Hardware for PLCs 211

AC

Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz.

address

The unique identification of the location of the data for an input device, output device, or other program instructions in the CPU's memory. Addresses are listed on I/O modules and correspond to field devices.

alternating current

AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz.

analog

A continuously variable electrical signal with a given range of values. Analog signals communicate information about variable conditions, such as pressure or temperature.

anti-static shoes

Protective footwear that reduces the accumulation of excess static electricity. Anti-static shoes provide electrical resistance.

anti-static wrist straps

Protective equipment worn on the wrists in order to reduce the effects of electrostatic discharge when working with or around PLC components. Anti-static wrist straps prevent the buildup of static electricity in a person's body.

backplane

The area on the back of a modular PLC rack where I/O modules, the CPU, and power supply are connected. The backplane provides communication paths for electrical signals.

binary digits

The smallest unit of information on a computer. Binary digits, or bits, consist of 1s and 0s.

bytes

A group of eight binary digits. A byte is used to express computer memory sizes.

capacitor

An electronic component that stores an electric charge and releases it as needed. Capacitors may be used in CPUs as a back-up power source.

central processing unit

CPU. The microprocessor device inside a computer that controls system activities. The central processing unit executes a PLC program and performs all logic activities for the system.

chassis

An enclosure with slots in it that is used to connect the multiple units of a modular PLC. A chassis typically allows room for additional units.

commands

A specific instruction given to a computer application to perform a task or function. Commands may be entered into a PLC via a peripheral device.

condensation

The collection of water droplets that can form on a cold surface due to contact with humid air. Condensation can damage electrical equipment and cause power failure.

contaminants

Foreign particles, such as dirt or dust, that can interfere with machine operation. Contaminants can damage the electronic components of a PLC.

CPU

Central processing unit. The microprocessor device inside a computer that controls system activities. The CPU executes a PLC program and performs all logic activities for the system.

DC

Direct current. Electricity that forms when electrons flow in one continuous direction. DC is used in many PLC circuits.

diagnostic routines

A set of data files in a PLC used for investigating or analyzing the cause of errors or failures in the PLC. Diagnostic routines check for a variety of issues in a PLC.

digital multimeter

A device that can measure voltage, current, and resistance. A digital multimeter is used when verifying the voltage of an input/output device for a PLC.

direct current

DC. Electricity that forms when electrons flow in one continuous direction. Direct current is used in many PLC circuits.

discrete

An electrical signal that is either on or off. Discrete signals are two-state signals.

distributed control system

A control system consisting of programmable logic controllers placed strategically throughout a manufacturing facility. Distributed control systems require a high-speed communication network.

electrical signals

Voltage, current, or frequency that communicates information between two devices. Electrical signals are either analog or discrete.

electrically erasable programmable read only

EEPROM. A type of memory storage that allows data to be read, easily changed, and permanently stored. Electrically erasable programmable read only memory can store data in a range of sizes.

electrostatic discharge

Current produced when two objects having a static charge are brought close enough together to produce an arc or discharge. Electrostatic discharge can cause skin irritation or destroy an electronic device.

enclosure

A physical barrier designed to contain and protect the electrical components of a PLC system. A steel enclosure has doors that open and close.

erasable programmable read only memory

EPROM. A type of memory storage that can be easily read but is difficult to change. Erasable programmable read only memory prevents the data from unauthorized changes.

Ethernet cables

A type of network cable used to connect devices to create a local area network. Ethernet cables are available in a variety of bandwidths.

Ethernet ports

A socket or opening on a CPU or computer that accepts an Ethernet cable. Ethernet ports allow a CPU to connect to another device or modem on a network.

fatal errors

An issue that prevents a PLC from functioning. Fatal errors are usually caused by major programming errors or hardware problems.

fiber optic cables

A cable packed with thin, flexible glass or plastic optical fibers that transmit light instead of electricity. Fiber optic cables can be used to connect remote I/O modules to a CPU.

field devices

A component that connects to the input/output module on a PLC and responds to a condition or performs a task. Each field device, or input/output device, has its own electrical requirement.

fixed PLC

A type of PLC with the CPU, I/O module, and the power supply housed in a self-contained unit. A fixed PLC is used for applications requiring basic process control.

flow sensor

A flow control device that measures the speed or quantity of a flowing liquid or compressed gas. Flow sensors can convert physical conditions into analog electrical signals.

gateway

A device used to join incompatible hardware components on a network. Gateway devices enable communication between PLC components and are used with an interface cable.

gigabytes

GB. A unit of memory equal to 1 billion bytes. Gigabytes are used to measure the storage capacity of a device.

handheld programmers

HHP. A type of programming device used with PLCs to enter program instructions. Handheld programmers have a small display screen and keypad all in one unit.

hard drive

A memory storage device that permanently stores and retrieves data on a computer. A hard drive is a rigid device.

hardware

The physical components that form a PLC system. Hardware components for a PLC include the power supply, input module, output module, and CPU.

human machine interfaces

HMI. A peripheral device connected to a PLC that allows an operator to monitor a program and interact with the PLC. A human machine interface may have a graphical or textual display.

I/O modules

Input/output module. The portion of a PLC that contains connection points for field devices and circuitry to convert electrical current. An I/O module may be input only, output only, or a combination.

I/O points

Input/output points. A connection point on an input/output module for a field device wire. I/O points allow for electrical connections between field devices and input/output modules.

input device

A type of field device connected to a PLC that responds to a condition in a process and sends an electrical signal to the input module. Input devices include components such as limit switches, push buttons, and sensors.

input devices

A type of field device connected to a PLC that responds to a condition in a process and sends an electrical signal to the input module. Input devices include components such as limit switches, push buttons, and sensors.

input module

The portion of a PLC that receives high-voltage signals from input devices. The input module reduces the incoming voltage and sends it to the CPU.

input signals

An electrical signal from an input device. Input signals enter the input module and are then sent to the CPU.

input/output devices

I/O devices. A component that connects to the input/output module on a PLC and responds to a condition or performs a task. Each input/output device, or field device, has its own electrical requirement.

input/output module

I/O module. The portion of a PLC that contains connection points for field devices and circuitry to convert electrical current. An input/output module may be input only, output only, or a combination.

input/output points

I/O points. A connection point on an input/output module for a field device wire. Input/output points allow for electrical connections between field devices and input/output modules.

integrated circuits

A complete electronic circuit within a single component. An integrated circuit in a CPU stores program data and allows communication with field devices.

interface cable

A hardware configuration that connects devices for the purpose of network communication. Interface cables include the standard RS-232 cable that enables serial communication.

isolator

A device that separates a component from a source of electrical power. An isolator in an input module allows the CPU to receive a lower voltage signal.

key switch

A component on a CPU that an operator turns to select operating modes. A key switch's position indicates the current operating mode for the CPU.

kilobytes

KB. A unit of memory equal to 1,024 bytes. Kilobytes are often used to measure the size of small digital files.

limit switch

A type of mechanical contact that detects the presence or absence of an object. Limit switches are a common type of discrete input device.

line voltage

The voltage transmitted by a conductor or circuit to an electrical device. Line voltage is typically 120 volts alternating current or 240 volts alternating current.

load power supply

A component that provides electrical energy to the field devices connected to a PLC. The load power supply must support the electrical ratings for a variety of field devices.

local area network

LAN. A communications network that connects computers and associated devices together over a short or long distance and for private use. A local area network is typically confined to a single room or building.

locally

A method of operating a CPU that requires using the key switch to change operating modes. Locally operating a CPU typically allow users to switch between program mode and run mode.

megabytes

MB. A unit of memory equal to 1 million bytes. Megabytes are often used to measure the size of digital image files.

memory

The portion of a computer where data is stored. Memory storage can be built into the CPU of a PLC or in the form of a removable device.

memory card

A storage device that allows data to be easily read and easily changed. Memory cards continue to store data if the power source is removed.

memory module

A form of electrically erasable programmable read only memory. A memory module allows a CPU to have expandable memory storage.

micro PLC

A type of fixed PLC with 32 or fewer I/O points. A micro PLC is a self-contained unit.

microprocessor

A computer processor on a small electrical circuit. A microprocessor is responsible for most of the functions of the CPU on a PLC.

milliampere

mA. A unit of current equal to one thousandth of an ampere. Milliamperes may be used to measure motor power.

modular PLC

A type of PLC configuration that has removable and interchangeable I/O modules, CPU, and power supply units. A modular PLC can process a large range of electrical signal levels to meet the needs of a particular application.

motor control circuit

A circuit designed to provide power and control to electric motors. A motor control circuit uses an output device, such as a motor starter, to control a motor's speed.

motor starter

A device that starts or stops a motor circuit. Motor starters are common discrete output devices.

multiplex

A type of wiring that allows multiple signals to be processed through a single channel. Multiplex wiring is used to reduce the number of wires when connecting components in a PLC network.

network

A group of hardware devices connected through a cable or by wireless methods. A network allows multiple devices to communicate and share information.

non-fatal errors

An issue that will not interfere with the basic operation of a PLC. A non-fatal error is usually a minor programming issue.

nonvolatile

A type of computer memory that preserves any data in memory even if the power source is turned off. Nonvolatile memory is often used for long-term data storage or secondary data storage.

operating cycle

A sequence of events that the CPU performs when it is in run mode. The operating cycle repeats continuously as long as the CPU is in run mode.

operating modes

A process that the CPU runs to complete actions. Operating modes allow a CPU to accept program instructions or run a program.

output device

A type of field device connected to a PLC that performs a desired action after receiving an electrical signal from the output module. Output devices include components such as motor starters, pilot lights, and solenoids.

output devices

A type of field device connected to a PLC that performs a desired action after receiving an electrical signal from the output module. Output devices include components such as motor starters, pilot lights, and solenoids.

output module

The portion of a PLC that carries out commands from the CPU. The output module sends signals to output devices.

output module

The portion of a PLC that contains output connection points that allow output devices to connect to the PLC and receive signals. An output module sends signals to output devices.

output signals

An electrical signal that the CPU sends to an output module. Output signals cause an output device to turn on or off.

output status files

The memory area in the CPU that stores the on/off status of output points. The output status file contains the results of the program scan in the CPU's operating cycle.

peripheral devices

A component connected to a PLC that allows operators to interact with and monitor programs in the PLC. Peripheral devices include programming devices and computers.

personal computer

PC. A processor-driven device that runs multiple programs and processes simultaneously. A personal computer may be used as a programming device for a PLC.

personal computers

PC. A processor-driven device that runs multiple programs and processes simultaneously. A personal computer may be used to program a PLC.

personal protective equipment

PPE. An article of clothing or a device worn to minimize exposure to hazards and prevent injury. Personal protective equipment for employees working with or around PLCs include clothing that reduces electrostatic discharge.

PLCs

Programmable logic controller. A processor-driven device that uses logic-based software to provide electrical control to machines and processes. A PLC is a solid-state device.

power cycle

The process of turning a device off and then turning it back on. Power cycling is usually a troubleshooting step when clearing PLC errors.

pressure transducer

A device that converts variations in pressure into an analog electrical signal. Pressure transducers are a type of analog input device.

preventive maintenance

Regularly scheduled service and upkeep performed while a machine or system is still in working order. Preventive maintenance can prolong equipment life and increase production.

program

The instructions entered into a CPU's memory that direct the PLC to control field devices. A program contains lines of instruction.

program mode

A CPU operating mode that is active when loading program instructions into the CPU or when editing an existing program. During program mode, the CPU does not run a program or perform the operating cycle.

programmable logic controller

PLC. A processor-driven device that uses logic-based software to provide electrical control to machines and processes. A programmable logic controller is a solid-state device.

programming devices

A peripheral device used to enter a program into a CPU's memory. Programming devices include personal computers and handheld devices.

proportional hydraulic valve

A control valve used in fluid systems that can change its output in response to an input value. A proportional hydraulic valve is a type of analog output device.

proprietary

Designed by a specific company for use only with its own systems or devices. Proprietary components include physical hardware, such as programming devices, and software components, such as programming software and networks.

proprietary networks

A communications network designed by a manufacturer for a specific brand of PLCs. Proprietary networks create distributed control systems.

proximity sensor

A device that detects a change in a physical condition and turns it into an electrical signal. Proximity sensors are common discrete input devices.

push button

A type of mechanical device that turns on or off to control an aspect of a machine or process. A push button is a common type of discrete input device.

rack

An enclosure with slots in it that is used to connect multiple parts of a modular PLC. A rack allows some PLC components to share a power source.

random access memory

RAM. A type of temporary memory storage that allows data to be immediately stored, accessed, and changed. Random access memory is lost in the event that power is removed.

read only memory

ROM. A type of memory storage that permanently stores data that can be read but not changed. Read only memory can be used to store PLC programs.

remote I/O network

A type of communications network that connects I/O modules and field devices to the CPU across short distances. A remote I/O network may be used for just one machine in a process or for numerous machines.

remote program mode

An operating mode on a CPU that is activated from a peripheral device. Remote program mode is active when loading program instructions into the CPU or when editing an existing program from a remote device.

remote run mode

An operating mode on a CPU that is activated from a peripheral device. In remote run mode, the CPU is executing the steps of a program.

remote test mode

An operating mode on a CPU that is activated from a peripheral device. In remote test mode, the CPU runs a program without energizing output devices.

remotely

A method of operating a CPU that requires using a connected peripheral device to change operating modes. Remote operating modes also require the key switch on the CPU to be in the REM position.

run mode

A CPU operating mode that is active when the CPU is executing the steps of a program. During run mode, changes cannot be made to a program.

rung

A horizontal line in a PLC ladder diagram that contains program instructions. A rung is read from left to right.

serial port

An interface on a CPU that is used to connect peripheral devices. A serial port allows data to be transmitted between connected devices.

shielded twisted pair cables

A type of cable used for PLC network communications that reduces electrical noise. Shielded twisted pair cables consist of braided wire layered between an outer plastic jacket and inner conducting wires.

single-step test mode

An operating mode on a CPU that executes a single rung of a program. Single-step test mode completes program instructions without energizing output devices.

software

The programs and instructions that control a computer's hardware functions and operations. Software is written in a programming language.

software keys

A number or code entered into a keypad to access functions on a CPU. Software keys, or passwords, allow an operator to switch between operating modes.

solenoid

A device that converts electrical energy into linear mechanical force. Solenoids are common discrete output devices.

stand-alone device

A computer or other device that is not permanently connected to a communications network. A stand-alone device can perform its function without requiring additional devices.

status LEDs

A light made of a semiconductive material that flashes different colors to provide information about a CPU or other PLC modules. Status LEDs indicate conditions such as the CPU's operating mode, network communication, or field device status.

system power supply

The PLC component that provides electrical energy to the CPU and I/O modules. The power supply sends low voltage, direct current to the CPU and I/O modules.

terminal block

An electrical connector on an I/O module in which the field device wires are clamped down. Terminal blocks are interchangeable and may be moved between modules.

test mode

An operating mode that allows a CPU to run a program without energizing output devices. Test mode may be used to check for program errors.

topology

The configuration or physical pattern of network cable and devices in a local area network. Common topologies include a bus, ring, and star shape.

transducer

A device that converts variable quantities into analog electrical signals. Transducers are used with analog input devices.

troubleshooting

A systematic approach to solving problems quickly and efficiently. Troubleshooting often involves a logical process of elimination to identify the source of a problem.

variable frequency drive

VFD. A device that converts incoming AC power into other desired frequencies to allow for AC motor speed control. A variable frequency drive is a common analog output device.

ventilation

The process of letting in fresh air. The ventilation in a PLC enclosure allows warm air to be drawn out of the top of the enclosure.

volatile

A type of computer memory that does not preserve data in the event of a power outage. Volatile memory includes random access memory.

volts

V. A measure of electrical pressure or potential. Volts may be abbreviated as V.

wireless local area network

WLAN. A communications network that sends and receives signals through the air instead of using cable. Wireless local area networks allow for the use of mobile devices and allow communication in areas that are impractical for wired networks.