Selecting a PLC

Criteria

• Number of logical inputs and outputs.• Memory• Number of special I/O modules• Scan Time• Communications• Software

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PLC CommunicationsSerial Communications

PLC communications facilities normally provides serial transmission of information.

Common Standards

RS 232

Used in short-distance computer communications, with the majority of computer hardware and peripherals. Has a maximum effective distance of approx. 30 m at 9600 baud.

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PLC CommunicationsLocal Area Network (LAN)

Local Area Network provides a physical link between all devices plus providing overall data exchange management or protocol, ensuring that each device can “talk” to other machines and understand data received from them.

LANs provide the common, high-speed data communications bus which interconnects any or all devices within the local area.

LANs are commonly used in business applications to allow several users to share costly software packages and peripheral equipment such as printers and hard disk storage.

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PLC CommunicationsRS 422 / RS 485

Used for longer-distance links, often between several PCs in a distributed system. RS 485 can have a maximum distance of about 1000 meters.

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Local-area network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area

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Various configurations, called topologies, have been used to administer LANs

Ring topology A configuration that connects all nodes in a closed loop on which messages travel in one direction

Star topology A configuration that centers around one node to which all others are connected and through which all messages are sent

Bus topology All nodes are connected to a single communication line that carries messages in both directions

A bus technology called Ethernet has become the industry standard for local-area networks

Figure 15.2 Various network topologies

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Metropolitan-area network (MAN) ) A network that connects two or more local-area networks over a potentially large geographic distance

Wide-area network (WANThe communication infrastructures that have been developed in and around large cities

Often one particular node on a LAN is set up to serve as a gateway to handle all communication going between that LAN and other networks

Communication between networks is called internetworking

The Internet, as we know it today, is essentially the ultimate wide-area network, spanning the entire globe

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The International Organization for Standardization (ISO) established the Open Systems Interconnection (OSI) Reference Model

Each layer deals with a particular aspect of network communication

Figure 15.5 The layers of the OSI Reference Model

Open Systems Interconnection model7 layer theoretical model of how a protocol

stack should be implemented

4123702 Data Communications System @YRU 19

4123702 Data Communications System @YRU 20

OSI model

Session layer: network dialog controller; establish, maintain, and synchronize communications between systems

Presentation layer: To handle the syntax and semantics of the information exchange between the two systems. data translation encryption/decryption compression

These duties are often handled by other layers now, hence the Internet model

4123702 Data Communications System @YRU 21

4123702 Data Communications System @YRU 22

Figure 2-9

Summary of OSI model

Many PLC manufacturers are embedding web servers into their equipment, even at the micro and nano levels.

Not only web connectivity make it possible to monitor a control system from anywhere it also things like system manuals to be saved for future use.

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The issue of security is at the uppermost and intruders must be kept at bay

A number of approaches have been developed to deal with this problem

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Sketch star,semistar and bus networks for the work cell .Robot 1 takes part from the input conveyer and put them in the lathe. When the lathe is through turning, parts are put onto the conveyer 1 or 2 by robot 1 depending upon the part no. While moving on output conveyer 1,the part is checked by a vision inspection.For conveyer 2 the part is removed from the conveyer by robot 2,machining by a mill and returned to the conveyer by robot 2.at the end of output conveyers 1 and 2,robot 3 puts finished parts in boxes on the output conveyer 315-25