PLC Installation Practices 340

analog

Characteristic of a continuously variable signal. Analog signals differ from digital signals in that small fluctuations in the analog signal convey meaningful information.

bandwidth

The amount of information or data that can be sent over a communications channel in a given period of time. The higher a channel's bandwidth, the more information it can carry.

battery low

An indicator lamp on a PLC signaling that the battery is low on power and needs to be recharged or replaced.

binary

A number system used for PLCs that has either 0 or 1 as a value. Using binary, a function is either on or off.

cable lubricant

A commercial lubricant used to facilitate network cable installation. Cable lubricant allows the cable to move easily through tight spaces and prevents the outer covering from being damaged.

CAT 5

100 to 1000 Mbps cable used for high level network applications that require a strong signal. CAT 1 through CAT 4 are used for analog, voice, and low level network applications.

central processing unit

The main processor of information in a PLC that performs all logic and math operations. Also called the controller.

centralized control system

A type of PLC system in which one PLC controls an entire process.

chassis

An enclosure with slots in it that is used to connect multiple parts of a PLC.

coaxial cable

A heavy-duty copper cable consisting of a central conductor wrapped in shielding and a protective jacket. Coaxial cable has a high resistance to electrical noise.

common

The screw terminal on a PLC that connects to the power source.

conduit

A tubular enclosure for holding wires or cables. Conduits should be used to minimize electrical noise in wires by keeping them separate.

connector

A device at the end of a cable used to join it with another cable or device.

CPU

Central Processing Unit. The main processor of information in a PLC that performs all logic and math operations. Also called the controller.

cross-talk

Interference between two network cables when the signal from one cable is unintentionally picked up by another cable nearby.

digital multimeter

A device that can measure voltage, current, or resistance. A digital multimeter is the most versatile and common meter used today for electrical maintenance.

DIN rail

A metal track or rail to which PLCs and other motor control devices can be easily attached or removed.

diode

A solid-state device that allows electricity to flow in only one direction.

DIP switches

Dual In-line Package switches. DIP switches use binary settings, on-off / yes-no, to configure various options on a device.

disable command

A command used to prevent an output device from operating. Disable is the opposite of the force command, and it can be used to prevent the operation of one or all of the output devices.

discrete

A signal that has been reduced from continuous signal to a sequence of individual quantities. Discrete devices have two states: ON and OFF.

distributive control system

A type of PLC system in which remote I/Os and control devices are divided among multiple PLCs.

electrical noise

An AC power line disturbance caused by sudden changes in the load. Electrical noise is a problem for solid state devices because they cannot distinguish between an intended electrical pulse and an unintended electrical spike.

electromagnetic interference

The disruption of transmission or reception of a signal caused by electrical and magnetic fields.

electrostatic discharge

A discharge of static electricity that can potentially cause injury or damage components, especially sensitive electrical components. Also known as ESD.

expandable I/O

An input/output section that can be expanded to allow for more inputs and outputs connected to a PLC.

fatal error

An error that prevents the PLC system from functioning. Fatal errors are usually caused by major programming errors or hardware problems.

fault

An indicator lamp on a PLC that lights up when there is an error in the PLC system. The PLC will typically de-energize all outputs when this LED is activated.

fiber optic cable

A cable packed with thin, flexible glass or plastic optical fibers that transmit light instead of electricity.

fixed I/O

An I/O section located onboard the PLC in a unit that also contains the processor, input/output modules, and power supply. With fixed I/Os, you cannot add more inputs or outputs than the number that have already been allotted.

force I/O command

A troubleshooting command used to override the input/output status of an application in order to test or debug a PLC program. The force command overrides any safety features in the PLC program.

heat sink

A conductive metal sheet with large fins that dissipate heat. Heat sinks protect solid-state devices from damage caused by thermal stress.

I/O module

The part of a modular PLC to which input and output devices are connected.

I/P transducer

Current/power transducer. An I/P transducer varies pressure and voltage.

input/output section

The jack where an input/output device is physically connected to a PLC. The input relays transfer signals to the internal relays. The output relays signals to external output devices.

isolation transformer

A transformer used to reduce or eliminate electrical noise by modifying the amount of voltage or current on the line.

master control relay

A safety relay used to de-energize supply power and provide a controlled safe stop to machinery.

modular PLC

A PLC configuration in which each component is split into a separate unit. A modular PLC lets you have as many inputs and outputs as you want.

NEC

The standard for minimum safe electrical installations. The National Electrical Code is adopted in some form as law in all 50 states.

network

A system in which two or more linked devices use a set of protocols to communicate with each other. PLC networks are often used to control remote I/Os.

NFPA 79: Electrical Standard for Industrial Machinery

A standard for electrical/electronic equipment developed by the National Fire Protection Association. NFPA 79 is also part of the National Electric Code (NEC).

polyethylene

A type of plastic often used for devices that house electrical wire and network cable.

power

An indicator light on a PLC signaling that the PLC has been turned on. This light should always be on when the PLC is operating.

power supply

A separate power source for a PLC that converts the standard incoming 120VAC to the low voltage DC that the CPU requires, usually around 5V.

printed circuit board

A layered construction of material used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive surface.

programmable logic controller

A processor driven device that uses logic-based software to provide electrical control to machines.

programming device

A device, usually a personal computer, used to enter instructions into the PLC.

raceway

A channel or track used to hold network cable.

rack

An enclosure with slots in it that is used to connect multiple parts of a PLC.

rack I/O

A larger type of PLC that is a collection of I/O cards that are linked together and stored in a rack. A rack I/O can handle thousands of inputs and outputs.

run

An operational switch on the CPU that allows the system to run, but no changes can be made to the program.

shielded twisted pair

Twisted pair cable that has a layer of braided wire between the outer plastic jacket and the inner conducting wires to protect them from electrical noise.

sink/source I/O module

An I/O module with circuitry that allows current to flow in either direction.

sinking

A wiring arrangement in which the I/O device provides current to the I/O module.

snubber circuit

An electrical circuit used to suppress electrical spikes. Snubbers are often used with inductive loads.

solid core

A type of twisted pair cable that is made up of one large central conductor. Solid core cable is inflexible and hard to install.

sourcing

A wiring arrangement in which the I/O module provides current to the I/O device.

status lamp

A light-emitting diode on a PLC that provides information on the operating condition of various parts of the PLC. Status lamps are used to show the status of inputs, outputs, and the operating conditions of the CPU.

stranded core

A type of twisted pair cable that consists of multiple connector wires within a single insulated casing. Stranded core is flatter and more flexible than solid core.

surge suppressor

A device that protects equipment from damaging electrical surges by regulating the voltage on the line.

thermal grease

A paste often applied between metal surfaces to increase the conduction of heat between them. Thermal grease should always be applied between a solid-state device and a heat sink.

thicknet

A relatively thick type of coaxial cable used for heavy industrial applications. Thicknet has a 0.4 in. (1 cm) diameter.

thinnet

A relatively thin type of coaxial cable used for lighter applications such as office networks. Thinnet has a 0.1 in. (0.4 cm) diameter.

tie-wrap

A thin plastic fastener used to tie and secure cable. One end of the tie-wrap has a hole through which the other end can be inserted to form a loop.

twisted pair cable

A type of cable used for communications. Twisted pair consists of pairs of copper wires twisted together.

unshielded twisted pair

A commonly used twisted pair cable that provides little protection from electrical noise.

user manual

A document created by the manufacturer of a device that provides detailed installation and operation instructions.

voltage spike

A sudden, short surge in voltage. Voltage spikes can be caused by lightning, power outages, short circuits, or power transitions in large equipment on the same power line.

watchdog timer

A timer that monitors how long it takes the CPU to complete a scan. Watchdog timers output an error message if the CPU scan takes too long.