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Overview of Data Communications


Topics

IntroductionHistoricalStandardsOSIProtocolsPhysical StandardsModern Instrumentation & Control


Introduction Transfer of Information From One Point to Another Transmitter/Receiver/Link Serial Links vs Parallel Links Digital Data vs Analog Mutual Agreement


Telegraph System Used Morse Code Plus and Minus Voltages CSMA/CD (Carrier Sense Multiple Access with Collision

Detection) Acknowledgement that the Message was Received


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


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


Standards Within A Communications System

Code - Morse - Hex - ASCIIPhysical - 232 - 422 - 485 Protocol - Modbus, Profibus, Hart,

Foundation Fieldbus, EthernetOpen Systems Interconnection Model


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


Representation of the OSI Model


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


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.


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


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


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


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


PLC Programmable Logic

Controller

Sequence Control and Interlocking

On/Off Inputs/outputs

Ladderlogic Type Programs


A Typical PLC System


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


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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