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Chapter 8. DATA COMMUNICATION• Need:

Design file exchange.

Part program downloading.

Person to person communication - e-mail, talk, video conferencing.

System control: commands, status data, sensor data

Remote login.

• 50% of plant floor computer system cost are allocated to networking costs.

• How to make control devices talk to each other.

• Solutions:

Point-to-point communication

Networking

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METHODS OF COMMUNICATION

• Point-to-Point: direct connection of two devices– Discrete I/O points – one bit at a time.

– Serial port –data byte transmitted in serial pulses.

– Parallel port – data byte transmitted in parallel.

• Network: multiple devices connected to the same media (cable, radio wave, etc.)

– Ethernet

– FieldBus

– MAP, etc.

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A COMPUTER

registersALU Control Unit

Memory

data bus

I/Oaddress bus

control bus

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AN I/O BUFFER

Data Bus

selectaddress bus

decoder

read/writefrom control bus

buffer

external device

internal to a computer

clock

Individual bits can be used for discrete I/O.

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ASCII CODE

High Bitslow 000 001 010 011 100 101 110

111

0000 NUL DLE SP 0 @ P \ p

0001 SOH DC1 ! 1 A Q a q

0010 STX DC2 " 2 B R b r

0011 ETX DC3 # 3 C S c s

0100 EOT DC4 $ 4 D T d t

0101 ENQ NAK % 5 E U e u

0110 ACK SYN & 6 F V f v

0111 BEL ETB ' 7 G W g w

1000 BS CHN ( 8 H X h x

1001 HT EM ) 9 I Y i y

1010 LF SUB * : J Z j z

1011 VT ESC + ; K [ k {

1100 FF FS , < L \ l |

1101 CR GS - = M ] m }

1110 SO RS . > N ^ n ~

1111 SI US / ? O _ oDE

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COMMUNICATION MODES• Simplex can be viewed as a communications "one-way street".

Data only flows in one direction. That is to say, a device can be a receiver or a transmitter exclusively. A simplex device is not a transceiver. A good example of simplex communications is an FM radio station and your car radio. Information flows only in one direction where the radio station is the transmitter and the receiver is your car radio. Simplex is not often used in computer communications because there is no way to verify when or if data is received.

• Duplex communications overcome the limits of Simplex communications by allowing the devices to act as transceivers. Duplex communication data flows in both directions thereby allowing verification and control of data reception/transmission. Exactly when data flows bi-directionally further defines Duplex communications.

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SIMPLEX AND DUPLEX (CONTINUE)

• Full Duplex devices can transmit and receive data at the same time.

• Half Duplex devices allow both transmission and receiving, but not at the same time. Essentially only one device can transmit at a time while all other half duplex devices receive.

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SERIAL COMMUNICATION

TX TX

RVRV

GND GND

Device 1 Device 2cable

UART UART

ParallelFrom the Data bus

DTE DCENull Modem connector does the swapping

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INTERFACEDTE: Data Terminal Equipment (terminal), PC, male connector

DCE: Data Circuit-terminating Equipment (modem, computer), female connector

DCE DCEDTE DTE

RS 232C, RS 422, X.21

RS232C 25 pin connector

DB25 connector

1 < -3V

0 > 3V

< 20 kbps

< 15 m

unbalanced signal

RS 422 37 pin or 9 pin

twisted pair balanced

100 kbps at 1200m

10 mbps at 12 m

or unbalanced (RS 423A)

3 kbps at 1000 m

300 kbps at 10 m

X.21

packet transmission mode

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TRANSMITTING THE LETTER 'S'

2 stop bits010100111 start

letter 'S' parity bit

1 start bit 7 data bit 1 parity bit 2 stop bits

time0

Volt

sec/bit

Baud rate = 1/ clock

Even parity

Some modem standards transfer morethan one bit per clock cycle.e.g. V.32bis transmit 6 bits per baud.

0: Space

1: Mark

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RS 232PIN NAME <TO DTE TO DCE> FUNCTION EIA CCITT

1 FG Frame Ground AA 101

2 TD > Transmitted Data BA 103

3 RD < Receive Data BB 104

4 RTS > Request to Send CA 105

5 CTS < Clear to Send CB 106

6 DSR < Data Set Ready CC 107

7 SG Signal Ground AB 102

8 CD < Carrier Detect CF 109

9 - Reserved - -

10 - Reserved - -

11 - Unassigned - -

12 (S)CD < Sec. Carrier Detect SCF 122

13 (S)CTS < Sec. Clear to Send SCB 121

14 (S)TD > Sec. Transmitted Data SBA 118

15 TC < Transmitter Clock DB 114

16 (S)RD < Sec. Received Data SBB 119

17 RC < Receiver Clock DD 115

18 - Unassigned - -

19 (S)RTS > Sec. Request to send SCA 120

20 DTR > Data Terminal Ready CD 108.2

21 SO < Signal Quality Detector CG 110

22 RI < Ring Indicator CE 125

23 > Data Rate Selector CH 111

Data Rate Selector CI 112

24 (E)TC > Ext. Transmitter Clock DA 113

25 - Unassigned - -

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RS 422RS422 is a "drop-in" replacement for most RS232 applications. It is full-duplex

and capable of long distance communications. Use balanced signal over twisted pair of wire, allows much faster speed and longer distance.

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COMPARISIONS

RS 232 RS 422 RS 485

Cabling single ended single endedmulti-drop

multi-drop

Number of Devices 1 transmit1 receive

5 transmitters10 receivers

32 transmitters32 receivers

Communication mode

full duplex full duplexhalf duplex

half duplex

Max. Distance 50 feet at 19.2 kbps 4000 feet at 100 kbps 4000 feet at 100 Kbps

Max. Data Rate 19.2 kbps for 50 feet 10 Mbps for 50 feet 10 Mbps for 50 feet

Signaling Unbalanced Balanced Balanced

Mark (data 1) -5 V min.-15 V max.

2 V min. (B>A)6 V max. (B>A)

1.5 V min. (B>A)5 V max. (B>A)

Space (data 0) 5 V min.15 V max.

2 V min. (A>B)6 V max. (A>B)

1.5 V min. (A>B)5 V max. (A>B)

Input Level Min. +/- 3 V 0.2 V difference 0.2 V difference

Output Current 500 mA 150 mA 250 mA

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Modem• Flow control

– Software through XON and XOFF

– Hardware through pin outs, I.e. request to send (RTS), clear to send (CTS), data terminal ready (DTR), and data set ready (DSR)

• Modulating the Signal– 1 baud = one audio signal transitions per second

– Frequency-shift keying (FSK): 1750 Hz for 1, 1080 Hz for 0. 1 baud = 1 bps

– Phase-shift keying (PSK): 0 and 1 are represented by the alteration of the carrier’s phase. Bit is coded at fixed phase shift, e.g. 0, 90, 180, and 270 degrees. More bits per baud.

– Quadrature-amplitude modulation (QAM): combine both phase and frequency. At 1700 Hz or 1800 Hz of 2400 baud, each phase-shift keyed. Six bps data per baud. Only 4 data bits usable.

– Trellis-coded quadrature-amplitude modulation (TCQAM or TCM): Use all 6 data bits. 14,400 bps modems use this.

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MODEM STANDARDS (reference only)CCITT V.XX standards (Consultative Committee for International Telephone

and Telegraph)

V.22, V.22 bis. : synchronous/asynchronous data transmission, full-duplex operation over 2 wire at 1,200 bps (2,400 and 1,200 bps for V.22 bis) data rate.

V.32 : synchronous/asynchronous data transmission, full-duplex operation over 2 wire at 9,600 bps data rate.

V.32 bis: synchronous/asynchronous data transmission, full-duplex operation over 2 wire at 14,400, 12,000, 9,600, 7,200, 4,800 bps data rate.

V.34 bis: synchronous/asynchronous data transmission, full-duplex operation over 2 wire at 28.8k, ...

Modem-connection negotiations (training and retraining), may reduce the data rate due to line noise. Fastrain: may go up the speed as well.

V.35

Duplex: full (two lines, two way), half (one line, one way)

Bell standard: Bell 103, 300 bps; Bell 201B: 2,400 bps, full duplex on 4 wire, or 1,200 bps, half duplex on 2 wire. Bell 201C: 2,400 bps, half duplex on 2 wire; Bell 208 A & B: 4,800 bps

Data compression: compress the data before transmission.

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PARALLEL INTERFACE ADAPTER

Data Bus Buffer

Control Register

Data Direction Register

Peripheral Interface

Chip Select and Read Write Control

Output Register

Datafrom computer

Data Bus

addressdecoder enable

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PC Parallel PortPin Descritpion I/O1 Strobe - Out2 +Data Bit 0 Out3 +Data Bit 1 Out4 +Data Bit 2 Out5 +Data Bit 3 Out6 +Data Bit 4 Out7 +Data Bit 5 Out8 +Data Bit 6 Out9 +Data Bit 7 Out10 -Acknowledge Out11 +Busy In12 +Paper End In13 +Select In14 -Auto Feed In15 -Error Out16 -Initialize Printer In17 -Select Input Out18 -Data Bit 0 returnn (GND) In19 -Data Bit 1 returnn (GND) In20 -Data Bit 2 returnn (GND) In21 -Data Bit 3 returnn (GND) In22 -Data Bit 4 returnn (GND) In23 -Data Bit 5 returnn (GND) In24 -Data Bit 6 returnn (GND) In25 -Data Bit 7 returnn (GND) In

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IEEE 488• Standard digital interface for programmable instrumentation

• HP interface (originally, is available on most instruments)

• GPIB (General Purpose Interface Bus)

• 1 mbps

• The IEEE-488 interface system consists of 16 signal lines and 8 ground lines. The 16 signal lines are divided into 3 groups (8 data lines, 3 handshake lines, and 5 interface management lines).

• Devices on the bus are: Listeners, Talkers, and Controllers

• up to 15 devices to be interconnected on one bus. Each device is assigned a unique primary address, ranging from 0-30, by setting the address switches on the device. A secondary address may also be specified,ranging from 0-30.

• Total cable length is limited to 20 meters with a maximum of 20 devices.

• TTL level signal.

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OTHER INTERFACES

Theoretical Actual First-Byte Latency

USB 1.5 Mbytes/s 1.25 Mbytes/s 20 ms

1394 50 Mbytes/s 9.6 Mbytes/s 6 ms

GPIB 1 Mbytes/s 1 Mbytes/s 110 µs

HS488 8 Mbytes/s 7.7 Mbytes/s 110 µs

Mostly for instrumentation. GPIB (IEEE 488) is still the major players.

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DATA EXCHANGE METHODS

• Polling

• Interrupt

• Direct Memory Access (hard disk to computer)

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POLLING

statusof inputport

load input byte tothe accumulator

yes

statusof outputport

send a byte tothe output port

yes

no

no

loop

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INTERRUPT

high priority interrupt

low priority interrupt

Main CPU Loop

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NETWORKS

No longer a point-to-point connection.

Many devices connected together and information can be passed by one device to any of the devices on the network.

Local area network - Ethernet, FDDI (Distributed Data Interface), ATM (Asynchronous Transfer Mode)

Wide area network

High speed local network

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GLOSSARY OF SELECTED TERMSBandwidth: frequency range used by the communication system.

Baseband: use voltage difference (digital)

Broadband: use coaxial cable and analog (RF) signals. Higher band width, multiple channels on the same cable. Digital signals are modulated on a carrier frequency.

CTV: 5 mbps per channel

Carrier from 5-300 M Hz

Carrier: A continuous frequency capable of being modulated or impressed with a second (information) signal.

DDS (Dataphone Digital Service): AT&T service in which data is transmitted in digital rather than analog form. Need no modem.

FDDI (Fiber Distributed Data Interface): ANSI standard for fiberoptic links with data rates up to 100 mbps. LED or laser light source; 2 km for unrepeated data transmission at 40 mbps.

ISDN (Integrated Services Digital Network): mixed digital-transmission services, basic rate at 144 kbps, and primary rates at 1.544 and 2.048 mbps.

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GLOSSARY OF SELECTED TERMS

Medium Access Control: controls which device on the network get the to send data to the medium.

CSMA/CD

Token Ring

Token Bus

Packet: small chunk of data.

Protocol: a set of rules that governs the operation of functional units to achieve communication.

TCP/IP: Transport protocols concurrently with existing Ethernet.

NFS: network file system - file system sharing, remote disk mounting.

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IDEAL LAN CHARACTERISTICS

• high speed: greater than 10 mega bits per second

• low cost: easily affordable on a microcomputer and/or machine controller

• high reliability/integrity: low error rates, fault tolerant, reliable

• expandability: easily expandable to install new nodes

• installation flexibility: easy to be installed in an existing environment

• interface standard: standard interface across a range of computers and controllers.

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COMMUNICATION NETWORK

• Protocol – defines both physical and software rules.

• Most widely accepted model is a 7 layered protocol model.

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ISO/OSI MODEL

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

medium

Device A Device B

Layer 7 Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1

User Program User Program

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LAYERED PROTOCOL2. DATA LINK LAYER

• flow control

• error control

Activate, maintain and deactivate the link. Error free transmission on the same network. Detecting noise. (media access, logical link)

3. NETWORK LAYER

provides the transparent transfer of data between transport entities. Responsible for establishing, maintaining, and terminating connections (between networks). Use globally unique node address.

4. TRANSPORT LAYER

Ensures that data units are delivered error-free, in sequence, without no losses or duplications.

Connection management

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LAYERED PROTOCOL5. SESSION LAYER

Controlling the dialogue between applications.

Dialogue type: two-way simultaneous (TWS), two-way alternate (TWA), one-way, etc.

Recovery after network breakage.

6. PRESENTATION LAYER

Syntax of the data exchanged between application entities.

e.g. teletext, videotex, encryption, virtual terminal.

7. APPLICATION LAYER

Common application services (CASE)

Specific application services (SASE)

Management

file transfer

job transfer

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A PACKET

Preamble

Physical layer message Data link layer message Network layer message Transport layer message Session layer message Presentation layer message Application layer message

Data

Checksum

Postamble

SYN code

SYN code

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CABLES

core wire

ground wire

insulators

Coaxial cable

Twist pair cable

Thick: CATV, RG-59, 75 ohmsThin: Ethernet, RG-58, 52 ohms

Unshielded:Shielded: signal loss, thick

Fiber-optic Cable

core

cladding

(62.5 micron)

noise immune

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LAN TOPOLOGIES

Ring

Star

Bus

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ETHERNET

Terminator Tap

Transceiver

Computer

T/S

C1

T/S

C2

50 ohm coaxial cable

T/S

C3

T/S

C4Repeater

digital

RF

T/S

bridge

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COLLISION DETECTION

A B

t0

A begin transmission

transmission time > 2a

A B

t0+a-e

B begin transmission before signal reach B

A B

t0+a

B detects collison

A B

t0+2a-e

A detects collison just before the end of transmission.

a is the time for signal to travel to B,

CSMA/CD (Carrier sensing) protocol

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ETHERNET CONNECTIONSStandard Ethernet (10BASE5)

• segment length 500 m

• cable 4 km

• transceiver cable 50 m

• between transceivers 2.5 m

• 100 transceivers per segment

• 50 ohm terminators

ThinNet Ethernet (10BASE2)

• segment length 185 m

• cable length 4 km

• T-connectors, 0.5 m between each

• 30 connections

• 50 ohm terminators

• T-connectors plugged directly to the Ethernet card.

Twisted-pair Ethernet (10BASE-T)

• segment length 100 m

• unshielded twisted-pair cable

• devices connected to a hub in a star configuration

• Hub connected to the standard Ethernet

• Use twisted-pair transceiver.

Hub

computers

computers

T-connector

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A TOKEN RING

repeater

computer

A

B

C

Ddirection of token and data packet

Only one token is passed around the network.

The device who has the token may transmit.

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A TOKEN BUS

Token passing network. Whoever has the token may transmit one ormore packets. When it is done, or the time has expired, it passes the token to the next station.

A C

BD

C D B A

D C A B

predecessor successor

Logical ring

E

D B

EEAdd a new node

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RS 485 BASED NETWORKS485 is sometimes termed as RS485 Multidrop LAN since it can connect several

devices in a LAN network environment. These devices are all connected to a single pair wire. Transmit and receive share the same two wires.

Officially the RS485 specification allows only 32 nodes (devices) on the LAN. However, I.C. manufacturers have developed RS485 drivers capable of allowing 128 to 255 nodes on an RS485 LAN.

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FIELD BUS

• Foundation Field Bus - A Fieldbus is a digital, two-way, multi-drop communication link among intelligent measurement and control devices. It serves as a Local Area Network (LAN) for advanced process control, remote input/output and high speed factory automation applications.

• The Fieldbus Access Sublayer (FAS) maps the Fieldbus Message Specification (FMS) onto the Data Link Layer (DLL).

• High Speed Ethernet (HSE) is the Fieldbus Foundation's backbone network running at 100 Mbit/second.

• Bus speed is 31.25 kbit/s.• Can communicate with other devices through HSE using

Ethernet protocols such as TCP/IP, SNTP, etc.

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FIELDBUS MODEL

Fieldbus message specification

Fieldbus access sublayer

Data link layer

Physical layer

Layer 7. Application

Layer 2. Data link

Layer 1. Physical

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MEDIA ACCESS CONTROL

• Token passing network

• Token controlled by a Link Active Scheduler (master of the network).

• Other devices on the network are basic devices.

• The device which holds the token can transmit to any other devices on the network.

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APPLICATIONS OF FIELDBUS

Sensors and actuators.

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NETWORK HIERARCHY

Plant wide

network

Control system and LANs

Fieldbus network

Office automation and department computers

Automation and display systems

Factory/plant instrumentation and control devices

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Allen-Bradley Networks

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DEVICE NET

• From Rockwell Automation, Allen-Bradley devision.

• Based on the Fieldbus.• Low level, 64-node multidrop network using

single cable to interface devices up to 500 m to PLC.

• Use either two twisted pair wires (24V) in a round cable, or two pair wires flat cable.

• Data rates, 500 kbps (100 m), 250 kbps (250 m), 125 kbps (500 m).

• Many sensors are device net ready.

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CONTROL NET

• By Rockwell Automation, Allen-Bradley

• For time critical applications.

• High-speed link between controller and I/O devices.

• 5 mbps

• 30 km or more using repeaters (in star, tree, or bus configuration), 1000 m w/o repeater.

• Physical layer can be fiber optic or 75 o cable.

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ETHERNET II

• Preamble 8 bytes• Destination 6 bytes• Source 6 bytes• Ethernet type 2 bytes (IPX/SPX, TCP/IP, etc)• Data 46 – 1500 bytes• Pad Character up to 46 bytes• Frame check sequence 4 bytes

• Frame length: 64-1518 bytes (not including preamble)

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NOVELL’S LAN

Physical layer – 3C5X9.com

LAN driver

Data link Layer – LSL.com

Network Layer – IPX0D1.com

ODI NDIS

Transport – TCP/IP.exe

Socket API - winsock.dll

Application layer - windows

ftp, http, telnet

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MAP 2.1 STANDARDLayer MAP implementation

Layer 7 ISO FTAM {DP} 8571

Application File Transfer Protocol

Manufacturing Messaging Format Standard (MMFS)

MAP Directory Services

MAP Network Management

Layer 6

Presentation NULL/MAP transfer

Layer 5 ISO Session{IS} 8327

Session Basic Combined Subset & Session Kernel, Full Duplex

Layer 4

Transport ISO Transport{IS} 8073

Class 4

Layer 3 ISO Internet{DIS} 8473

Network Connectionless, SubNetwork Dependent Convergence Protocol

Layer 2 ISO Logical Link Control {DIS} 8802/2 (IEEE 802.2)

Data Link Type 1, Class 1

ISO/IEEE 802.4 Token Passing Bus Medium Access Control

Layer 1 ISO Token Passing Bus{DIS} 8802/4 (IEEE 802.4)

Physical 10 Mbps Broadband

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An Integrated Corporate Communication Network

Bridge

CorporateTOP network

Gat

eway

Gatew

ay IBMSNA networkCorporate

Ethernet

DivisionTOP network

CAD/CAM CRT

Finance/accounting

Office

MAPBackbone

Gateway

MAPSub Network

Router

Data baseTerminal

server

CRT

OfficeT

OP

network

Gatew

ayVendor

Network

Gateway

Robots

Machines

PLCs

PLCsRobots

Corporate Offices

Factory

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ISDN: Integrated Service Digital Network

• B channel: 64 kbps - basic user channel

• D channel: 16 or 64 kbps - control, setup, calls on B.ch. common chan

• H0: 384 kbps

• H11: 1.536 Mbps

• H12: 1.92 Mbps

Basic access: two B-channels and one D channel - total 144 kbps

Primary access: T-1 transmission at 11.544 Mbps, 23 B-channels, 1 D channel

terminalequip.

ISDNswitch

packet-switched capabilities

circuit-switched

nonswitched

common-channel signaling

terminalequip.

ISDNswitch

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Frame Relay and ATM

• On top of the physical layer

• Frame Relay:– Use on ISDN line, assume less transmission error. Less

overhead than X.25 packet-switching.

– Block data transfer, up to 2 Mbps

• ATM: Asynchronous Transfer Mode– For broadband ISDN

– packet switching

» X.25 - control, data on the same channel, variable length

» ATM - separate, fixed length (cells), 10s and 100s Mbps

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COMMUNICATION REQUIREMENTS

10 Kbps 56 Kbps1.54 Mbps

10 - 16 Mbps

100 Mbps

1.2 Gbps

Ordinary Phone Basic rateISDN

Primary rateISDN

Ethernet andToken Ring

Fast Ethernet,FDDI

AsynchronousTransfer Mode(ATM)

Speech grade audio(32-64 Kbps)

Low quality compressed video(50 Kbps - 1.5 Mbps)

High fidelity audio(64 Kbps - 1.5 Mbps)

Medium quality compressed video(1.5 Mbps - 6 Mbps)

High quality compressed video(6 Mbps - 24 Mbps)

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COMMUNICATION ON UNIX

NETWORK INTERFACE LAYERPROTOCOL LAYERSOCKET LAYER

IN buffer

Out buffer

Application

e.g. ftp telnet

Protocol

Networkinterface

out packet

In packet

Hardware

determines the route of travel

when a communication is desired, create a socket

get

protocol

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EXAMPLE

s = socket(AF_INET, SOCK_STREAM,0); /* create a socket*/

connect(s,&server, sizeof(server)); /* establish connection */

write(s,buf,sizeof(buf)); /* send data */

close(s); /* close socket*/

internet domain for TCP protocol

#include <sys/types.h>#include <sys/socket.h>sockaddr server;

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TCP/IP PROTOCOL

UDP: User datagram protocolFTP: File transfer protocolSMTP: Simple mail transfer protocolTELNET: Virtual terminal protocol

TCP: Transmission control protocolIP: Internetwork protocol

Application

Presentation

Session

Transport

Network

Data Link

Physical

Layer 7

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

User Program

UDPFT PSMT PTELNET

T CP

I P

ETHERNET

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DATA COMMUNICATIONALTERNATIVES

• Phone and fax

• BBS (bulletin board system) run your own.

• Commercial information vendors: CompuServe, Prodigy, America Online, GEnie

• Internet connection

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WHAT IS INTERNET?

Internet is a loosely connected wide area network. It is a group of worldwide information resources open to everyone on the network. Some characteristics of the internet:

• Origin: Arpanet sponsored by US DOD in the 1970s.

• Who may participate? Anyone who pays a nominal fee to connect to a nearby network and agrees to follow a set of rules.

• Who runs the network? Nobody is in charge.

• Who pays for it and to whom? The organization who is connected to the network must pay it own segment of the network. There is no central organization to collect the payment.

• What kind of hardware is needed to run the network? Any kind of computer hardware.

• How to connect to a network? Find a closest node and negotiate the connection.

• What is the limitation of using it? No direct commercial use.

• What is most widely used operating system on the net? Unix.

• How big is the network? Too big and growing to be even bigger every minute.

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INTERNET ROUTERS

INTERNET

networklayer

datalink

physicallayer

networklayer

datalink

physicallayer

networklayer

datalink

physicallayer

Network A Network B Network C