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Overview of Data Communications
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Topics
IntroductionHistoricalStandardsOSIProtocolsPhysical StandardsModern Instrumentation & Control
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Introduction Transfer of Information From One Point to Another Transmitter/Receiver/Link Serial Links vs Parallel Links Digital Data vs Analog Mutual Agreement
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Telegraph System Used Morse Code Plus and Minus Voltages CSMA/CD (Carrier Sense Multiple Access with Collision
Detection) Acknowledgement that the Message was Received
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Historical BackgroundTeletypes Needed Baudot Code
Equal Number of Bits for All Characters
Hardware Control Using Break CharacterPositive Voltage Held Longer than One
Character
Baud Rate defined as the possible number of changes in state per second
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OSI Model Developed by the ISO Universal Framework for Building A Communications Systems 7 Layer Model 3 Layer Model Very Common for Industry Missing the 8th Layer
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Standards Within A Communications System
Code - Morse - Hex - ASCIIPhysical - 232 - 422 - 485 Protocol - Modbus, Profibus, Hart,
Foundation Fieldbus, EthernetOpen Systems Interconnection Model
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Common Codes
Hex - Used for “Number” ProtocolsOnly Has 0-9 and A-F CharactersLimited but Very Efficient
ASCII - Used in Most New ProtocolsHas All the Letters, Number and PunctuationHigh Overhead but Very Versatile
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Representation of the OSI Model
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Protocols Rules for Transmission of Data (Packets) Packet Can Include:
Sync Byte Destination Address Source Address Data Error Detection
Modbus - Profibus - Canbus - Ethernet - TCP/IP HDLC - DH+ - HART - DNP3 - Interbus - FTP
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Basic Structure of an Information Frame Defined
SyncByte
DestinationAddress
SourceAddress
Data to beTransmitted
Error DetectionByte
SYNCHRONIZING BYTE Is Used To Synchronize The Receiver.
DESTINATION ADDRESS Is Where The Packet Is Going.
SOURCE ADDRESS Is Where The Packet Has Coming From.
DATA Is The Information Contained In The Packet.
ERROR BYTE Is The Result Of The Error Correction Calculation.
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A Typical Serial Data Communications Link
RS-232 RS-232
Terminal Modem
Transmitted Data Received DataAnalog Signal
Modem Communication Link Terminal
DTE DCEDCE DTE
• DTE = Data Terminal Equipment
• DCE = Data Communication Equipment
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Voltage StandardsEIA/RS-232 Interface Standard
Half / Full Duplex - Point to Point
EIA/RS-422 Interface StandardFull Duplex - Point to Point
EIA/RS-485 Interface StandardHalf Duplex - Multidrop
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Modern Instrumentation Distributed Control System (DCS)
All Electronic
Programmable Logic Controller (PLC) Came from Relay Equipment
Smart Instrumentation Intelligent Electronic Devices ( IED ) Super Smart Sensors
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Distributed Control System
Based Around a Data HighwayModular Distributed & IntegratedEach Module Performs Specific Dedicated
Task“Good” on Analog ControlEasy Connection to PLC’s / supervisory
Computer Interfaces
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PLC Programmable Logic
Controller
Sequence Control and Interlocking
On/Off Inputs/outputs
Ladderlogic Type Programs
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A Typical PLC System
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Smart InstrumentationMicroprocessor Based Instrument
Embedded Controllers
Data Communications CapabilityRS- 232, RS 485 or Fiber OpticAbility to Multi-drop With Other
Instruments on Same HighwayWave of the Future...Eg Fieldbus
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Graphical Representation of A Complete System
Top Layer is EthernetLower Level is RS-485 Data Highway
Network, RS-232 and Fiber OpticDCS Data HighwayInterconnects Using a Gateway
(See Figure 1.5 Page 13)
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