Networking for PLCs 261

access method

A network routine that controls data transfers and prevents collisions. The access method for a LAN requires network devices to sense when other devices are transmitting data.

address

The unique identification of the location of the data for a networked device. Addresses are necessary for devices to send and receive data.

antenna

A rod or wire that converts radio wave frequencies into alternating current or vice versa. Antennas can send or receive signals on a wireless local area network.

bandwidth

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

binary digit

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

bridge

An interconnecting device with two ports that forwards data between two segments or two local area networks. Bridges forward data without reading it.

buffer

A component that surrounds the core in a fiber optic cable to isolate it and protect it from physical damage. A buffer may be contained within the cable or be a plastic coating around the cable.

bus topology

A type of network topology in which all the nodes are connected to a common line with two terminating resistors. A bus topology sends data in one direction.

bytes

A group of eight binary numbers. Bytes are used in networks and PLCs to send data.

cable connector

The part of a cable that plugs into a port to connect devices. A cable connector is a type of interface used with transmission media.

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.

centralized control

A type of industrial network configuration that designates one component as the controller to manage the communication and execution of tasks for all other components. Centralized control systems include remote I/O configurations.

cladding

The protective layer surrounding the core of a fiber optic cable. The cladding protects the core from contaminants, supports the optical fibers, and reduces loss of light from the core.

coaxial cable

A type of transmission medium used for PLC networks that consists of a central conductive copper wire surrounded by an insulating layer and a shielding layer, all enclosed in a jacket. Coaxial cable has high electrical noise resistance.

code

A system that uses numbers or symbols to represent information other than simple numerical quantities. Codes allow operators to enter information into a PLC.

collision detection

The ability of network nodes to sense when other devices are transmitting data. Collision detection is part of a network access method.

collision domains

A network segment where nodes send data packets at the same time. Collision domains are reduced by adding network segments.

communication link

A channel that connects two or more devices on a network and allows them to send and receive data. A communication link may be physical or virtual.

conductive

The ability to produce electricity. A conductive copper cable is used in a coaxial cable.

configuration table

Information contained within a router that identifies the location of network devices and the pathways to those devices. A configuration table allows a router to send data to the correct place.

CSMA/CD

Carrier sensing multiple access with collision detection. A LAN access method in which devices check the network to see if it is clear before transmitting data. CSMA/CD can use a lot of bandwidth.

daisy chain topology

A type of network topology in which nodes connect directly to each other in a linear or circular formation. A daisy chain may be used to add nodes to an existing topology.

data collisions

The result of simultaneous data packet transmission between two or more devices on a portion of a network. Data collisions force network devices to resend data packets.

decentralized control

A type of industrial network configuration that connects multiple PLCs and I/O modules throughout a facility. A decentralized control system, or distributed control system, allows all networked devices to share data.

DeviceNet

An industrial network and protocol that operates using a distributed control system. DeviceNet started out as proprietary but is now an open protocol.

dielectric material

An electrical insulator that transmits electric force without conductivity. Dielectric material is used around the copper wire in coaxial cable to help protect against electrical noise.

distributed control system

DCS. A type of industrial network configuration that connects multiple PLCs and I/O modules throughout a facility. A distributed control system, or decentralized control system, allows all networked devices to share data.

electrical noise

A power line disturbance caused by sudden changes in the load. Electrical noise is problematic to a PLC because the components cannot differentiate between an intended electrical spike and an unintended electrical spike.

electromagnetic interference

Electrical noise in a circuit that can interrupt or degrade the performance of the circuit. Electromagnetic interference can be reduced by using fiber optic cable.

encodes

The process of converting data into a format that allows it to be transmitted by computers. Encoding is necessary because computers only recognize binary data.

ethernet cable

A type of transmission medium that is able to withstand harsh industrial environments. Ethernet cables are used to connect devices on local area networks (LAN).

fiber optic cable

A cable used for PLC networks that is packed with a thin glass or plastic core, a layer of cladding, a buffer, and a plastic jacket. In a fiber optic cable, data travels as particles of light rather than as digital signals.

fieldbus

A type of industrial network and protocol that uses a distributed control system so that multiple PLCs can communicate. Fieldbus is an open protocol designed to be used with other networks.

firewall

A security barrier in a network that restricts access to authorized users and blocks unwanted content. A firewall is often included in a router.

gateway

A device used to join two incompatible hardware components or protocols on a network. Gateway devices are available as preconfigured or configurable.

header

A data field within a packet that includes the addresses of the sender and receiver of the data message. The header helps ensure that the data packet is sent to the correct address.

host computer

A computer that controls communication with the other computers or devices on a network. A host computer can also perform specialized functions in coordination with other computers.

hub

An interconnecting device with multiple ports that broadcasts a signal to connected nodes on a network. A hub continues to send data to other active nodes if one or more nodes malfunctions.

human machine interface

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 connect to an industrial network.

hybrid topology

Two or more topologies used for one network. Hybrid topologies allow for flexible and customizable setups.

I/O devices

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

I/O modules

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

industrial networks

An internet or intranet network used to connect devices for factory automation and process control applications. Industrial networks must meet standards set by the Institute of Electrical and Electronics Engineers.

input/output devices

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

input/output modules

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

Institute of Electrical and Electronics Engineers

IEEE. An organization composed of engineers, scientists, and students. The Institute of Electrical and Electronics Engineers sets standards for the computer and electronics industries.

insulated

Covered in a material that prevents the passage of electricity. Insulated copper wire is used in twisted pair cable.

interconnecting devices

A network hardware device that connects components and helps transmit data signals between components and parts of a network. Interconnecting devices include repeaters, hubs, switches, and bridges.

interdependent

To relate to or be reliant on. Interdependent nodes in a ring topology must all fully function to transmit data.

interface

A hardware device that connects devices for the purpose of network communication. An interface can also join protocols.

internet

A global communication network accessible by anyone. Internet networks allow an unlimited number of devices to connect.

Internet Engineering Task Force

IETF. The regulatory body that defines open network protocols. The Internet Engineering Task Force maintains the TC/IP protocol suite.

intranet

A local or private communication network accessible only to authorized users. Some intranet networks allow access to the internet.

jacket

A plastic protective outer layer on a cable. The jacket protects the inner wires in a cable from damage and prevents signal interference.

LAN

Local area network. A communication network that connects computers and associated devices together over a short or long distance. A LAN uses a wired connection.

LANs

Local area network. A communication network that connects computers and associated devices together over a short or long distance. A LAN uses a wired connection.

linear

A straight or nearly straight line. Linear topologies transmit data in a single direction.

local area network

LAN. A communication network that connects computers and associated devices together over a short or long distance. A local area network uses a wired connection.

logical link

A communication channel that connects devices and conveys a data signal between devices. A logical link is used with electromagnetic waves.

logical topology

The path that data takes as it travels between nodes on a network, regardless of the physical layout of the components. The logical topology of a network may differ from its physical topology.

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.

network

A group of hardware devices connected via cable or wireless methods. A network allows two or more devices to communicate and share information.

network security

The practice of preventing unauthorized access to a network or network resources. Network security blocks unauthorized users from accessing or altering secure information.

nodes

An active electronic device on a network. Nodes are able to create, send, or receive data on a network.

packets

A unit of data that a network protocol sends through the network to the correct destination. Packets consist of bits or bytes of data.

packet-switching

A method of sending data through a network that involves separating data into small units and reassembling it into the proper order when it reaches its destination. Packet-switching techniques optimize a network's bandwidth capacity.

peer-to-peer

An industrial network configuration that connects multiple PLCs so they can share resources without using a central computer. In a peer-to-peer network, all PLCs can initiate communication and share data.

peripheral devices

A physical device connected to a computer or PLC that performs an auxiliary function. Peripheral devices include printers and programming devices.

physical link

A communication channel that connects devices and conveys a data signal between devices. A physical link is used with various types of cables or wires.

physical topology

The arrangement or configuration of network cables and nodes in a network. Common physical topologies include linear and circular setups.

PLCs

Programmable logic controllers. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A PLC can replace many physical relays and hard-wired connections in a process.

port

A connection point on a device that allows devices to connect via cable. Interconnecting devices may have two or more ports.

preconfigured gateway

A gateway device designed and equipped with software to be used with proprietary network components. Preconfigured gateway devices are a part of specific PLC networks.

programmable logic controllers

PLCs. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A programmable logic controller can replace many physical relays and hard-wired connections in a process.

programming device

A peripheral device connected to a PLC that is used to enter instructions into a PLC's memory. Programming devices include personal computers and handheld programmers.

proprietary network

A communication network designed by a manufacturer for a specific brand of PLC. A proprietary network typically creates a distributed control system.

proprietary protocols

A type of network protocols that can only be used by specific networks and devices. Proprietary protocols for PLC networks are developed by PLC manufacturers.

protocol

A set of rules and standards that state how data should be arranged and coded for transmission on a network. Protocols are standardized by different regulatory bodies.

radio wave

A wavelength in the electromagnetic spectrum. Radio wave signals are used with many applications, such as wireless networks, radio communication, and television broadcasting.

real-time communication

A characteristic of a PLC that allows it to control a process without significant delay. Real-time communication is often in the order of microseconds or milliseconds.

remote I/O

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

repeater

An interconnecting device that retransmits a signal throughout a network to extend the length of the signal. A repeater has two ports and does not read data.

resistors

A device that absorbs a data signal on a bus topology. Resistors are on both ends of a trunk in a bus topology so that the data signal does not travel backwards.

ring topology

A type of network topology in which all the nodes are connected in a closed loop. In a ring topology, a data signal travels in one direction through each node before reaching its destination.

router

An interconnecting device that can connect two types of networks with different protocols, such as a local area network and a wireless network. A router determines where information packets should go and sends them to their destination by the shortest, most efficient route.

scanner module

A PLC component used in some proprietary networks to allow the PLC to communicate with field devices. A scanner module is installed with other PLC modules on a rack.

segment

A portion of a network that is physically connected to the rest of the network but kept separate by an interconnecting device. A segment can be used to better control data transmission.

shield

An inner layer of material surrounding wires that provides protection from electrical noise. The shield is often surrounded by an outer plastic jacket.

shielded twisted pair cable

A type of twisted pair cable used for PLC networks that consists of pairs of insulated copper wires, a layer of braided or woven wire, and an outer plastic jacket. Shielded twisted pair cable offers more protection against electrical noise than unshielded twisted pair cable.

standard protocols

A type of network protocols that are open and can be used by anyone. Standard protocols are published and maintained by a regulatory body.

star topology

A type of network topology in which each node connects to an interconnecting device. A star topology is the most common for wireless networks.

suite

A group of compatible computer programs that are designed to work in unison with each other. Suites are usually sold or used as a single unit.

switches

An interconnecting device with multiple ports that reads data and sends it to the correct nodes on a network. Switches create separate segments at each port.

transmission control protocol/internet protocol

TC/IP. A suite of communication protocols used for internet or intranet networks. Transmission control / internet protocol breaks a data message into packets to send through a network and then reassembles the message when it reaches its destination.

transmission medium

The physical or logical method through which data travels between network nodes. Transmission media include different types of copper-based cable and devices equipped with antennas.

trunk

The common line of a bus topology that supports all data transmission. The trunk in a bus topology has all the nodes connected to it.

Twisted pair cables

A type of transmission medium used for PLC networks that consists of twisted pairs of copper wires. Twisted pair cables include shielded and unshielded varieties.

unshielded twisted pair cable

A type of twisted pair cable used for some PLC networks that consists of two insulated copper wires twisted around each other surrounded by a protective jacket. Unshielded twisted pair cable protects against a small amount of electrical noise.

wireless local area network

WLAN. A communication 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 communication in areas that are impractical for wired networks.

wireless router

A device in a wireless local area network (WLAN) that forwards data to its intended destination on the network. A wireless router receives radio wave signals from devices equipped with antennas.

WLAN

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