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

Подождите немного. Документ загружается.

33-6 Industrial Communication Systems

Periodic.communication.is.used.for.the.exchange.of.cyclic.process.data.that.support.the.services.

of.the.peripheral.message.specication.(PMS).through.a.process.data.channel..PMS.is.a.subset.of.the.

manufacturing.message.specication.(MMS).. is.enables.direct.access.to.the.cyclically.transmitted.

process.data.and.is.able.to.transmit.process-relevant.data.quickly.and.eciently..From.the.application.

point.of.view,.it.acts.as.a.memory.interface.

Instead,

.asynchronous.communication.is.used.to.transfer.conguration.les.provided.by.the.network.

management.channel.(peripheral.network.management,.PNM).through.a.parameter.channel.. e.data.

transmitted.in.the.parameter.channel.have.low.dynamics.and.occur.relatively.infrequently.(e.g.,.updating.

text.in.a.display).and.are.used.for.manufacturer-independent.conguration,.maintenance,.and.startup.of.

the.INTERBUS.system.

Network

.management.is. used,. for.example,.to.start/stop. INTERBUS.cycles,. to.execute. a.system.

reset,.and.fault.management..Furthermore,.logical.connections.between.devices.can.be.established.and.

aborted.via.the.parameter.channel.

INTERBUS

.must.then.transmit.both.parameter.data.and.time-critical.process.data.simultaneously,.

avoiding.any.interference.between.the.two.data.ows..To.this.aim,.the.data.format.of.the.summation.

frame.has.been.extended.by.including.a.specic.time.slot.inside.the.time.assigned.to.each.node..Whereas.

process.data.are.fully.exchanged.at.every.cycle,.parameter.data.telegrams,.which.are.up.to.246.bytes.long.

and.are.not.featured.by.real-time.constraints,.will.be.exchanged.in.several.consecutive.bus.cycles.by.

inserting.a.dierent.part.of.the.data.in.the.time.slot.provided.for.the.addressed.devices.. e.peripherals.

communication.protocol.(PCP).performs.this.task:.It.inserts.a.part.of.the.telegram.in.each.summation.

frame.and.recombines.it.at.its.destination.(see.Figure.33.6).. e.parameter.channels.are.activated,.if.

necessary,.and.do.not.aect.the.transfer.of.I/O.data.. e.longer.transmission.time.for.parameter.data.

that.is.segmented.into.several.bus.cycles.is.sucient.for.the.low.time.requirements.that.are.placed.on.the.

transmission.of.parameter.information.

INTERBUS

.uses.a.master/slave.procedure.for.data.transmission.. e.parameter.channel.follows.the.

client/server.paradigm..It.is.possible.to.transmit.parameter.data.between.two.slaves.(peer-to-peer.com-

munication).

. is.means.that.both.slaves.can.adopt.both.the.client.and.server.function..With.this.func-

tion,

.layer.2.data.is.not.exchanged.directly.between.the.two.slaves.but.is.implemented.by.the.physical.

master/slave.structure,.that.is,.the.data.are.rst.transmitted.from.the.client.to.the.master.and.then.

forwarded.to.the.server.from.the.master.. e.server.response.data.are.also.transmitted.via.the.master..

However,.this.diversion.is.invisible.for.slave.applications.

.task.of.a.server.is.described.using.the.model.of.a.virtual.eld.device.(VFD).

.VFD.model.unambiguously.represents.that.part.of.a.real.application.process,.which.is.visible.

and.accessible. through. the.communication..A.real.device.contains.process. objects..Process.objects.

Data

Service

Header

Loopback

word

Node 1

n – 5

n – 4

n – 3

n – 2

Cycle n – 1

Node 2

Summation frame (bus cycle)

Node k FCS

….

......

FIGURE 33.6 Transmission.of.acyclic.parameter.data.with.a.segmentation.and.recombination.mechanism.

INTERBUS 33-7

include.the.entire.data.of.an.application.process.(e.g.,.measured.values,.programs,.or.events).. e.pro-

cess

.objec

.are.ente

red

.in.the.objec

.dict

ionary

.(OD).as.comm

unication

.objec

ts.

. e.OD.is.a.stan

dard-

ized

.publ

.list

.in.whic

.comm

unication

.objec

.are.ente

red

.with.thei

.prop

erties.

.To.ensu

.that.data.

are.exch

anged

.smoo

thly

.in.the.netw

ork,

.addi

tional

.item

.must.be.stan

dardized,

.in.addi

tion

.to.the.OD,.

whic

.can.be.acce

ssed

.by.each.devi

ce.

. is.incl

udes

.devi

.feat

ures

.such.as.the.manu

facturer

.name.or.

den

.devi

.func

tions

.that.are.manu

facturer

.inde

pendent.

. ese.sett

ings

.are.used.to.achi

eve

.a.clos

and.manu

facturer-independent

.repr

esentation

.of.a.real.devi

.from.the.poin

.of.view.of.the.comm

uni-

cation

.sys

tem.

. is.kin

.of.mod

eling

.is.kno

.as.a.VFD

33.3 Diagnostics

 e.system.diagnostics.plays.an.important.role.in.real-world.applications..In.increasingly.complex.

syst

ems,

.erro

.must.be.loca

ted

.quic

kly

.usin

.syst

.diag

nostics

.and.must.be.clea

rly

.indi

cated

.to.the.

user

.For.this.reas

on,

.a.good.erro

.diag

nostics,

.in.addi

tion

.to.dete

cting

.erro

rs,

.must.incl

ude

.reli

able

erro

.loca

lization.

.For.usua

.mess

age-oriented

.eld

bus

.syst

ems,

.whic

.are.base

.on.a.bus.stru

cture,

only.one.mess

age

.is.ever.tran

smitted

.to.a.devi

.at.any.time

.An.erro

.whic

.aec

.the.syst

.via.

a.spec

ic

.devi

ce,

.can.even.dest

roy

.mess

ages,

.whic

.are.not.addr

essed

.to.the.faul

.devi

.itse

lf,

.but.

may.be.dire

cted

.towa

.othe

.remo

.devi

ces.

.So.it.is.ther

efore

.virt

ually

.impo

ssible

.to.dete

rmine

.the.

exac

.erro

.loca

tion.

INTERBUS

.uses.the.CRC.algo

rithm

.in.each.devi

.to.moni

tor

.the.tran

smission

.path

.betw

een

.two.

devi

ces

.and

.in.the.eve

.of.CRC.err

ors,

.can.det

ermine

.in.whi

.seg

ment

.the.err

.occ

urred.

.orde

.to.main

tain

.data.comm

unication

.in.the.syst

em,

.the.mast

.must.be.able.to.cope.at.leas

.with.

the.fol

lowing

.err

ors:

•

. Cabl

.bre

•

. Fail

ure

.of.a.dev

ice

•

. Shor

.cir

cuit

.on.the.lin

•

. Diag

nostics

.of.tem

porary

.int

erference

.(e.

g.,

.cau

sed

.by.ele

ctromagnetic

.int

erference)

.all.eld

bus

.syst

ems,

.in.the.even

.of.a.line.inte

rruption,

.the.devi

ces

.aer.the.inte

rrupt

.are.no.long

reac

hable.

. e.erro

.loca

lization

.capa

bility

.depe

nds

.on.the.tran

smission

.syst

.used

.In.line

.bus.

systems,.messages.are.broadcast.to.all.devices..However,.in.case.of.a.line.break,.these.messages.are.lost.

beca

use

.the.devi

ces

.are.no.long

.able.to.resp

ond.

.Bus.oper

ation

.cann

.be.main

tained

.beca

use,

.due.to.

the.inte

rruption,

.the.cond

ition

.of.the.physi

cal

.bus.term

ination

.using.a.term

ination

.resis

tor

.is.no.long

met.. is.can.lead.to.ree

ctions

.with

.the.bus.con

guration.

. e.resu

lting

.inte

rference

.leve

.will.make.

corr

ect

.oper

ations

.not.possi

ble.

.Aer.a.cert

ain

.peri

od,

.the.mast

.will.dete

.the.data.loss.but.will.not.

be.able.to.prec

isely

.dete

rmine

.the.erro

.loca

tion.

. e.wiri

.diag

rams

.must.be.cons

ulted,

.so.that.the.

serv

ice

.or.mai

ntenance

.per

sonnel

.can.det

ermine

.the.pro

bable

.err

.loc

ation

.(se

.Fig

ure

.33.

7).

Unlike

.bus.syst

ems,

.the.indi

vidual

.devi

ces

.in.the.INTE

RBUS

.syst

.are.netw

orked,

.so.that.each.

one.beha

ves

.as.a.sepa

rate

.bus.segm

ent.

.Foll

owing

.a.fata

.erro

.the.outg

oing

.inte

rfaces

.of.all.devi

ces

are.fed.back.inte

rnally

.via.a.bypa

.swit

ch.

.In.the.even

.of.a.line.inte

rrupt

.betw

een

.the.devi

ces,

.the.

mast

.acti

vates

.each.sepa

rate

.devi

.in.turn

.To.do.this

.the.mast

.open

.the.outg

oing

.inte

rface,

star

ting

.from.the.rst.devi

.up.unti

.the.erro

.loca

tion,

.thus.clea

rly

.iden

tifying

.the.inac

cessible

devi

ce.

. e.bus.mast

.can.then.clea

rly

.assi

.the.erro

.loca

tion.

.If.a.devi

.fail

.the.eld

bus

.beha

ves

in.the.same.way.as.for.a.line.inte

rrupt.

.Howe

ver,

.the.func

tional

.capa

bility

.of.the.rema

ining

.stat

ions

die

.in.bus.and.ring.syst

ems.

Short

.circ

uits

.on.the.line.are.a.majo

.chal

lenge

.in.a.bus.syst

em.

.In.the.even

.of.a.dire

.or.indi

rect

(e.g

.via.gro

und)

.sho

.cir

cuit

.on.the.lin

.the.tra

nsmission

.pat

.is.blo

cked

.for.the.ent

ire

.sec

tion.

.bus.syst

ems,

.one.tran

smission

.line.is.used.for.all.devi

ces,

.whic

.mean

.that.the.mast

.cann

reac

.segm

ent

.part

.eith

er.

.Sign

.ree

ctions

.due.to.impe

dance

.mism

atching

.will.make.comm

unica-

tion

.impo

ssible.

. is.consi

derably

.redu

ces

.furt

her

.erro

.loca

lization.

.Line

.bus.syst

ems

.also.supp

ort

33-8 Industrial Communication Systems

a.division.into.dierent.segments..Repeaters,.which.are.placed.at.specic.points,.can.then.perform.

diagnostic.functions..However,.a.repeater.cannot.monitor.the.entire.system;.it.can.only.cover.a.dened.

number.of.devices.per.segment..Furthermore,.the.use.of.repeaters.incurs.additional.costs.and.increased.

conguration.eort.

.the.INTERBUS.system,.the.user.is.supported.by.the.physical.separation.of.the.system.into.dierent.

bus.segments..As.described.for.the.line.interrupt,.the.devices.are.activated.by.the.master.in.turn.and.the.

ring.is.closed.prior.to.the.short.circuit,.which.means.that.subsystems.can.be.started.up.again.. e.error.

location.can.be.reported.in.clear.text.at.the.bus.master.

.summary,.the.INTERBUS.diagnostic.features.are.essentially.based.on.the.physical.segmentation.

of.the.network.into.numerous.point-to-point.connections.. is.feature.makes.INTERBUS.particularly.

suitable.for.use.with.ber.optics,.which.are.used.increasingly.for.data.transmission.in.applications.with.

large.drives,.welding.robots,.etc..In.linear.systems,.the.use.of.ber.optics—as.in.bus.segmentation—

requires

.expensive.repeaters,.which.simulate.a.ring.structure.

33.4 Performance Evaluation

 is.section.considers.the.performance.of.INTERBUS.with.respect.to.the.relevant.parameters,.such.as.

the.number.of.I/O.nodes.and.the.amount.of.cyclic.I/O.data.

.following.equation.applies.to.the.INTERBUS.layer.2.cycle.time.T

T n T T

IB Bit SW

= ⋅ + +13 6( ) ×

where

.is.the.total.length.of.the.SR.[bytes]

Bit

.the.time.needed.for.transmitting.a.bit

.the.soware.runtime.of.the.master

Master

(b)

Master

(a)

FIGURE 33.7 e.behavior.of.(a).bus.systems.and.(b).ring.systems.in.the.event.of.a.cable.break.

INTERBUS 33-9

 e.frame.overhead.of.6.bytes.is.caused.by.a.2.byte.long.loopback.word.and.a.4.byte.long.FCS..Each.octet.

at.layer.2.is.transmitted.within.a.13.bit.data.telegram.at.layer.1..Depending.on.the.total.amount.of.input.

and.output.data,.the.total.payload.size.can.be.calculated.by

n PL PL

Input

i k

Output

i k













= =

∑ ∑

max ,

where

.is.the.total.payload.size:.sum.of.all.user.data.[bytes].of.all.devices.k.with.n.≤.512.bytes.and.k.≤.512

Bit

.is.the.bit.time:.2.μs.at.0.5.Mbps.or.0.5.μs.at.2.Mbps

.is.the.soware.runtime.of.the.master.(depending.on.implementation)

.is.the.layer.2.payload.size.of.the.ith.device.[bytes].where.1.≤.i ≤ k.and.k.≤.512

.Figure.33.8,.the.INTERBUS.cycle.time.at.the.MAC.level.with.a.bit.rate.of.2.Mbps.and.a.soware.run-

time

.of.0.7.ms.is.illustrated.as.a.function.of.the.number.of.nodes.k.and.their.corresponding.payload.size.PL.

33.5 Summary

INTERBUS.is.a.serial.bus.system.developed.for.the.time-critical.data.transfer.at.the.eld.level.of.industrial.

automation.systems.. e.physical.topology.of.INTERBUS.is.based.on.a.ring.structure,.that.is,.all.devices.are.

actively.integrated.in.a.closed.transmission.path.. e.data.forward.and.return.lines.in.the.INTERBUS.system.

are.led.to.all.devices.via.a.single.cable.. is.means.that.the.general.physical.appearance.of.the.system.is.an.

“open”.tree.structure..Due.to.a.total.frame.approach,.the.cyclic.data.transfer.of.INTERBUS.is.very.ecient.

in.comparison.to.eldbus.systems.based.on.an.individual.frame.approach..As.a.result,.small.cycle.times.can.

be.achieved.with.small.bit.rates..Acyclic.messages.are.transmitted.using.precongured.time.slots.within.the.

total.frame.in.conjunction.with.a.segmentation.and.recombination.mechanism.at.layer.2.

references

. 1.. IEC,.Digital.data.communication.for.measurement.and.control—Fieldbus.for.use.in.industrial.

control.systems,.2001,.IEC.61158.Ed.3,.Type.8.(INTERBUS),.IEC,.Geneva.

. 2.. INTERBUS.Club,.www.interbusclub.com,.visited.May.2009.

. 3.. Baginski,.A..and.Müller,.M.,.INTERBUS Basics and Practice,.Hüthig-Verlag,.Heidelberg,.Germany,.

2002.(German).

4.5

0 25 50 75

Number of nodes k

100 125 150

3.5

2.5

(ms)

1.5

0.5

PL = 1 byte

PL = 4 byte

FIGURE 33.8 INTERBUS.cycle.times.as.a.function.of.the.number.of.nodes.and.the.used.payload.size.

34-1

34.1 Introduction

WorldFIP.is.a.eldbus.that.has.been.mainly.designed.for.time-critical.applications.featured.by.periodic.

proc

esses.

.Its.name.come

.from.the.eld

bus instrumentation protocol

.that.was.init

ially

.stan

dardized

.as.

Fren

.nati

onal

.Stand

ard

.and.subs

equently

.incl

uded

.into.the.Euro

pean

.Fiel

dbus

.Stand

ard

.CENE

LEC

EN50

170.

. e.name.has.been.chan

ged

.into.Worl

dFIP

.to.make.it.more.attr

active

.for.the.inte

rnational

mark

et.

.basi

.idea.behi

.Worl

dFIP

.is.to.cons

ider

.all.vari

ables

.rele

vant

.to.the.appl

ication

.proc

ess

.as.

a.dist

ributed

.data

base

.that.is.peri

odically

.refr

eshed

.in.orde

.to.main

tain

.the.valu

.of.the.upda

ted

vari

ables.

. is.prov

ides

.a.dete

rministic

.fram

ework

.for.the.excha

nge

.of.proc

ess

.vari

ables

.prod

uced

.by.

sens

ors,

.proc

essed

.by.PLCs.or.proc

ess

.comp

uters,

.and.cons

umed

.by.actu

ators.

. e.new.conc

ept

.of.

prod

ucer/consumer,

.as.oppo

sed

.to.the.trad

itional

.clie

nt/server

.conc

ept,

.was.intr

oduced

.rst.time.in.

Worl

dFip.

.the.eld

bus

.must.carr

.not.only.peri

odic

.proc

ess

.vari

ables

.(whi

.are.usua

lly

.a.few.byte

.long

)

.but.

also.mana

gement

.data.(e.g

.con

guration

.les

.whic

.can.be.very.long

.die

rent

.kind

.of.tra

.must.

be.con

sidered:

•

. Peri

odic

. tra

. for. the. upda

ting

. of. proc

ess

. vari

ables.

. In. Worl

dFIP-based

. appl

ications,

. it. is.

assu

med

.tha

.abo

.99%.of.tra

c

.bel

ongs

.to.thi

.cat

egory.

•

. Aper

iodic

.tra

c

.for.the.exc

hange

.of.pro

cess

.var

iables.

•

. Aper

iodic

.tra

.for.the.exch

ange

.of.mess

ages.

. is.last.kind.of.tra

.is.gene

rated

.by.mana

ge-

ment

.les

.whic

.acco

rding

.to.thei

.dime

nsions

.must.be.hand

led

.in.a.die

rent

.way.with.resp

ect

to.pro

cess

.var

iables.

.laye

red

.arch

itecture

.adopt

.in.Worl

dFIP

.is.base

.on.only.thre

.laye

.(as.show

.in.Figu

.34.1

whic

.is.usua

.for.real

-time

.netw

orks:

.physi

cal

.laye

.(PHL

.data.link.laye

.(DLL.=.medi

.acce

.con-

rol

.[MA

.+.log

ical

.lin

.con

trol

.[LL

.),.and.app

lication

.lay

.(AL

WorldFip

34.1. Introduction.....................................................................................34-1

34.2

. Physi

cal

.Lay

er...................................................................................34-2

34.3

. Data.Lin

.Lay

er................................................................................34-2

Transmission.of.Cyclic.Trac. •. Bus.Arbitrator.Table. •. .

Transmission.of.Asynchronous.Trac

34.4. Application.Layer.............................................................................34-6

34.5

. Timi

.Pro

perties

.of.Wor

ldFIP

.Net

works..................................34-7

Concept.of.Producer/Distributor/Consumer. •. Buer.Transfer.

Timings. •. Bus.Arbitrator.Table. •. WorldFIP.Aperiodic.Buer.

Transfers. •. Setting.the.WorldFIP.BAT:.Rate.Monotonic.Approach

References...................................................................................................34-18

Francisco Vasques

University of Porto

Orazio Mirabella

University of Catania

34-2 Industrial Communication Systems

34.2 Physical Layer

PHL.uses.Manchester.type.encoding.. ree.versions.have.been.dened:.a.rst.version,.called.S1,.oper-

ates

.at.50.kbps.using.twisted.pairs;.the.second.version,.S2,.operates.at.1.Mbps;.and.the.third.version,.

S3,.operates.at.2.5.Mbps..Both.S2.and.S3.versions.use.coax.or.ber.optics..Two.interesting.features.of.

PHL.concern.the.use.of.intrinsic.safety.barriers.for.operations.in.hazardous.environments.and.the.

possibility.to.have.power.on.the.bus.that.can.be.used.by.eld.devices.without.the.need.of.a.dedicated.

power.line.

With

.reference.to.the.topology.adopted.in.WorldFIP,.a.minimum.system.is.set.up.by.a.single.trunk.

with.several.stations,.of.which.one.has.the.role.of.bus.arbitrator.(BA).and.manages.the.access.to.the.

medium.. ree.kinds.of.devices.can.be.connected.to.the.bus:

•

. Junction boxes:. ese.devices.are.multiport.repeaters.that.connect.a.cluster.of.eld.devices.to.

the.bus.

•

. Field concentrators:. ese.devices.are.gateways.that.connect.eld.devices.that.use.the.WorldFIP.

protocol.or.dierent.protocols..Moreover,.they.can.be.used.to.interconnect.several.independent.

WorldFip.trunks.

•

. Repeaters:. ese.devices.are.used.to.extend.the.bus.length.by.connecting.two.or.more.trunks.

34.3 Data Link Layer

DLL.is.split.into.a.MAC.and.LLC.sublayers..MAC.is.based.on.a.time.division.multiple.access.protocol.

(TDMA),.where.a.special.station.called.the.BA.makes.the.scheduling.of.all.transmissions.through.the.

distribution.of.a.specic.authorization..Any.of.the.several.WorldFIP.stations.active.on.the.bus.can.oper-

ate

.as.the.BA.(this.allows.for.redundancy.in.case.of.a.fault.in.the.current.BA).but.at.each.time.instant.

only.one.BA.can.be.in.the.bus.

.presence.of.a.centralized.control.for.the.access.to.the.physical.medium.is.an.important.feature.

of.the.MAC.protocol.. e.BA.knows.the.transmission.requirements.of.all.the.devices.present.in.the.

system.and.can.distribute.the.bandwidth.available.in.such.a.way.so.as.to.satisfy.their.time.constraints..

 is.approach.is.particularly.ecient.in.cases.when.most.of.the.trac.is.periodic.since.the.BA.can.

authorize.the.transmission.at.the.correct.time..On.the.other.hand,.this.approach.is.ineective.for.

asynchronous.trac.that.cannot.be.known.a.priori.and.cannot.be.scheduled..For.asynchronous.traf-

c,

.the.protocol.must.use.an.approach.that.allows.the.BA.to.became.aware.of.additional.requests.and.

update.its.schedule.

Application

layer

MPS SUB MMS

Empty

TDMA protocol

Manchester II biphase coding

Power on the bus/intrinsec safety

Read/write/transfer of buer

Request/transfer of ack/unack

Fieldbus

management

Layers 3–6

Data

link

layer

Physical

layer

FIGURE 34.1 Layered.architecture.of.WorldFip.

WorldFip 34-3

LLC.sublayer.exploits.the.communication.channel.made.available.by.MAC.and.provides.two.main.

kinds.of.services:

•

. Services.to.read/write/transfer.process.data.that.are.contained.into.suitable.reception/transmission.

buers

•

. Services.to.request.the.conrmed/unconrmed.transmission.of.message.les

Figure

.34.2.shows.the.basic.mechanism.adopted.in.WorldFip.for.data.transfer,.which.is.performed.

in.several.steps:

•

. e.user.(producer).passes.to.the.AL.the.information.to.be.transferred.to.remote.consumers.

•

. e.AL.entity.stores.the.information.inside.a.buer.that.can.be.read.by.the.DLL.entity.

•

. e.DLL.entity.through.the.physical.medium,.transfers.such.information.into.the.remote.buers.

of.the.various.consumers.

•

. e.information.is.read.by.the.remote.ALs.and.delivered.to.the.consumer.processes.

•

. is.sequence.is.repeated.cyclically.and.is.well.suited.for.periodic.data.transfer.

34.3.1 transmission of Cyclic traffic

As.stated.earlier,.the.task.of.the.DLL.is.to.read.a.buer.and.to.transfer.its.data.through.the.bus..Since.

the.bus.is.a.shared.medium,.in.order.to.avoid.collisions,.it.is.necessary.to.organize.suitably.the.various.

transmissions.. is.is.the.task.of.the.BA,.a.station.that.carries.out.a.key.role.in.the.system.by.providing.

the.authorization.for.each.buer.transfer.

.mechanism.adopted.for.the.transmission.of.cyclic.trac.is.shown.in.Figure.34.3.and.is.executed.

through.the.following.steps:

•

. e. BA. broadcasts,. on. the. bus,. an. identier. (ID_DAT). that. implicitly. species. all. stations.

involved.in.the.current.transaction..One.station.will.be.the.producer.of.the.information,.whereas.

the.other.ones.will.be.the.consumers.

•

. e.identier.is.received.by.both.the.producer.and.consumers.

•

. e.producer.sends.its.message.(RP_DAT).on.the.bus.

•

. e.consumers.that.are.involved.into.the.transaction.accept.the.data.

.sequence.illustrates.clearly.the.role.of.BA.which,.through.a.single.message,.identies.the.role.of.all.

stations.participating.to.the.next.phases..All.devices.receive.the.identier.but.only.one.will.identify.itself.as.

a.producer,.whereas.all.the.others.will.be.consumers.. e.architecture.of.the.BA.is.shown.in.Figure.34.4..As.

we.can.see,.the.BA.is.more.than.simply.a.scheduler.as.it.must.also.cope.with.asynchronous.requests.from.

nonscheduled.trac..For.this.reason,.the.BA.must.monitor.the.bus.and.check.for.some.additional.informa-

tion

.that.the.producer.will.incorporate.in.its.frame.with.reference.to.some.additional.bandwidth.request.

Data producer

Applicat.

layer

Data link

layer

Physical

layer

Physical flow

Logical flow

Data consumer Data consumer

FIGURE 34.2 e.basic.producer–consumer.data.transfer.

34-4 Industrial Communication Systems

 rough.the.monitoring.of.the.current.transmission,.the.BA.can.check.some.control.bits.that.indicate.

an.additional.request.from.the.producer..If.any,.the.BA.will.proceed.through.an.analysis.of.the.scan.

table.that.contains.the.time-ordered.list.of.all.the.identiers.to.be.circulated.on.the.bus.. rough.this.

analysis,.the.BA.will.identify.free.portions.of.bandwidth.to.be.assigned.to.the.asynchronous.trac.

34.3.2 Bus arbitrator table

The.BA.is.the.network.conductor..After.the.system.has.been.configured,.the.BA.is.in.possession.

of.the.variables.list.to.be.analyzed.and.their.time.constraints..If.the.configuration.is.validated.and.

it.satisfies.the.time.constraints.imposed.by.the.application.process,.the.BA.can.generate.the.Scan.

table.and.use.it.again.and.again.in.order.to.authorize.cyclic.traffic..Scan.of.the.variables.is.deter-

ministic,

.that.is,.WorldFIP.can.guarantee.that.a.variable.featured.by.a.certain.periodicity.will.be.

analyzed.at.the.right.time.instants..As.an.example,.Table.34.1.shows.a.list.of.six.periodic.variables.

that.need.to.be.scheduled.

Result of the

configuration process

SCAN table

Transmitter

Message req.

identifier

Asynchronous

traffic identifier

Analyzer of the control field

Physical layer

Receiver

Application layer

FIGURE 34.4 Architecture.of.the.bus.arbitrator.

Producers/consumers nodes

(a)

(b)

ID_DAT

RP_DAT

Bus arbitrator

FIGURE 34.3 The.sequence.of.messages.required.for.a.transaction..Transmission.of.the.authorization.from.

(a).the.bus.arbitrator.and.(b).transmission.of.a.data.reply.from.a.producer.

WorldFip 34-5

 e. BA.must. rst. analyze. the. single. variables.and. achieve. their.refresh. period. (which. will. be.

expressed.in.milliseconds).and.the.data.type.(e.g.,.8.bit.integer.number.and.short.oating.point.num-

ber)

.in.order.to.compute.the.time.required.by.each.variable.. is.time.is.made.up.of.three.compo-

nents:

.the.time.required.by.the.BA.to.transmit.the.ID.frame.that.polls.a.producer,.the.turnaround.

time.(i.e.,.the.time.between.the.transmission.of.an.ID.frame.by.the.BA.and.the.reply.by.an.informa-

tion

.producer),.and.the.time.required.by.the.producer.to.send.the.data.frame.. e.values.reported.in.

Table.34.1.are.based.on.a.transmission.bit.rate.of.1.Mbps.and.a.turnaround.time.of.20.ms.(the.range.

allowed.by.WorldFIP.with.a.bit.rate.1.Mbps.is.from.10.to.70.ms).

From

.the.values.reported.in.Table.34.1,.we.can.infer.that.transmission.of.single.variables.is.not.very.

ecient.in.WorldFip..In.fact,.we.have.a.heavy.transmission.overhead.since.a.single.variable.requires.two.

frames.and.some.turnaround.time..Of.course,.the.situation.becomes.better.when.the.dimension.of.the.

data.increases.as.the.rate.(data.eld)/(transmission.overhead).is.improved.

Figure

.34.5.shows.a.possible.schedule.of.the.variables.listed.in.Table.34.1.to.be.stored.into.the.bus.

arbitrator.table.(BAT)..Time.on.X-axis.is.divided.into.elementary.cycles.that.correspond.to.the.time.

required.to.transmit.variables.with.the.shortest.refresh.period.. is.organization.of.the.data.scan.sim-

plies

.the.construction.of.the.scheduling.table.as.we.divide.the.time.in.xed.slots.and.then.try.to.allocate.

all.variables.into.the.various.time.slots.without.violating.their.time.constraints..Time.on.Y-axis.is.the.

sum.of.the.time.required.to.transmit.all.the.variables.that.are.scheduled.inside.a.time.slot.

.Figure.34.5,.we.can.see.that.in.the.rst.elementary.cycle,.all.the.variables.are.transmitted..In.the.

subsequent.cycles,.the.variables.are.transmitted.according.to.a.distribution.based.on.their.time.con-

straints.

.We.observe.that.no.elementary.cycle.overcomes.the.value.of.5.ms,.so.that.the.schedule.does.not.

violate.any.time.requirement.

5 ms

Time available for acyclic trac

F A B D E

A A C

A A E D B A F A

Time

A A D

C A E

FIGURE 34.5 Possible.schedule.of.the.variables.listed.into.Table.34.1.

TABLE 34.1 List.of.Variables.to.Be.Scheduled

Variable.

Name

Refresh.Period.

(ms) Data.Type

Transaction.

Time.(µs)

A 5 INT_8 170

B 10 INT_8 170

C 15 INT_16 178

D 15 SFPOINT 194

E 20 INT_16 178

F 30 UNS_32 194