls a bit in the input image table file; each output is
G controlled by a bit in the output image table file.
Immediate input instruction A programmable logic controller
Gate A circuit having two or more input terminals and one output instruction that temporarily halts the user program scan so that the
terminal, where an output is present when and only when the prescribed processor can update the input image table file with the current status
inputs are present. of one or more user-specified input points.
Gateway A device or pair of devices that connects two or more Immediate output instruction A programmable logic controller
communication networks. This device may act as a host to each network instruction that temporarily halts the user program scan so that the
and may transfer messages between the networks by translating current status of one or more user-specified output points can be updated
their protocols. to current output image table file status by the processor.
Glitch A voltage or current spike of short duration that adversely Impedance The total resistive and inductive opposition that an
affects the operation of a PLC. electric circuit or device offers to a varying current at a specified
Gray code A binary coding scheme that allows only 1 bit in the data frequency. Impedance is measured in ohms (V) and is denoted by the
word to change state at each increment of the code sequence. symbol Z.
Gray-encoder module Converts the Gray-code signal from an input Increment The act of increasing the contents of a storage location
device into straight binary. or value in varying amounts.
Ground A conducting connection between an electric circuit or Index A reference used to specify an element within an array.
equipment chassis and the earth ground. Indirect addressing An addressing mode in which the address of
Ground loop A condition in which two or more electrical paths the instruction serves as a reference point-instead of the actual address.
exist within a ground line. Inductance A circuit property that opposes any current change.
Ground potential Zero voltage potential with respect to the ground. Inductance is measured in henries and is represented by the letter H.
H Industrial terminal The device used to enter and monitor the
program in a PLC.
Half-duplex A mode of data transmission that communicates in two Input Information transmitted from a peripheral device to the input
directions but in only one direction at a time.
module and then to the data table.
Handshaking The method by which two digital machines establish Input devices Devices such as limit switches, pressure switches,
communication.
pushbuttons, and analog and/or digital devices that supply data to a
Hard contacts Any type of physical switch contacts. programmable logic controller.
Hard copy Any form of a printed document such as a ladder diagram Input/output (I/O) address A unique number assigned to each
program listing, paper tape, or punched cards. input and output. The address number is used when programming,
Hard drive An inflexible recording disk used as a computer disk drive. monitoring, or modifying a specific input or output.
Hardware The mechanical, electric, and electronic devices that Input/output (I/O) module A plug-in assembly that contains
make up a programmable logic controller and its application. more than one input or output circuit. A module usually contains two
Hardwired The physical interconnection of electric and electronic or more identical circuits. Normally, it contains 2, 4, 8, 16, 32, or
components with wire. 64 circuits.
Hexadecimal A number system having a base of 16. This numbering Input/output (I/O) scan time The time required for the processor to
system requires 16 elements for representation, and thus uses the monitor inputs and control outputs.
decimal digits zero (0) through nine (9) and the first six letters of the Input/output (I/O) update The continuous process of revising each
alphabet, A through F. and every bit in the input and output tables, based on the latest results

Glossary 399

pet73842_gls_395-406.indd 399 03/11/15 4:09 PM