Wilamowski B.M., Irwin J.D. The Industrial Electronics Handbook. Second Edition: Industrial Communication Systems

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36-8 Industrial Communication Systems

36.5 Modbus Serial

A.Modbus.serial.network.comprises.multiple.devices.connected.to.a.physical.medium.(e.g.,.a.serial.bus)..

Due.to.the.shared.nature.of.the.physical.medium,.a.mechanism.is.required.to.regulate.medium.access..

For.this.reason,.Modbus.serial.follows.the.master–slave.interaction.model.(Figure.36.5),.with.the.Modbus.

client.becoming.the.master,.and.the.Modbus.servers.taking.the.role.of.slaves.. e.master.is.responsible.for.

initiating.the.communication.by.sending.requests.to.the.slaves,.one.request.at.a.time.. ese,.in.turn,.reply.

to.the.master.with.the.requested.data.. e.request/reply.exchange.can.be.performed.in.one.of.two.ways:

•

. Unicast mode:. e.master.sends.a.request.to.a.specic.slave.. e.slave.processes.this.request.and.

replies.to.the.master.

•

. Broadcast mode:. e.master.sends.a.request.to.all.slaves.. e.slaves.process.this.request,.but.do.

not.reply.to.the.master.

.master.only.starts.a.request/reply.exchange.once.the.previous.exchange.has.nished.

Each

.Modbus.serial.network.may.only.have.one.master.. e.number.of.slaves.is.limited.to.247..A.device.

can.be.both.master.and.slave,.but.not.at.the.same.time.. erefore,.a.device.can.change.its.role.whenever.

appropriate..In.practice,.roles.are.usually.static.since.the.Modbus.protocol.itself.does.not.specify.how.the.

changing.of.roles.may.be.managed.

36.5.1 Frames

 e.request/reply.exchange.is.performed.using.request.and.reply.APDUs,.which.are.transmitted.within.

frames.that.always.have.the.same.structure.(Figure.36.6):

•

. Address:. is.eld.contains.the.slave.address..In.a.request.frame,.it.identies.the.destination.

device,.while.in.a.reply.frame.it.identies.the.sender..Each.slave.has.a.unique.address.in.the.range.

1–247..Address.0.is.used.(by.the.master).for.broadcasting.messages.. e.range.248–255.is.reserved.

•

. Modbus APDU:. is.eld.is.the.application.layer.PDU.(Section.36.4).

•

. CRC or LRC:. is.eld.is.used.for.error.detection.purposes.. e.content.depends.on.the.transmis-

sion

.mode.(RTU.or.ASCII).being.used..For.details,.see.Section.36.5.4.

MasterSlave n Slave 2 Slave 1

Request

Modbus network

Request

Unicast mode

Broadcast mode

FIGURE 36.5 Request/reply.exchange.

Address CRC or LCRFunction code Data

Modbus serial frame

Modbus application layer PDU (APDU)

FIGURE 36.6 Modbus.serial.frame.

Modbus 36-9

In.Modbus.serial,.frames.exchanged.between.master.and.slave.devices.can.be.transmitted.in.one.

of.two.mod

es:

.RTU.or.ASC

II.

.Eac

.mod

.def

ines

.dif

ferent

.way

.to.cod

.the.fra

.con

tents

.(i.

e.,

byt

es)

.for.tra

nsmission.

.Wha

.mos

.dis

tinguishes

.bot

.mod

.are.the.sma

ller

.fra

mes

.pro

duced

.by.

the.RTU.mod

.Con

sequently,

.for.the.sam

.bau

.rat

.the.RTU.mod

.ach

ieves

.a.hig

her

.thr

ough-

put.

.The

.are

.how

ever,

.cir

cumstances

.tha

.may.lea

.one.to.cho

ose

.the.ASC

.mod

.The

.wil

.be.

dis

cussed

.bel

ow.

All

.dev

ices

.mus

.imp

lement

.the.RTU.mod

.wit

.the.ASC

.mod

.bei

.opti

onal.

.A.dev

ice

.may.sup

port

.bot

.mod

es,

.but.nev

.simu

ltaneously.

.With

.a.net

work

.all.dev

ices

.mus

.use.the.sam

.tra

nsmis-

sion

.mo

de;

.th

.co

mmunication

.ca

nnot

.oc

cur

.ot

herwise

.sin

.bo

.mo

des

.ar

.in

compatible.

36.5.2 rtU Mode

 e.RTU.mode.uses.an.asynchronous.approach.for.data.transmission..Each.byte,.within.a.frame,.is.

tra

nsmitted

.usi

.an.11.bi

.ch

aracter

.(F

igure

.36

.7):

•

. 1.st

art

.bi

.(S

T),

.us

.fo

.th

.in

itial

.sy

nchronization

•

. 8.bi

ts,

.th

.da

.co

ded

.in.bi

nary

.wi

.th

.le

ast

.sig

nicant

.bi

.se

.r

•

. 1.pa

rity

.bi

.(P

T),

.us

.fo

.er

ror

.de

tection

•

. 1.sto

.bi

.(S

P),

.to.en

sure

.a.mi

nimum

.id

.ti

.be

tween

.co

nsecutive

.ch

aracter

.tr

ansmissions

All

.dev

ices

.mus

.sup

port

.the.eve

.par

ity

.che

cking

.met

hod,

.but.the.use

.may.cho

ose

.any.oth

.met

hod

(ev

en,

.odd

.or.no.par

ity)

.on.dev

ices

.tha

.sup

port

.the

.How

ever,

.all.dev

ices

.wit

hin

.the.sam

.net

work

mus

.all.be.con

gured

.to.use.the.sam

.met

hod.

.If.the.no.par

ity

.met

hod

.is.use

.the

.the.numb

.of.stop.

bit

.mu

.be.2.. i

.gu

arantees

.th

.1.by

.wi

.al

ways

.be.co

ded

.wi

.11.bi

ts.

.abs

ence

.of.fra

.del

imiters

.(he

ader

.or.tra

iler)

.may.lea

.to.syn

chronization

.err

ors

.dur

ing

.fra

rec

eption,

.i.e

.the.rec

eiver

.may.hav

.di

culty

.in.ide

ntifying

.the.beg

inning

.or.the.end.of.eac

.fra

me.

.To.

ove

rcome

.thi

.it.bec

omes

.nec

essary

.tha

.the.tim

.int

erval

.bet

ween

.con

secutive

.cha

racters

.(wi

thin

.the.

sam

.fra

me)

.doe

.not.exc

eed

.a.cer

tain

.val

ue;

.oth

erwise

.the.nex

.cha

racter

.cou

.be.easi

.int

erpreted

.by.

the.rec

eiver

.as.the.beg

inning

.of.a.new.fra

me.

.In.the.sam

.man

ner,

.if.con

secutive

.fra

mes

.are.sep

arated

by.a.ver

.sma

.tim

.int

erval,

.the

.cou

.be.int

erpreted

.as.a.sing

.fra

me.

.In.bot

.cas

es,

.rec

eived

.dat

wil

.be.misi

nterpreted

.whi

.wil

.lea

.to.com

munication

.err

ors.

.To.avo

.thi

.iss

ue,

.the.tra

nsmission

.of.

a.fr

ame

.mu

.me

.th

.fo

llowing

.ti

ming

.re

quirements

.(F

igure

.36

.8):

•

. Eac

.fra

.mus

.be.tra

nsmitted,

.one.cha

racter

.ae

.the.oth

.wit

.a.min

imum

.idl

.int

erval

bet

ween

.the

.of.1.5.cha

racter

.tim

es.*

.If.thi

.int

erval

.is.exc

eeded,

.the.rec

eiver

.con

siders

.tha

.the.

fra

.is.in

complete

.an

.sh

ould

.di

scard

.it

•

. Con

secutive

.fr

ames

.mu

.be.se

parated

.by.an.id

.in

terval

.of.at.le

ast

.3.

.ch

aracter

.ti

mes.

Time.necessary.for.the.transmission.of.1.5.×.11.bits.

Address

Function Data

0 . . . . 252 bytes

CRC

1 byte 1 byte 2 bytes

ST 1 2 3 4 5 6 7 8 PT SP

Character

FIGURE 36.7 RTU.frame.transmission.

36-10 Industrial Communication Systems

From.this.description,.it.is.easy.to.understand.that.some.care.must.be.taken.when.implementing.an.

RTU.driver,.particularly.with.regard.to.the.accuracy.of.the.timers.used..Although.ecient,.the.RTU.

mode.requires.a.tight.control.of.timing..For.more.details.on.implementation.aspects,.readers.are.sug-

gested

.to.consult.[4].

36.5.3 aSCII Mode

 e.ASCII.mode.also.employs.an.asynchronous.transmission..In.this.case,.however,.each.byte.is.trans-

mitted

.as.two.ASCII.characters.(Figure.36.9).

.coding.process.is.as.follows:.each.8.bit.byte.is.divided.into.two.nibbles.of.4.bits.each..One.nibble.

contains.the.4.most.signicant.bits.in.the.byte,.and.the.other.the.4.least.signicant.bits..Each.nibble.is.

then.coded.using.its.hexadecimal.representation.on.the.ASCII.alphabet..For.example,.a.byte.with.value.

0xA1.(or.161.in.decimal).is.transmitted.as.the.characters.“A”.followed.by.“1.”. e.character.“A”.is.sent.as.

the.byte.with.value.0x41.which.is.the.ASCII.representation.of.the.“A”.character..Likewise,.the.character.

“1”.is.transmitted.as.its.ASCII.value.of.0x31.

Each

.ASCII.character.is.transmitted.using.10.bits.as.follows:

•

. 1.start.bit.(ST)

•

. 7.data.bits,*.the.ASCII.character.(“0”-“9,”.“A”-“F”).with.the.least.signicant.bit.sent.rst

•

. 1.Parity.bit.(PT)

•

. 1.Stop.bit.(SP)

.requirements.for.parity.checking.are.the.same.as.those.of.the.RTU.mode.

Contrary

.to.the.RTU.mode,.a.frame.transmitted.in.the.ASCII.mode.has.a.header.and.a.trailer.. e.

header.identies.the.beginning.of.a.frame.and.is.represented.by.the.“:”.ASCII.character.(0x3A).. e.trailer.

identies.the.end.of.the.frame,.and.comprises.two.ASCII.characters:.CR.(Carriage.Return—0x0D).and.LF.

(Line.Feed—0x0A),.transmitted.in.this.order.

.use.of.headers.and.trailers.helps.the.reception.process,.since.the.receiver.has.only.to.wait.for.

the“:”.character.to.detect.the.beginning.of.a.frame..Conversely,.the.end.of.a.frame.is.detected.when.the.

Each.ASCII.character.is.coded.using.7.bits.

≤1.5 chars

Frame 1

≥3.5 chars ≥3.5 chars

Char

Frame 2

FIGURE 36.8 RTU.timing.requirements.

Address Function Data “CR” “LF”

2 chars 2 chars (0 . . . 252) × 2 chars 2 chars

ST 1 2 3 4 5 6 7 PT SP

1 char

LCR

2 chars

Character

FIGURE 36.9 ASCII.frame.transmission.

Modbus 36-11

sequence.“CR”.“LF”.occurs..Since.this.mode.does.not.specify.a.maximum.time.interval.between.con-

secutive

.cha

racters,

.thi

.imp

lies

.tha

.thi

.int

erval

.cou

.the

oretically

.ass

ume

.any.val

ue.

.How

ever,

.thi

cou

.lea

.to.a.situ

ation

.in.whi

.the.rec

eiver

.wou

.blo

ck,

.e.g

.whe

.the.sen

der

.cra

shes

.dur

ing

.a.fra

tra

nsmission,

.whe

reupon

.the.rec

eiver

.wou

.wai

.ind

enitely

.for.the.rem

aining

.cha

racters.

.In.ord

.to.

pre

vent

.the

.situ

ations,

.the.rec

eiver

.sho

uld

.imp

lement

.a.tim

eout

.mec

hanism.

.Whe

.a.tim

eout

.occ

urs,

the.rec

eived

.dat

.are.dis

carded

.and.the.rec

eiver

.wai

.for.the.nex

.fra

me.

.In.[4]

.it.is.sug

gested

.to.use.1.s.

for.the.tim

eout

.val

ue.

.How

ever,

.it.is.pos

sible

.to.use.oth

.val

ues

.dep

ending

.on.the.net

work

.topo

logy.

Fur

ther

.de

tails

.ca

.be.fo

und

.in.[4

From

.the.dis

cussion,

.it.is.eas

.to.und

erstand

.tha

.the.ASC

.mod

.is.simp

ler

.to.imp

lement

.and.doe

not.pos

.any.par

ticular

.tim

ing

.req

uirements.

.How

ever,

.the.pri

.to.pay.is.the.dou

bling

.of.the.fra

.size.

whe

.co

mpared

.to.th

.RT

.mo

de.

36.5.4 Error Detection

Error.detection.mechanisms.are.located.at.two.levels,.namely,.the.character.level.and.frame.level.

.the.cha

racter

.lev

el,

.a.par

ity

.che

cking

.met

hod

.is.use

. e.sen

der

.com

putes

.the.par

ity

.of.eac

.cha

acter

.usin

.the.par

ity

.met

hod

.cho

sen

.(ev

en,

.odd

.or.no.par

ity)

.and.set

.the.par

ity

.bit.acc

ordingly.

. e.

rec

eiver,

.usin

.the.sam

.met

hod

.of.the.sen

der,

.com

putes

.the.exp

ected

.par

ity

.bit

.and.com

pares

.it.wit

the.rec

eived

.par

ity

.bit

.If.the

.are.di

erent,

.an.err

.has.occ

urred

.and.the.ent

ire

.fra

.is.dis

carded.

.It.is.

eas

.to.see.tha

.the.err

.det

ection

.cap

ability

.of.thi

.met

hod

.is.qui

.lim

ited.

.For.exa

mple,

.whe

.two.bit

wit

hin

.a.ch

aracter

.ar

.i

pped

.(d

.to.a.tr

ansmission

.er

ror),

.th

.er

ror

.wi

.no

.be.de

tected.

Error

.det

ection

.at.the.fra

.lev

.use

.a.CRC.(cy

clic

.red

undancy

.che

ck)

.in.the.RTU.mod

.or.LRC.

(lo

ngitudinal

.red

undancy

.che

ck)

.in.the.ASC

.mod

.Alt

hough

.the

.met

hods

.are.con

ceptually

.dif

ferent,

.the.met

hodology

.for.the

.use.is.the.sam

. e.sen

der

.com

putes

.the.CRC.or.LRC.val

.usi

a.pred

ened

.alg

orithm

.whi

.is.app

lied

.to.the.fra

.con

tents:

.Addr

ess

.+.APD

.el

ds.

. is.val

.is.

the

.app

ended

.to.the.fra

me.

. e.rec

eiver

.com

putes

.the.CRC.or.the.LRC.usi

.the.sam

.alg

orithm,

and.com

pares

.the.com

puted

.res

ults

.wit

.the.rec

eived

.one

.If.the

.are.di

erent,

.it.con

cludes

.tha

.the.

fra

.has.err

ors

.and.sho

uld

.the

refore

.be.dis

carded.

. e.CRC.val

.is.com

puted

.thr

ough

.a.pol

ynomial

div

ision

.of.the.fra

.usi

.+.X

.+.1.as.the.generator.polynomial..It.produces.a.16.bit.value,.

whi

.is.tra

nsmitted

.wit

.the.low

.byt

.rs

. e.LRC.val

.is.com

puted

.usi

.a.mod

ulo-2

.sum.

(XO

R-sum)

.of.all.byt

.of.the.fra

.and.prod

uces

.an.8.bit.val

ue.

.A.com

parison

.bet

ween

.bot

.met

hods

sho

.tha

.the.CRC.has.a.hig

her

.err

.det

ection

.cap

ability.

. e.CRC.and.LRC.val

ues

.are.tra

nsmitted

usi

.the.sam

.met

hodology

.use

.for.the.rem

aining

.cha

racters.

.Det

ails

.abo

.CRC.or.LRC.com

puta-

tion

.can.be.fou

.in.[4]

36.5.5 Physical Layer

Modbus.serial.supports.two.physical.layers:.RS485.[5].and.RS232.[6].. e.RS485.consists.of.a.multi-drop.

net

work

.abl

.to.sup

port

.mul

tiple

.dev

ices

.(ty

pically

.32.dev

ices

.per.net

work

.seg

ment).

.Sev

eral

.seg

ments

can.be.con

nected

.by.usin

.rep

eaters,

.thu

.inc

reasing

.the.max

imum

.numb

.of.dev

ices

.in.the.net

work.

.It.

sup

ports

.1.mas

ter

.(at.a.tim

.and.up.to.247.sla

ves,

.and.it.can.be.use

.for.lar

ger

.dis

tances

.(up.to.100

0m)

wit

.hig

.bau

.rat

es.

. e.RS23

.is.a.poi

nt-to-point

.sol

ution

.wit

.jus

.1.mas

ter

.and.1.sla

ve.

.It.is.use

.for.

sho

.di

stances

.(<

.m).wi

.lo

.to.me

dium

.ba

.ra

tes.

All

.dev

ices

.mus

.sup

port

.an.RS4

.int

erface,

.whi

.the.RS23

.is.opti

onal.

.Bot

.tra

nsmission

.mod

(RT

.and.ASC

II)

.can.be.use

.on.eit

her

.int

erface

.typ

.Mor

eover,

.all.dev

ices

.wit

hin

.a.net

work

.mus

hav

.the.sam

.con

guration,

.com

prising

.the.fol

lowing

.par

ameters:

.tra

nsmission

.mod

.(RT

.or.ASC

II),

par

ity

.typ

.(ev

en,

.odd

.or.no.par

ity),

.and.bau

.rat

.(bi

ts/s).

.All.dev

ices

.mus

.sup

port

.9.6.and.19.

.kbp

rat

es,

.wit

.the.lat

ter

.bei

.the.def

ault

.val

ue.

.Fur

ther

.det

ails

.abo

.the.cha

racteristics

.of.eac

.int

erface

and.th

eir

.ap

plication

.to.Mo

dbus

.ne

tworks

.ca

.be.fo

und

.in.[4

36-12 Industrial Communication Systems

36.6 Modbus tCP

A.Modbus.TCP.network.consists.of.multiple.devices.connected.through.a.TCP/IP.network,.interact-

ing

.following.the.client–server.model..A.client.sends.a.request.to.a.server,.which.in.turn.responds.to.

the.client.with.the.requested.data.. is.transaction.(request/response.exchange).is.performed.by.send-

ing

.Modbus.TCP.frames.through.a.TCP.connection.previously.established.between.the.client.and.the.

server..Connection.establishment.and.management.are.handled.by.the.TCP/IP.protocol.and.occur.inde-

pendently

.of.the.Modbus.protocol..A.Modbus.server.listens.on.port.502.for.requests.from.clients.that.

wish.to.establish.a.new.connection.with.the.server.. is.port.is.presently.reserved.(and.registered).for.

Modbus.applications.[2].

Some

.Modbus.server.devices.may.support.multiple.connections.from.distinct.clients.simultaneously..

Similarly,.clients.may.establish.multiple.connections.to.distinct.servers..Additionally,.the.Modbus.TCP.

protocol.allows.a.client.to.send.multiple.Modbus.requests.to.the.same.server.over.the.same.connection,.

even.before.the.arrival.of.the.replies.to.previous.requests..A.device.may.be.both.a.client.and.server.at.

the.same.time.

36.6.1 Frames

Modbus.TCP.frames.(request.and.response).consist.of.a.MBAP.header.(Modbus.Application.Protocol.

header).plus.the.application.layer.APDU.(Figure.36.10).

.MBAP.header.comprises.several.elds:

•

. Transaction identier:.Since.a.client.can.issue.several.concurrent.transactions.over.the.same.TCP.

connection,.the.responses.are.not.guaranteed.to.arrive.in.the.same.order.that.the.requests.were.

sent..It.is.therefore.necessary.to.have.an.identier.for.each.transaction.in.order.to.match.the.

request.and.response.frames.. e.client.initializes.this.eld.when.it.performs.a.request.. e.server.

echoes.this.value.in.the.response.frame.

•

. Protocol identier:. is.is.used.to.identify.the.protocol;.it.currently.always.has.the.value.0.

•

. Length:. is.eld.indicates.the.size.(in.bytes).of.the.unit.identier.eld.plus.the.APDU..Data.trans-

fer

.in.a.TCP.connection.is.performed.as.a.stream.of.bytes,.which.could.lead.to.a.situation.where.

several.frames.are.waiting.to.be.read.in.the.reception.buer..To.identify.frame.boundaries.in.these.

situations,.the.frame.length.must.be.known.

•

. Unit identier:. is.eld.is.used.to.identify.the.destination.device.(a.slave)..It.is.used.mainly.by.

gateways.between.Modbus/TCP.and.Modbus.serial.networks,.where.the.gateway,.upon.receiving.

a.Modbus/TCP.frame,.needs.to.know.the.identication.of.the.slave.on.the.Modbus.serial.net-

work

.that.should.receive.that.frame..Modbus/TCP.devices.that.are.not.gateways.usually.ignore.

this.eld.

Unlike

.Modbus.serial,.Modbus.TCP.frames.do.not.have.an.error.detection.eld.. is.was.considered.

unnecessary.as.the.TCP/IP.stack.already.includes.several.error.detection.mechanisms..Further.details.

about.the.implementation.of.Modbus.TCP.can.be.found.in.[2].

Function code

Data

Modbus TCP frame

Modbus application layer PDU (APDU)

Transaction

identier

Protocol

identier

Length

Unit

identier

MBAP header

2 bytes 2 bytes 2 bytes 1 byte

FIGURE 36.10 Modbus.TCP.frame.

Modbus 36-13

36.7 Example

A.typical.example.of.a.small-scale.project.using.a.Modbus.network.is.the.automation.of.a.rope-making.

machine.. is.machine.consists.of.two.physically.separated.parts:.a.braiding.component.that.winds.the.

twines.into.a.rope.and.a.traction.component.that.pulls.out.the.built.rope.at.the.correct.speed..Each.com-

ponent

.is.driven.by.an.electrical.motor.connected.to.a.variable.speed.drive.. e.braiding.component.addi-

tionally

.contains.several.digital.and.analog.sensors.to.detect.possible.faults.(e.g.,.twine.breaking).and.

digital.and.analog.actuators.(e.g.,.status.signaling.lights).. e.automation.logic.is.handled.by.a.single.small.

PLC,.and.an.HMI.graphical.terminal.is.used.to.interact.with.the.operator.(Figure.36.11)..A.SCADA.graphi-

cal

.interface.on.the.plant.manager’s.computer.also.connects.to.the.PLC.over.the.Modbus.TCP.network.in.

order.to.obtain.manufacturing.data.(e.g.,.up-time,.meters.of.rope.produced).

.this.project,.the.PLC.communicates.with.the.two.variable.speed.drives.over.a.Modbus.serial.net-

work,

.with.the.PLC.acting.as.the.master.and.the.drives.acting.as.slaves.. e.digital.and.analog.I/Os.are.

handled.by.an.RTU,.which,.along.with.the.graphical.terminal.and.the.SCADA,.connects.to.the.PLC.

over.a.Modbus.TCP.network.(TCP/IP.over.Ethernet).. e.PLC.acts.as.a.client.to.the.Modbus.server.in.

the.RTU.and.as.a.server.to.the.Modbus.clients.in.the.graphical.terminal.and.SCADA..Note.that.on.the.

Modbus.TCP.network:.(1).the.PLC.acts.as.both.a.client.and.a.slave.simultaneously,.(2).three.clients.coex-

ist

.on.the.same.network,.and.(3).the.server.on.the.PLC.is.accessed.simultaneously.by.two.clients.(the.

graphical.terminal.and.the.SCADA).

.order.to.show.in.more.detail.how.the.data.are.actually.accessed.remotely.over.the.Modbus.protocol.

the.connection.between.the.PLC.and.the.RTU.will.be.explained.further..A.typical.example.of.a.Modbus.

slave.device.is.the.RTU.of.the.STB.series.of.products.by.Schneider.Electric.. ese.RTUs.are.modular,.

consisting.of.a.network.adapter,.followed.by.one.or.more.physical.interface.modules.. e.following.

example.(Figure.36.12).considers.an.RTU.with.a.Modbus/TCP.network.interface.(NIP.2212),.followed.

by.the.following.modules:.power.supply.(PDT.3100),.four.digital.inputs.(DDI.3420),.four.digital.outputs.

(DDO.3410),.two.analog.inputs.(AVI.1270),.and.two.analog.outputs.(AVO.1250).

Although

.the.Modbus.protocol.denes.bit-sized.memory.areas,.this.particular.RTU.does.not.make.

use.of.them;.in.fact,.it.only.uses.the.holding.registers.memory.area..It.maps.the.four.digital.inputs.

onto.the.four.least.signicant.bits.of.one.16.bit.holding.register,.the.four.digital.outputs.onto.the.four.

least.signicant.bits.of.another.register,.and.each.of.the.analog.inputs/outputs.onto.their.own.register..

Additionally,.it.uses.some.registers.to.store.status.information.

PLC

Graphical

terminal

Drive 1

Master/client

Modbus serial network

Drive 2

RTU

SCADA

Modbus/TCP network

Slave/server

Braider

machine

Traction

machine

FIGURE 36.11 Example.machine.and.automation.equipment.

36-14 Industrial Communication Systems

Figure.36.13.shows.a.table.with.the.exact.mapping.of.I/Os.and.status.data.to.the.holding.register.

memory.area,.for.an.RTU.congured.as.in.Figure.36.11..All.addresses.in.this.table.start.with.the.digit.4,.

which.is.the.method.used.by.this.RTU’s.supplier.to.indicate.that.these.refer.to.registers.in.the.holding.

.holding.registers.used.are.grouped.into.two.contiguous.areas..One.area,.starting.at.address.0001,.

will.be.used.as.an.output.memory.area,.i.e.,.the.Modbus.client.may.write.new.data.to.these.registers,.which.

will.then.be.used.by.the.RTU.to.drive.the.physical.outputs.. e.data.stored.in.the.second.contiguous.area,.

starting.at.address.5392,.will.be.updated.by.the.RTU.in.such.a.way.as.to.reect.the.actual.values.in.the.physi-

cal

.inputs.and.outputs..Although.the.Modbus.protocol.allows.a.Modbus.client.to.write.new.data.values.to.

Node

Number

Module

Name

STB DDI 3420

STB DDO 3410

STB AVI 1270

STB AVO 1250

Input

Address

45392

45393

45394

45395

45396

45397

45398

45399

45400

45401

Input

Value

dec

3248

3264

I/O data values

Output

Address

40001

40002

40003

Output

ValueFormat Format

FIGURE 36.13 Mapping.of.I/Os.onto.Modbus.holding.registers.for.the.example.RTU.

FIGURE 36.12 Physical.aspect.of.RTU.with.conguration.as.described.in.the.example.

Modbus 36-15

holding.registers,.this.particular.RTU.will.ignore.any.attempt.to.write.to.registers.in.this.second.contiguous.

area..Registers.in.this.second.contiguous.area.should.therefore.be.considered.as.being.“read.only.”

.may.be.seen.from.this.table,.the.four.digital.inputs.of.the.rst.DDI.3420.module.are.mapped.onto.

the.holding.register.at.address.5392.. e.least.signicant.4.bits.of.the.following.register.at.address.5393.

will.contain.status.information.(short-circuit,.power.failure).related.to.these.same.inputs.

.Modbus.master.changes.the.outputs.on.the.next.module,.the.DDO.3410.with.four.digital.outputs,.

by.writing.the.desired.state.to.the.holding.register.at.address.0001.. is.DDO.3410.module.uses.another.

two.registers.in.the.input.area:.the.rst.(at.address.5394).echoes.the.value.stored.in.the.register.0001,.

while.the.following.(at.address.5395).stores.the.actual.status.of.the.physical.outputs.

.AVI.1270.module.uses.the.registers.5396.and.5398.to.store.the.input.voltage.of.each.analog.input,.

while.the.other.two.registers,.at.addresses.5397.and.5399,.store.status.information.(over.and.under-

voltage

.errors.and.warnings).. e.AVO.1250.uses.the.registers.at.addresses.0002.and.0003.to.receive.

the.voltage.values.that.should.be.applied.to.the.analog.outputs,.and.the.registers.at.5400.and.5401.store.

status.information.(over.and.under-voltage.errors).

.example.of.a.possible.exchange.of.Modbus.message.frames.between.the.PLC.and.this.particular.

RTU.(functioning.as.Modbus.server).is.described.in.Figure.36.14..In.this.exchange,.the.PLC.uses.the.

Modbus.function.“Write.Multiple.Registers”.to.change.the.analog.output.values.stored.in.registers.0002.

and.0003.to.the.values.0xE124.and.0xE248,.respectively..Note.that.the.starting.address.of.0002.is.sent.

on.the.wire.as.0001.since.this.particular.RTU.follows.the.Modbus.specication.to.the.letter,.and.all.

documentation.assumes.that.the.memory.areas.start.at.address.1,.which.is.then.sent.on.the.wire.as.0.

acronyms

APDU. Application.layer.protocol.data.unit

ASCII

. American.Standard.Code.for.Information.Interchange

CRC

. Cyclic.redundancy.check

EIA

. Electronics.Industries.Association

FIFO

. First.in.rst.out

HMI

. Human–machine.interface

. Internet.protocol

LAN

. Local.area.network

LRC

. Longitudinal.redundancy.check

MBAP

. Modbus.application.protocol

PLC

. Programmable.logic.controller

RTU

. Remote.terminal.unit

SCADA

. Supervisory.control.and.data.acquisition

TCP

. Transmission.control.protocol

TIA

. Telecommunications.Industry.Association

10 00 01 00 02 4 E1 24

Modbus TCP frame

Modbus application layer PDU (APDU)

Transaction

identier

MBAP header

00 00 0B 01

Protocol

identier

Length

Unit

Func.

code

Start

address

count

Byte

count

Data for

E2 48A3 6E

00 00 00 06 01A3 6E 00 01 00 02

Request (from PLC to RTU)

Reply (from RTU to PLC)

Data for

FIGURE 36.14 Example.Modbus.transaction.between.the.PLC.and.the.RTU.

36-16 Industrial Communication Systems

references

. 1.. Modbus.Application.Protocol.Specication,.v1.1b,.Modbus-IDA,.North.Graon,.MA,.December.28,.

2006.

. 2.. Mod

bus

.Mess

aging

.on.TCP/IP.Imp

lementation

.Guide.v1.0b,.Mod

bus-IDA,

.Nor

.Gra

on,

.MA,.

Oct

ober

.24,.2006.

. 3.. Open.Mo

dbus/TCP

.Sp

ecication,

.An

.Swa

les,

.Sc

hneider

.Ele

ctric

.SA,.Fra

nce,

.Ma

rch

.29,.1999.

. 4.. MOD

BUS

.over.Seri

.Line.Spe

cication

.and.Imp

lementation

.Guide.V1.02,.Mod

bus-IDA,

.Nor

Gra

on,

.MA,.2006.

. 5.. Elec

trical

. Cha

racteristics

. of. Genera

tors

. and. Recei

vers

. for. Use. in. Bal

anced

. Digit

. Mul

tipoint

Syst

ems,

.ANSI/TI

A/EIA-485-A-1998.

. 6.. Int

erface

.Betw

een

.Dat

.Termin

.Equi

pment

.and.Dat

.Circui

t-Terminating

.Equi

pment

.Emplo

ying

Seri

.Bin

ary

.Da

.In

terchange,

.ANSI/TI

A/EIA-232-F-1997.

37-1

37.1 Introduction

 e.distribution.of.automation.functions.in.instrumentation.and.control.(I&C).architectures.arose.in.

the.late.1970

.with.prop

rietary

.netw

ork

.solu

tions

.desig

ned

.to.t.an.appl

ication’s

.perf

ormance.

.To.ensu

inte

roperability

.betw

een

.die

rent

.prod

ucts

.from.die

rent

.vend

ors,

.the.IEC.6115

.stan

dard

.has.been.

crea

ted.

.Howe

ver,

.due.to.manu

facturers’

.pres

sure,

.this.lead.only.to.a.coll

ection

.of.seve

ral

.die

rent

.spec

cations

.cor

responding

.to.eac

.ven

dor’s

.com

mercial

.oe

Meanwhile,

.Ethe

rnet

.(IEE

.802.

.[12]).beca

.the.stan

dard

.solu

tion

.in.desk

top

.comm

unication

syst

ems

.that.impl

ements

.the.rst.two.laye

.of.the.OSI.Mode

.(see.Chapt

.1).whil

.TCP/

.proto

cols

(RFC.791.[15

]

.and.793.[16

.cf..Cha

pter

.61.of.thi

.boo

.did.so.for.lay

ers

.3.and.4.

til

.the.late.1990

.Ethe

rnet

.coul

.not.be.used.at.the.plan

.oor.beca

use

.of.its.nond

eterministic

medi

.acce

.cont

rol

.meth

od,

.call

.CSMA/

.(cf..Chap

ter

.2)..Even.thou

.the.appa

rition

.of.full

duplex

.link

.and.swit

ching

.tech

nology

.turn

.Ethe

rnet

.into.a.cand

idate

.for.plan

.oor.comm

unica-

tion,

.it.stil

.coul

.not.guar

antee

.a.dete

rministic

.resp

onse

.time

.main

.beca

use

.fram

.coul

.stil

.be.

dela

yed

.or.lost.insid

.swit

ches.

.Such.a.beha

vior

.would.not.meet.the.requ

irements

.of.real

-time

.cont

rol

syst

ems.

.Henc

.to.over

come

.this.prob

lem,

.vend

ors

.deve

loped

.seve

ral

.die

rent

.“Ind

ustrial

.Ethe

rnet”

solut

ions.

Figure

.37.1.illu

strates

.the.usua

.netw

ork

.segm

entation

.in.an.indu

strial

.info

rmation

.syst

em.

. e.two.

high

est

.leve

ls—corporate

.LAN.and.Indu

strial

.LAN.leve

.3—do.not.supp

ort

.time

-constrained

.appl

cation.

.Stand

ard

.Ether

net

.is.gene

rally

.depl

oyed

.at.thos

.leve

ls.

.On.the.cont

rary,

.Indu

strial

.Ether

net

solu

tions

.are.desig

ned

.to.addr

ess

.the.Indu

strial

.LAN.requ

irements

.of.the.autom

ation

.netw

ork

.and.eld

buses.

.Indeed,.real-time.and.critical.applications.are.performed.at.those.automation.levels.

.chapt

.prop

oses

.not.to.focu

.on.one.spec

ic

.solu

tion

.but.to.give.an.over

view

.of.seve

ral

.avai

lable

comm

ercial

.sol

utions.

Industrial Ethernet

37.1. Introduction..................................................................................... 37-1

37.2

. Indu

strial

.Ethe

rnet.......................................................................... 37-2

What.Does.Ethernet.Mean?. •. What.Does.Industrial.

Mean?. •. Classication.of.Industrial.Ethernet.Solutions

37.3. Standardized.Solutions.of.IEC.61158.and.IEC.61784................ 37-5

EtherNet/IP. •. Foundation.Fieldbus.High-Speed.Ethernet. •. .

SERCOS.III. •. “Exotic.Solutions”:.EPA,.Tcnet,.Vnet/IP,.

PNETonIP

37.4. Features.of.Major.Industrial.Ethernet.Solutions........................ 37-9

37.5

. Synt

hesis............................................................................................37-9

Abbreviations...............................................................................................37-9

References................................................................................................... 37-10

Gaëlle Marsal

EDF Research and

Deve

lopment

Denis Trognon

EDF Research and

Deve

lopment