computer integrated manufacturing: control networks & field-bus systems

8/10/2019 Computer Integrated Manufacturing: Control Networks & Field-Bus Systems

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Communications Networks in

ManufacturingThe emphasis on CIM is on thecommunication linkage made by thecomputer between the various softwaresubsystems that drive the manufacturingoperations & support functions.

CIM requires an information processingthat ties all of these needs together on-line.



Manufacturing ComputerizationDesign engineering function has …

NC machining function has … Production & inventory control functionhas …

Process planning function has …



InterfacingDifferent standards, vendors and users of machines must

be compatible to link together in order to be

interconnected in CIM system.Information passed between microprocessors &automation equipment is by using serial or parallel I/Odevices.

Use serial or parallel communications (basically aninterconnector device from one device to another).RS232C the most popular serial interface to connectcomputer peripherals.

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InterfacingFor faster data transfer rate, a parallelinterface is required that is able to managetransfer of information between severaldevices. The standard is IEEE488.



LOCAL AREA NETWORKSLAN is the primary means of interconnecting devices inan automated system to achieve CIM.

What is a network? A network consists of two or morecomputers that are linked in order to share resources (suchas printers and CD-ROMs), exchange files, or allowelectronic communications.

The computers on a network may be linked throughcables, telephone lines, radio waves, satellites, or infraredlight beams.

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LOCAL AREA NETWORKSThe three basic types of networks include:

Local Area Network (LAN) – look at basictypes of topology.Wide Area Network (WAN)Wireless Local Area Networks – allows

equipment such as data collection devicessuch as bar code readers to have networkconnections. Slower than hard wirednetworks.

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TOPOLOGYWhat is a Topology?

The physical topology of a network refers to the

configuration of cables, computers, and other peripherals.Types of topologies:

Linear Bus Star

Star-Wired Ring Tree

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http://fcit.usf.edu/Network/chap5/chap5.htm












STAR NETWORKS/TOPOLOGYThe central control/station is called a “server” because itsnetwork role is to make the interconnection between the

various robots, NC machines & automation devices thatneed to communicate.Suitable for situations that are not subject to frequentconfiguration changes. All message pass through a centralstation.This is not a popular type of LAN for CIM. Telephonesystems in office buildings usually have this type oftopology.

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Star layout for a LAN. All messages must be routedthrough a central server.

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TOPOLOGYStar

A star topology is designed with each node (file server, workstations,and peripherals) connected directly to a central network hub or

concentrator .Data on a star network passes through the hub or concentrator beforecontinuing to its destination. The hub or concentrator manages andcontrols all functions of the network. It also acts as a repeater for thedata flow. This configuration is common with twisted pair cable ;

however, it can also be used with coaxial cable or fiber optic cable .Advantages of a Star Topology

Easy to install and wire. No disruptions to the network then connecting or removing devices.Easy to detect faults and to remove parts.

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http://fcit.usf.edu/Network/glossary.htm
















TOPOLOGYDisadvantages of a Star Topology

Requires more cable length than a linear topology.If the hub or concentrator fails, nodes attached are disabled.

More expensive than linear bus topologies because of the cost of theconcentrators.

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RING TOPOLOGY

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All individual user stationsare connected in a continuous

ring. This message isforwarded from one station tothe next until it reaches itsassigned destination. If anystation fails, the entirenetwork will shut down.



BUS TOPOLOGYAll stations have independent access. Thissystem is reliable and easier than the othertwo to service.

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TOPOLOGYLinear Bus

A linear bus topology consists of a main run ofcable with a terminator at each end. All nodes (file server, workstations, and peripherals) areconnected to the linear cable.

Advantages of a Linear Bus Topology• Easy to connect a computer or peripheral to alinear bus.• Requires less cable length than a star topology.Disadvantages of a Linear Bus Topology• Entire network shuts down if there is a break in

the main cable.• Terminators are required at both ends of thebackbone cable.• Difficult to identify the problem if the entirenetwork shuts down.• Not meant to be used as a stand-alone solution

in a large building

.

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(1) ETHERNETThe most popular type of LAN. Uses a simple coaxial cable asa bus . How can control of messages be managed when the onlyinterconnector is a simple two-wire cable? The answer isthrough software. Each device on the network has a“transceiver” responsible for encoding each message the deviceis sending and for decoding each message it receives.Each batch of information is called a “packet – represent

packages of data to be put on the network for delivery to one ormore devices” that contains all source and destinationinformation to enable it to be properly received by only thedevice for which it is intended.

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ETHERNETOne of the many protocols . A protocol is aset of rules that governs thecommunications between computers on anetwork. These rules include guidelinesthat regulate the following characteristics

of a network: access method, allowed physical topologies, types of cabling, andspeed of data transfer.

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ETHERNETSuppose two devices want to put a message on theEthernet at the same time. The answer is that the

messages will actually collide. A protocol calledCSMA/CD (carrier sense multiple access with collisiondetection) handles this problem.Other access-control methods are the token ring &

token bus . Only the device that has the token is allowedto transmit, while all of the other devices only receive.

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ETHERNETWhen message collide, both of the sendingdevices are notified by the CSMA/CD, which

jams the entire network so that all devices willstop transmitting. After waiting an appropriatetime, device transceivers can again attempt totransmit messages.

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Ethernet LAN network. Two networks are joined bya repeater to form one Ethernet.

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(2) TOKEN RINGSAnother strategy for constructing a LAN avoids the problem ofmessage collisions by providing a message carrier that travelsaround to all devices connected to the network – “token ring”.A single electronic token moves around the ring from onecomputer to the next. If a computer does not have informationto transmit, it simply passes the token on to the nextworkstation. If a computer wishes to transmit and receives an

empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the computer forwhich the data is meant. At this point, the data is captured by

the receiving computer.

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TOKEN RINGSThe Token Ring protocol requires a star-wiredring using twisted pair or fiber optic cable. It can

operate at transmission speeds of 4 Mbps or 16Mbps.Disadvantage: Each device must have a repeater,slower than Ethernet because messages can besent only by the device that has the token, andonly if the token in free at that time.



Token ring LAN. The token follows a fixed path andmakes a complete loop.

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(3) TOKEN BUSESThis is a combination of the concepts oftoken rings and the Ethernet bus.There are 2 routing modes for the token.When carrying a message, the token goesdirectly from source to destination. Whenthe token is empty, it makes the completeloop, following the chain, while looking fornew messages to send.



NETWORK CABLECable is the medium through which information usuallymoves from one network device to another. The type ofcable chosen for a network is related to the network’stopology, protocol and size.Types of cable:

Unshielded Twisted Pair (UTP) Cable

Shielded Twisted Pair (STP) Cable Coaxial Cable Fiber Optic Cable

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NETWORK CABLEUnshielded Twisted Pair (UTP) Cable

Twisted pair cabling comes in two varieties: shielded and unshielded.Unshielded twisted pair (UTP) is the most popular and is generallythe best option for school networks.The quality of UTP may vary from telephone-grade wire to extremelyhigh-speed cable. The cable has four pairs of wires inside the jacket.Each pair is twisted with a different number of twists per inch to helpeliminate interference from adjacent pairs and other electrical devices.

The tighter the twisting, the higher the supported transmission rateand the greater the cost per foot.

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NETWORK CABLECoaxial Cable

Coaxial cabling has a single copper conductor at its

center. A plastic layer provides insulation between thecenter conductor and a braided metal shield (See fig. 3).The metal shield helps to block any outside interferencefrom fluorescent lights, motors, and other computers.

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Protocol SummaryPROTOCOL CABLE SPEEED TOPOLOGY

ETHERNET TWISTED PAIR,COAXIAL ,

FIBER

10Mbps(data transmission

on rate)

STAR

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Next chapter:

Communication standards - MAP



Networking Hardware



Networking HardwareSwitch: A concentrator is a device that provides a centralconnection point for cables from workstations, servers,and peripheralsRepeaters: Since a signal loses strength as it passes alonga cable, it is often necessary to boost the signal with adevice called a repeater. The repeater electricallyamplifies the signal it receives and rebroadcasts it.Repeaters can be separate devices or they can beincorporated into a concentrator. They are used when thetotal length of your network cable exceeds the standardsset for the type of cable being used.



Networking HardwareBridges: A bridge is a device that allows you to segmenta large network into two smaller, more efficient networks.If you are adding to an older wiring scheme and want thenew network to be up-to-date, a bridge can connect thetwo.Routers: A router translates information from onenetwork to another; it is similar to a super intelligent

bridge. Routers select the best path to route a message, based on the destination address and origin. The routercan direct traffic to prevent head-on collisions, and issmart enough to know when to direct traffic along backroads and shortcuts.

computer integrated manufacturing: control networks & field-bus systems

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