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

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39-1

39.1 Introduction

Ethernet. POWERLINK. (EPL). is. a. real-time. Ethernet. (RTE).network. originally. developed. by. B&R.

GmbH.[1].and.curr

ently

.mana

ged

.by.the.Ether

net

.POWE

RLINK

.Stand

ardization

.Grou

.(EPS

.[2]..

 e.rst.versi

.of.EPL.was.deve

loped

.by.B&R.in.2001.and.subs

equently

.publ

ished

.as.an.open.stan

dard,

name

.versi

.2.0.[11]

.by.the.EPSG.in.2003

.Such.a.versi

.has.been.then.enco

mpassed

.in.the.IEC.6178

Inte

rnational

.Stand

ard

.[7].wher

.it.is.refe

rred

.as.Comm

unication

.Pro

.(CP).#1.of.the.Comm

unication

Pro

.Fami

.(CPF

)

.#13..A.more.rece

.versi

.of.the.EPL.spec

ication

.[6].is.curr

ently

.down

loadable

from.the.EPSG.Web.site..Start

ing

.from.the.tigh

.timi

.requ

irements

.of.indu

strial

.comm

unication

syst

ems

.[15]

.the.EPL.proto

col

.has.been.deve

loped

.in.orde

.to.prov

ide

.very.fast.comm

unication

.cycl

(dow

.to.100.μs).with.low.jitt

.(bel

.1.μs).and,.at.the.same.time

.main

taining

.comp

atibility

.with.

lega

.Ether

net.

.An.appl

ication

.laye

.proto

col

.has.been.intr

oduced

.on.top.of.the.EPL.comm

unication

stac

.In.part

icular,

.the.well

-established

.CANO

pen

.stan

dard

.[10].has.been.chos

en,

.ensu

ring

.in.such.a.

way.comp

atibility

.with.a.wide.numbe

.of.alre

ady

.depl

oyed

.comm

unication

.syst

ems.

.In.this.cont

ext,

EPL.is.oe

.ref

erred

.as.“CA

NOpen

.ove

.Ethe

rnet.”

39.2 EPL Protocol

 e.communication.architecture.of.EPL.is.shown.in.Figure.39.1.

.can.be.seen

.EPL.make

.use.of.the.nati

.Ethe

rnet

.stac

.and,.in.part

icular,

.it.is.den

.as.a.data-

link

laye

.(EPL.DLL).prot

ocol,

.plac

.on.top.of.the.Ethe

rnet

.medi

.acce

.cont

rol

.(MAC

)

.[8].that,.as.it.is.well.

know

.reli

.on.a.CSMA

/CD

.(carr

ier

.sens

.mult

iple

.acce

.with.coll

ision

.dete

ction)

.tech

nique.

. e.phys

ical

laye

.of.the.EPL.is.the.same.as.that.of.the.Ethe

rnet.

.Spec

ically,

.the.100B

ASE-X

.half

-duplex

.tran

smission

mode.has.been.chos

en.

.More

over,

.both.RJ-4

.and.M12.conn

ectors

.can.be.used.at.the.user

’s

.disc

retion.

. e.

EPL.spec

ication

.also.reco

mmends

.the.use.of.stan

dard

.patc

.cable

.(twi

sted

.pair

.scre

ened

.twis

ted

.pair

AWG2

6).

.Fina

lly,

.conc

erning

.inst

allation

.guid

elines,

.EPL.foll

ows

.the.reco

mmendations

.of.IAON

.[5].

Ethernet POWERLINK

39.1. Introduction.....................................................................................39-1

39.2

. EPL.Pro

tocol.....................................................................................39-1

39.3

. Fram

.Map

ping................................................................................39-3

39.4

. Netw

ork

.Con

gurations.................................................................39-4

39.5

. Redu

ndancy

.Asp

ects.......................................................................39-5

Medium.Redundancy. •. MN.Redundancy

39.6. Security.Aspects...............................................................................39-7

POWERLINK.Safety

39.7. Performance.Analysis.....................................................................39-8

Jitter. •. Turn-Around.Time. •. Cycle.Time. •. Acyclic.Trac

References...................................................................................................39-10

Paulo Pedreiras

University of Aveiro

Stefan Schoenegger

B&R Industrial Automation

Lucia Seno

Italian National

Rese

arch

Cou

ncil

Stefano Vitturi

Italian National

Rese

arch

Cou

ncil

39-2 Industrial Communication Systems

EPL.denes.two.types.of.stations,.namely.the.managing.node.(MN).and.controlled.nodes.(CNs).. e.

MN.represents,.usually,.the.“controller”.of.an.automation.system.(i.e.,.the.device.that.implements.the.

automation.tasks),.whereas.the.CNs.are.typically.eld.devices.such.as,.for.example,.sensors/actuators..

Each.network.contains.exactly.one.MN.and.up.to.240.CNs.connected.in.dierent.topologies,.as.will.be.

illustrated.in.the.next.section..Several.EPL.compliant.devices.are.o-the-shelf.available.from.dierent.

vendors.which.encompass.most.of.the.components.used.in.industrial.automation.systems.such.as,.for.

example,.programmable.logic.controllers.(PLCs),.computer.numerical.control.(CNC).machines,.sen-

sors,

.actuators,.etc..Moreover,.since.April.2008,.the.open.source.soware.codes.of.both.POWERLINK.

MN.and.CN.are.fully.downloadable.[4],.allowing.for.the.free.implementation.of.POWERLINK.nodes.

 anks

.to.its.performance.gures,.EPL.may.be.employed.in.several.elds.and,.in.particular,.for.appli-

cations

.with.very.tight.timing.constraints.such.as,.for.example,.coordinated.motion.control.systems.[9].

.DLL.of.EPL.relies.on.a.time.division.multiple.access.(TDMA).technique.managed.by.the.MN,.which.

allows.stations.to.orderly.access.the.physical.medium.and.hence.avoiding.collisions.even.if.nonswitched.

Ethernet.congurations.are.employed..In.practice,.the.network.operation.is.based.on.a.periodic.cycle.(EPL.

cycle).with.constant.duration..As.can.be.seen.in.Figure.39.2,.the.EPL.cycle.is.split.into.four.sections.

. rst. section,. designated. start. period,. is. initiated. by. the. MN,. which. broadcasts. the. SoC.

(start.of.cycle).frame.to.synchronize.all.the.CNs..Subsequently,.the.isochronous.period.is.entered,.

TCPUDP

Higher layer

protocols

EPL

application layer

protocol

EPL data link layer

Legacy Ethernet MAC

Legacy Ethernet physical layer

FIGURE 39.1 Ethernet.POWERLINK.communication.architecture.

Start

period

CN #1

Idle

period

Asynchronous

period

Isochronous period

SoC

CN #2 CN #n CNs

MN or

CN #x

Asnd/Eth

MN MN MN

Ethernet POWERLINK cycle

SoA

PReq/PRes

CNs

FIGURE 39.2 Ethernet.POWERLINK.cycle.

Ethernet POWERLINK 39-3

where.MN.and.CNs.exchange.cyclic.data.. is.is.accomplished.by.means.of.two.dierent.frames,.

name

.PReq.(Pol

.Requ

est)

.and.PRes.(Pol

.Resp

onse).

.PReq.is.issu

.by.the.MN.towa

.a.spec

ic

CN.and.carr

ies

.the.outp

.data

. e.addr

essed

.CN,.aer.an.elab

oration

.time

.send

.as.a.resp

onse

the.PRes.fram

.cont

aining

.the.inpu

.data

.Such.a.latt

.fram

.is.not.sent.excl

usively

.to.the.MN.but,.

inst

ead,

.it.is.broa

dcasted

.on.the.netw

ork

.so.that.all.CNs.rece

ive

.it..Aer.rece

iving

.the.PRes.fram

the.MN.wait

.for.a.quie

.time

.and.then.it.move

.on.the.next.CN..It.is.wort

.obse

rving

.that.not.all.

the.CNs.are.nece

ssarily

.poll

.at.each.EPL.cycl

.Inde

ed,

.the.stan

dard

.supp

orts

.two.comm

unication

clas

ses,

.refe

rred

.resp

ectively

.as.cont

inuous

.and.mult

iplexed,

.whic

.spec

ify

.the.way.in.whic

.CNs.are.

addr

essed.

.In.the.cont

inuous

.clas

.a.CN.is.poll

.at.ever

.cycl

.wher

eas

.in.the.mult

iplexed

.clas

.a.

CN.is.poll

.ever

.n.cycl

.(wit

.n.>.1)..Both.comm

unication

.clas

ses

.may.be.hand

led

.cont

empora-

neously

.in.an.EPL.cycl

.so.that.it.is.poss

ible

.to.have.a.poll

ing

.sche

.in.whic

.some.CNs.that.are.

quer

ied

.cont

inuously

.(i.e

.ever

.EPL.cycl

e),

.wher

eas

.some.othe

.are.poll

.with.a.peri

.whic

.is.

an.inte

ger

.mult

iple

.of.the.EPL.cycl

.the.end.of.the.isoch

ronous

.peri

od,

.the.MN.broa

dcasts

.the.SoA.(sta

.of.acyc

lic)

.fram

.info

rming

.all.

CNs.that.the.asyn

chronous

.peri

.is.star

ted.

.In.this.peri

od,

.only.one.asyn

chronous

.mess

age

.may.be.sent.

eith

.by.the.MN.or.by.one.of.the.CNs..In.prac

tice,

.duri

.the.isoch

ronous

.peri

od,

.the.MN.may.coll

ect

requ

ests

.for.asyn

chronous

.tran

smissions

.(fro

.itsel

.and/

.the.CNs).and.then

.accor

ding

.to.a.prio

rity

schem

.it.select

.the.node.whic

.is.gran

ted

.to.tran

smit.

. e.select

.node

.whic

.is.info

rmed

.by.the.

MN.via.the.SoA.fram

.may.tran

smit

.a.fram

.belo

nging

.to.one.of.two.die

rent

.cate

gories,

.name

.EPL.

AsyncSend.frame.and.legacy.Ethernet.message.. e.AsyncSend.frame.type.is.dened.by.the.EPL.protocol;.

it.can.be.used.for.acyc

lic

.proc

ess

.data.(typ

ically

.alar

ms)

.and.it.may.be.deli

vered

.eith

.in.unic

ast

.or.in.mul-

icast

.mode..Conc

erning

.lega

.Ethe

rnet

.mess

ages,

.they.are.used.for.gene

ral

.purp

ose

.comm

unication,

.and.

they.may.be.of.what

ever

.type.alth

ough,

.to.this.rega

rd,

.the.EPL.stan

dard

.recom

mends

.the.use.of.the.UDP/

.prot

ocol

.suit

.(rat

her

.than.the.most.popu

lar

.TCP/

IP)

.sinc

.UDP.intr

oduces

.a.lowe

.over

head.

Finally,

.the.idle.peri

.is.ente

red

.at.the.end.of.the.asyn

chronous

.peri

od.

. e.purp

ose

.of.this.time.

inte

rval

.is.to.guar

antee

.that.ther

.are.no.mess

age

.over

runs,

.i.e.,.no.mess

ages

.cros

.the.EPL.cycl

.boun

aries.

. us,.in.this.peri

od,

.ther

.is.no.acti

vity

.on.the.netw

ork

.since.all.stat

ions

.simpl

.wait.for.the.begi

ning

.of.a.new.cycl

.Tabl

.39.1.summ

arizes

.the.most.rele

vant

.para

meters

.of.the.EPL.proto

col

.as.deri

ved

from.the.sta

ndardization

.doc

uments.

.corr

ect

.exec

ution

.of.the.EPL.cycl

.requ

ires

.that.all.stat

ions

.conn

ected

.to.the.netw

ork

.are.comp

li-

ant

.with.the.EPL.proto

col.

. is.mode.of.oper

ation

.is.refe

rred

.in.the.stan

dard

.docu

ment

.as.“EPL.mode

.”

Neve

rtheless,

.a.spec

ic

.mode.of.oper

ation

.(nam

ely

.“basi

.Ether

net

.mode

”)

.allo

.for.the.coex

istence

on.the.same.netw

ork

.of.EPL.stat

ions

.as.well.as.lega

.Ether

net

.ones

.For.thes

.con

gurations,

.howe

ver,

as.men

tioned

.in.the.sta

ndard

.doc

ument,

.the.str

ict

.det

erminism

.of.the.EPL.cyc

.is.no.lon

ger

.ens

ured.

39.3 Frame Mapping

 e.EPL.frames.are.encapsulated.in.standard.Ethernet.protocol.data.units.as.shown.in.Figure.39.3.

.can.be.seen

.the.Eth

ernet

.typ

.el

.is.set.to.the.hex

adecimal

.valu

.88A

.whi

.uni

quely

.ide

nties

EPL.. e.rst.octe

.of.the.EPL.fram

.(“me

ssage

.type

,”

.i.e.,.the.rst.octe

.in.the.Ethe

rnet

.data.eld

)

.speci

es

the.type.of.fram

. e.code

.used.in.orde

.to.speci

.the.die

rent

.fram

.are.repo

rted

.in.Tabl

.39.2

TABLE 39.1 EPL.Parameters

Parameter.Description Value

Transmission

.spe

ed 10–100

.Mbps

Maximum

.numb

.of.CNs 240

Dura

tion

.of.the.sta

.perio

45.μs

Quiet

.time.of.MN

.μs

Ela

boration

.time.of.each.CN

.μs

39-4 Industrial Communication Systems

 e.second.and.third.octets.contain,.respectively,.the.source.and.destination.nodes.of.the.message.in.

the.EPL.network.. e.subsequent.octets.accommodate.the.data.to.be.exchanged.

39.4 Network Congurations

 e.connections.between.the.EPL.nodes.can.be.realized.by.means.of.either.traditional.Ethernet.hubs.or.

switches..Actually,.the.EPL.standard.recommends.the.use.of.hubs.since.they.ensure.both.limited.jitter.

(about.500.ns).and.low.path.delay.(about.40.ns)..Nonetheless,.switches.can.also.be.employed.as.connec-

tion

.devices,.provided.that.additional.jitter.and.latency.are.taken.into.account.during.the.EPL.system.

conguration.

.availability.of.standard.Ethernet.components.allows.for.the.implementation.of.several.dier-

ent

.EPL.congurations..Moreover,.particular.CNs.may.be.equipped.with.an.integrated.hub.in.order.

to.simplify.the.implementation. of.complex.network.topologies.with.various.hub.levels.in.which.

traditional.star.congurations.are.combined.with.linear.chains.of.CNs.as.shown,.for.example,.in.

Figure.39.4.

Since

.the.EPL.protocol.is.purposely.developed.to.avoid.frame.collisions,.the.Ethernet.standard.

constraint.of.a.maximum.round.trip.time.of.5.12.μs.(at.100.Mbps).has.not.to.be.necessarily.fullled.. is.

reveals.particularly.helpful.for.linear.congurations,.oen.deployed.in.low-level.industrial.applications.

that,.due.to.their.structures,.may.have.relevant.round.trip.times.

.the.other.hand,.the.tight.timing.of.the.EPL.protocol.introduces.further.important.constraints..

In.particular,.in.order.to.detect.possible.transmission.errors.and/or.device.failures,.the.correct.opera-

tion

.of.the.protocol.requires.that,.aer.issuing.a.PReq.frame,.the.MN.has.to.receive.the.PRes.from.the.

addressed.CN.within.a.specied.time-out.. e.default.time-out.value.is.25.μs..However,.since.it.clearly.

TABLE 39.2 Codes.of.the.Ethernet.POWERLINK.

Frames

Ethernet.POWERLINK.Frame

Hexadecimal.

Code

Start

.of.cyclic.(SoC) 01

Poll.request.(PReq) 03

Poll.response.(PRes) 04

Start.of.asynchronous.(SoA) 05

Asynchronous.send.(ASnd) 06

DEST.

address

SRC.

address

Ethernet

type

Data FCSPAD

Start frame.

delimiter

Preamble

7 1 6 6 2 Up to 1500 > = 0 4

Standard ethernet protocol data unit

Number

of octects:

EPL

message

type

EPL

source

EPL

destination

EPL data

1 1 1 Up to 1496

Ethernet POWERLINK frame

Number

of octects:

FIGURE 39.3 Ethernet.POWERLINK.frame.

Ethernet POWERLINK 39-5

depends.on.the.network.conguration.as.well.as.on.the.employed.components,.the.EPL.standard.

species.that.it.can.be.either.globally.overwritten.or.set.appropriately.for.each.single.CN.

39.5 redundancy aspects

 e.redundancy.features.of.EPL.are.explicitly.dened.by.the.POWERLINK.high-availability.services.

[12].which.represent.an.extension.to.the.basic.specication.[11].and.which.maintain.full.compatibility.

with.it.. e.high-availability.services.of.POWERLINK.have.the.objective.of.providing.full.network.

functionality.even.in.case.of.any.upcoming.single.failure.on.any.of.the.POWERLINK.components.(MN,.

CNs,.and.physical.infrastructure).. e.main.focus.of.this.concept.is.to.provide.a.very.fast.switchover.

time.in.case.of.failure..In.particular,.POWERLINK.high.availability.includes.a.hot standby.functional-

ity.

. e.set.of.services.provided.by.POWERLINK.high.availability.rely.on.two.specic.aspects,.namely.

medium.redundancy.and.MN.redundancy.

39.5.1 Medium redundancy

 is.feature.is.provided.by.introducing.a.second.physical.cabling.in.the.network..All.devices.are.con-

nected

.to.both.cables,.and.all.information.can.be.transmitted.on.both.links.by.each.node.(MN.as.well.

as.CNs).implementing.the.high-availability.service.set.. e.receive.direction.is.bundled.in.the.so-called.

link.selector.functionality.that.is.part.of.the.slave.device.. is.functionality.receives.both.incoming.data.

streams.and.evaluates,.based.on.predened.algorithms,.which.signal.and.information.to.choose.. e.

decision.algorithm.can.take.a.single.attribute,.such.as.the.incoming.time,.or.can.use.several.attributes,.

such.as.CRC.or.link.quality..Each.link.selector.is.requested.to.include.its.link.status.into.the.PRes.frame..

 is.information.can.be.used.by.the.MN.to.give.feedback.to.the.application.regarding.the.quality.of.the.

network.. e.functionality.of.the.link.selector.requires.that.the.maximum.variation.of.the.path.delay.on.

the.network,.taken.between.any.two.specic.nodes.in.both.networks,.shall.not.exceed.5.12.μs.(transmis-

sion

.time.for.64.byte.on.a.Fast.Ethernet.network).

Figure

.39.5.shows.an.example.of.medium.redundancy..As.can.be.seen,.two.links.are.used.for.commu-

nicating

.with.the.CNs..If.a.problem.occurs.on.the.rst.link,.then.the.link.selectors.of.the.CNs.that.do.not.

receive.the.signal.correctly.on.the.rst.link.switch.immediately.to.the.second.one.

39.5.2 MN redundancy

MN.redundancy.introduces.an.extension.to.the.standard.MN.functionality.to.provide.high-availability.

services.at.the.application.layer..In.order.to.realize.a.high-availability.system,.two.or.more.redundant.

Hub

Managing

node

CN CN CN

Hub Hub Hub

FIGURE 39.4 Example.of.Ethernet.POWERLINK.conguration.

39-6 Industrial Communication Systems

managing.nodes.(RMNs).need.to.be.available.in.the.network..In.this.direction,.two.new.entities.have.

been.dened.. e.active.managing.node.(AMN).is.one.(and.the.unique).of.the.RMNs.that.currently.

hosts.and.executes.the.functionality.of.the.MN.. e.stand-by.managing.node.(SMN).is.one.of.the.RMNs.

that.is.in.stand-by.state.regarding.MN.functionality..From.the.network.point.of.view,.a.SMN.behaves.

like.a.CN,.with.the.dierence.that.the.SMN.is.constantly.monitoring.the.network.and.hosting.also.the.

current.status.of.all.participants..In.case.of.any.failure.of.the.AMN,.one.of.the.RMNs.will.immediately.

takeover.the.functionality.of.the.AMN.. e.takeover.process.will.not.last.more.than.one.single.EPL.

cycle.. e.takeover.process.can.introduce.jitter,.upper.bounded.by.the.maximum.path.delay.of.the.net-

work.

.All.devices.connected.to.the.network.shall.be.capable.of.tolerating.this.single.jitter.of.the.network..

Since.more.than.one.SMN.could.be.available.in.the.network,.an.election.mechanism.has.to.executed.

before.the.beginning.of.network.operation..Such.a.technique.is.based.on.the.(unique).priorities.of.the.

dierent.SMNs.. e.priorities.could.be.either.assigned.with.an.engineering.tool.(and.congured.within.

the.object.dictionary).or.derived.from.the.unique.node.address.of.the.SMNs.

.further.way.of.providing.basic.redundancy.service.is.to.use.a.ring.structure.for.the.cabling.

architecture..A.closed.ring.always.oers.the.possibility.of.having.two.independent.transmission.paths.

from.every.node.to.every.other.node.in.the.network..For.some.branches,.this.is.a.cost-eective.approach,.

as.the.critical.section.is.the.physical.connection.between.the.cable.and.the.socket,.inuenced.by.such.

factors.of.the.rough.industrial.environment.as,.for.example,.temperature,.humidity,.and.vibration.. is.

feature.should.not.be.confused.with.the.full.redundant.network.described.above.. e.ring.redundancy.

for.EPL.networks.is.actually.an.optional.product.feature.of.the.MN.

.can.be.noticed.in.Figure.39.6,.EPL.products.(MN.as.well.as.CNs).that.support.ring.redundancy.

need.to.be.equipped.with.two.Ethernet.ports..In.case.of.a.noninterrupted.transmission.path,.the.closed.

ring.passes.the.sent.frame.through.the.network.back.to.the.MN.. e.MN.will.recognize.this.frame.as.

“recently.sent”.and.simply.will.lter.it..If.such.a.frame.is.not.received.within.a.time-out,.then.the.MN.

will.conclude.that.a.problem.has.occurred.(typically.a.cable.break)..In.this.case,.the.MN.will.send.the.

Redundant

managing node

EPL

link selector

Redundant

managing node

EPL

link selector

First link Second link

Hub

FIGURE 39.5 Example.of.Ethernet.POWERLINK.medium.redundancy.

PLC

managing node

Drive Drive Drive

Sensor Sensor

FIGURE 39.6 Example.of.Ethernet.POWERLINK.ring.redundancy.

Ethernet POWERLINK 39-7

data.of.the.next.EPL.cycle.on.both.connections.and.make.use.of.the.alternative.transmission.path..If.the.

ring.is.rees

tablished,

.the.MN.will.dete

.the.obvi

ous

.coll

ision

.and.logi

cally

.unlo

.the.ring.topol

ogy

agai

.dir

ectly.

39.6 Security aspects

 e.functional.principle.of.POWERLINK,.when.operating.in.EPL.mode,.clearly.separates.the.real-time.

doma

.from.any.non-

real-time

.(oc

.one.. e.acces

.to.the.POWE

RLINK

.real

-time

.doma

.is.gran

ted

by.dedic

ated

.gate

way

.func

tionality

.of.the.netw

ork.

. ese.gate

ways

.typi

cally

.prov

ide

.a.basi

.NAT.(net

work

addr

ess

.tran

slation)

.mecha

nism

.and.ther

efore

.hide

.the.POWE

RLINK

.IP.addr

ess

.from.bein

.acces

sed

with

out

.havi

.the.exac

.know

ledge

.abou

.the.gate

way

.con

guration.

. is.prin

ciple

.prov

ides

.basic.

secu

rity

.meas

ures

.alre

ady

.by.desi

gn,

.thou

.most.gate

way

.prod

ucts

.coul

.incl

ude

.furt

her

.rew

all

func

tionality

.on.top.

39.6.1 POWErLINK Safety

POWERLINK.safety.is.a.protocol.for.a.so-called.one.channel.safe.data.transmission.up.to.safety.

inte

grity

.leve

.(SIL

)

.3.. e.prot

ocol

.has.been.spec

ied

.in.2007.by.the.Safe

.Work

ing

.Grou

.of.EPSG

and.the.rst.prod

ucts

.comp

liant

.with.the.spec

ication

.have.been.cert

ied

.by.the.TÜV.Rhei

nland

Grou

.in.2008

POWERLINK

.safe

.work

.acco

rding

.to.the.“Bla

.Chan

nel”

.prin

ciple,

.mean

ing

.that.the.safe

meas

ures

.are.comp

letely

.inde

pendent

.of.the.unde

rlying

.prot

ocol.

.As.a.rest

riction,

.howe

ver,

.the.unde

lying

.prot

ocol

.has.to.prov

ide

.secu

rity

.meas

ures

.for.the.netw

ork.

.Obvi

ously,

.POWE

RLINK

.safe

.has.

been.deve

loped

.in.orde

.to.opti

mize

.its.inte

gration

.with.the.EPL.prot

ocol

.(in.this.dire

ction,

.both.the.

doma

.sepa

ration

.and.the.high.cycl

.upda

.rate.reve

aled

.part

icularly

.help

ful).

.Nonet

heless,

.it.may.

be.empl

oyed

.eec

tively

.on.top.of.any.othe

.prot

ocol

.as.well

.POWE

RLINK

.safe

.allo

.to.impl

ement

safe

.netw

ork

.mana

gement

.(SNM

.tech

niques,

.a.safe.con

guration

.mana

ger

.(SCM

)

.for.netw

ork

part

icipant

.and.the.safe.exch

ange

.of.proc

ess

.info

rmation

.via.safe.proc

ess

.data.objec

.(SPD

O).

.With.

POWE

RLINK

.safe

ty,

.it.is.poss

ible

.to.exch

ange

.devi

ces

.and.they.will.be.auto

matically

.boot

.and.con-



gured

.by.the.syst

.with

out

.any.user.inte

rvention.

. e.SCM.is.resp

onsible

.for.para

metrization

.and.

con

guration

.of.all.safe

.node

.in.the.netw

ork.

.Its.main.func

tionality

.is.to.veri

.duri

.boot

-up

.of.

the.netw

ork

.if.the.curr

ent

.para

meter

.set.of.each.node.matc

hes

.the.expe

cted

.para

meters

.as.con

gured

by.the.user

.In.the.case.ther

.is.a.mism

atch,

.the.SCM.down

loads

.the.expe

cted

.para

meter

.set.to.the.

devi

.and.rebo

ots

.it..All.safe

.para

meters

.are.stor

.on.a.data

base

.on.the.SCM..For.any.give

.safe.

conf

iguration

.of.the.syst

em,

.the.SCM.is.able.to.iden

tify

.wron

gly

.plug

ged

.modu

les

.by.comp

aring

the.vendor.ID.and.the.product.code.of.any.booting.device.with.the.centrally.located.database..A.mis-

atch

.woul

.prev

ent

.the.devi

.from.boot

ing,

.and.the.prot

ocol

.may.then.indi

cate

.the.fail

ure

.to.the.

appl

ication

.laye

POWERLINK

.safe

.devi

ces

.can.be.used.in.a.mixed.mode.with.both.safe.and.nons

afe

.devi

ces

.conne

cted

to.the.same.netw

ork

.infra

structure.

. is.is.oen.refe

rred

.to.as.inte

grated

.safe

.tech

nology.

.comm

unication

.mode

.of.the.POWE

RLINK

.safe

.proto

col

.is.base

.on.the.well

-known

.prod

ucer/

consumer

.tech

nique.

.Ever

.safe.node.can.be.prod

ucer

.and/

.cons

umer

.of.any.safe

ty-related

.info

rma-

tion.

. e.uniq

.iden

tier

.of.safe.node

.as.well.as.safe.data.is.prov

ided

.by.the.safe

.addr

ess

.(SAD

R).

POWE

RLINK

.safe

.syst

ems

.can.be.free

.set.up.in.ever

.possi

ble

.topol

ogy

.prov

ided

.by.the.tran

sport

medi

.Each.POWE

RLINK

.safe

.netw

ork

.requ

ires

.one.singl

.SCM.. e.numbe

.of.safe

.node

.for.one.

SCM.is.limi

ted

.to.a.maxi

mum

.of.1023

. is.is.then.refe

rred

.to.as.one.safe

.doma

.(SD)

.For.a.huge.

safe

.netw

ork,

.a.maxi

mum

.of.1023.SDs.can.be.conn

ected

.via.safe

.doma

.gate

ways

.(SDG

s).

.From.the.

poin

.of.vie

.of.one.SD,.the.SDG.app

ears

.simp

.as.a.saf

ety

.nod

Data

. tran

smission

. of. safe

ty-related

. data. is. prov

ided

. by. the. dedi

cated

. safe

. fram

. form

at.

. e.

maximum.data.that.can.be.transferred.with.one.frame.is.254.byte..Each.user.data.are.duplicated.and.

39-8 Industrial Communication Systems

transmitted.within.two.subframes.. e.CRC.type.used.(either.CRC8.or.CRC16).depends.on.the.size.of.

the.user.data

.All.syst

ematic

.fail

ures

.will.be.reco

gnized

.by.the.POWE

RLINK

.safe

.proto

col

.incl

uding

dela

.rep

etition,

.wro

.seq

uence,

.ins

ertion,

.los

.and.mas

querade

.[11

39.7 Performance analysis

While.the.performance.of.real-time.networks.can.be.assessed.according.to.a.multitude.of.performance.

crit

eria,

.in.the.cont

ext

.of.EPL.the.most.comm

.metr

ics

.are.jitt

er,

.turn

-around

.time.(TAT

.and.cycl

time

.Othe

.para

meters,

.such.as.the.late

ncy

.of.the.asyn

chronous

.mess

ages,

.are.also.comm

only

.used.to.

asse

.the.proto

col

.perf

ormance.

. is.sect

ion

.addr

esses

.the.perf

ormance

.anal

ysis

.of.the.EPL.proto

col,

illu

strating

.how.the

.par

ameters

.can.be.est

imated

.and.how.key.net

work

.desi

.opti

ons

.ae

.the

39.7.1 Jitter

Jitter,.dened.as.the.maximum.time.deviation.that.may.occur.on.cyclic.events,.is.dened.in.the.EPL.

stan

dard

.as.bein

.in.the.subm

icrosecond

.rang

. e.jitt

.in.EPL.netw

orks

.is.dete

rmined

.by.the.MN,.

whic

.is.resp

onsible

.for.corr

ect

.send

ing

.of.the.SoC.mess

age,

.as.well.as.by.the.netw

ork

.comp

onents.

Whil

.the.MN.jitt

.is.intr

insic

.to.the.hard

ware/soware

.impl

ementation

.char

acteristics,

.the.jitt

.due.

to.the.netw

ork

.comp

onents

.depe

nds

.on.the.natu

.of.the.comp

onents

.them

selves

.and.of.the.netw

ork

topol

ogy.

.Alth

ough

.the.jitt

.indu

ced

.by.a.singl

.hub.(or.by.a.limi

ted

.numbe

.of.such.devi

ces)

.may.seem.

negl

igible,

.as.stat

.abov

.EPL.perm

its

.the.viol

ation

.of.the.IEEE.802.

.topol

ogy

.guid

elines,

.open

ing

.the.

way.for.the.use.of.tree.or.line

.topol

ogies

.with.seve

ral

.casc

aded

.hubs

.Desp

ite

.thei

.conv

enience,

.such.

topol

ogies

.have.a.nega

tive

.impa

.on.the.glob

.netw

ork

.jitt

er,

.whic

.depe

nds

.on.the.high

est

.numbe

.of.

hubs.tha

.exi

.bet

ween

.the.MN.and.any.CN.. e.jit

ter

.upp

.bou

.can.be.com

puted

.as

. J J N J

NET mn hub hub

= + × . (39.1)

where

.is.the.MN.jitter

hub

.is.the.hub-induced.jitter.(homogeneous.hubs.assumed)

hub

.is.the.number.of.hubs.between.the.MN.and.any.CN

For

.illu

strative

.purp

oses,

.cons

ider

.a.MN.and.a.CN.inte

rconnected

.by.a.sing

.hub..In.this.con

gura-

tion,

.cons

idering,

.for.exam

ple,

.the.POWE

RLINK

.inte

rface

.desc

ribed

.in.[3],.J

.=.50.ns,.whereas.a.

typi

cal

.valu

.for.the.hub-

induced

.jitt

.is.J

hub

.=.70.ns.. us,.the.jitter.upper.bound.becomes,.trivi-

ally,

.=.120.ns..For.a.two-level.tree.topology,.the.jitter.upper.bound.becomes.J

.=.190.ns,.with.an.

incr

ease

.of.58.3

.In.thes

.cond

itions,

.a.line

.topo

logy

.has.to.be.limi

ted

.to.13.devi

ces

.in.orde

.to.

guar

antee

.a.jitt

.valu

.belo

.1.μs.

39.7.2 turn-around time

Another.important.performance.criterion.is.the.TAT,.dened.as.the.time.elapsed.between.the.transmission.

of.a.requ

est

.and.the.recep

tion

.of.the.corr

esponding

.resp

onse.

. e.TAT.has.a.noti

ceable

.impa

.on.the.

band

width

.util

ization

.eci

ency

.sinc

.this.corr

esponds

.to.netw

ork

.idle.time

.for.jitt

er,

.the.TAT.depe

nds

.on.cons

tructive

.aspec

.of.the.netw

ork

.comp

onents

.and.devi

ces

.as.well.

as.on.the.netw

ork

.topo

logy.

.In.sowa

re-based

.impl

ementations,

.the.EPL.stac

.is.enti

rely

.execu

ted

.in.a.

micr

oprocessor,

.requ

iring

.sever

.proc

essing

.step

.to.decod

.the.requ

ests

.and.issu

.the.corr

esponding

repl

ies.

. e.assoc

iated

.late

ncy

.depe

nds

.on.the.dela

.intr

oduced

.by.sever

.fact

ors

.such.as.code.execu

tion

time

.DMA.techn

iques,

.inte

rrupt

.hand

ling

.proc

edures

.and.bus.prot

ocols

.that.may.lead.to.resp

onse

.time

Ethernet POWERLINK 39-9

typically.in.the.order.of.some.microseconds.. e.EPL.protocol.may.also.have.a.total.hardware.support,.

typ

ically

.via.FPG

.tech

nology.

.In.thi

.case

.the.EPL.pac

ket

.han

dler,

.the.Eth

ernet

.con

troller,

.and.the.hub.

(if.inc

luded)

.are.com

pletely

.emb

edded

.in.har

dware.

. e.pro

cessing

.of.inc

oming

.req

uests,

.in.thi

.case

.is.

ent

irely

.car

ried

.out.in.har

dware,

.and.the.int

erconnection

.bet

ween

.the.Eth

ernet

.con

troller

.and.the.EPL.

pac

ket

.han

dler

.can.use.dedi

cated

.buse

.lea

ding

.to.ext

remely

.sho

rter

.res

ponse

.tim

es,

.bel

.1.μs.

milarly

.to.th

.ji

tter,

.th

.use.of.tr

.or.li

near

.to

pologies

.wi

.sev

eral

.ca

scaded

.hu

.al

.ha

.a.ne

gative

imp

act

.on.the.TAT.sin

.eac

.hub.sit

uated

.bet

ween

.the.MN.and.a.CN.add

.its.own.del

ay.

.Mor

eover,

.it.has.

to.be.not

iced

.tha

.the.hub

-induced

.del

.aec

.bot

.the.MN.req

uests

.and.the.CN.rep

lies,

.dou

bling

.in.suc

a.way.its.imp

act

.in.eac

.que

.of.the.CN.

.ge

neral,

.th

.TA

.in.a.tr

ansaction

.in

volving

.no

.i.ca

.be.up

per

.bo

unded

.as

. TAT

CN hub hub prop Req Res

i i i

D D N t C C= + × × + + +2 . (39.2)

where

is.the.nominal.response.time.of.the.ith.CN

hub

.is.the.nominal.hub.delay.(homogeneous.hubs.assumed.for.simplication)

hub

is.the.number.of.hubs.involved.in.a.transaction.between.the.MN.and.the.ith.CN

prop

.is.the.wire-propagation.time

Req

.is.the.transmission.time.of.the.request.message

Res

.is.the.transmission.time.of.the.corresponding.response.message

.an.exa

mple,

.a.tra

nsaction

.in.whi

.the.MN.pol

.a.CN.hav

ing

.a.sho

.amo

unt

.of.dat

.to.tra

nsmit

(up.to.36.dat

.byt

es)

.may.be.con

sidered.

.In.thi

.cas

Req

.=.C

Res

.=.6.72.μs..Assuming.also.a.fast.CN.

(i.

e.,

.wit

.har

dware

.sup

port)

.yie

lding

.a.1.μs.res

ponse

.tim

.a.sing

.hub.wit

.400.ns.del

ay,

.and.tha

.the.

seg

ment

.(wi

re)

.len

gth

.is.of.100.m.(le

ading

.to.t

prop

.=.500.ns),.the.TAT,.given.by.Equation.39.2,.is.(in.seconds):

TAT = × + × × + × + × . × =

− − − − −

1 10 2 400 10 500 10 2 6 72 10 16 24 1 0

6 9 9 6 6

( ) . ×

If.a.two

-hub

.top

ology

.is.con

sidered,

.the.abo

.com

puted

.val

.of.TAT.is.inc

reased

.by.800.ns,.cor

responding

to.a.rel

ative

.deg

radation

.of.abo

.4.9

.on.the.TAT

.For.a.lin

ear

.top

ology

.wit

.10.dev

ices,

.the.tot

.inc

rease

wou

.be.aro

und

.22.

2%.

. ese.per

formance

.gu

res

.sho

.how.the.net

work

.top

ology

.des

ign

.may.hav

.a.

sub

stantial

.imp

act

.on.the.TAT

.use.of.swi

tch

.dev

ices

.may.sign

icantly

.wor

sen

.the.per

formance.

.Ind

eed,

.cut

-through

.swi

tches

ind

uce

.a.lat

ency

.of.at.lea

.12.μs.sinc

.mes

sage

.for

warding

.onl

.sta

rts

.ae

.the.rec

eption

.of.the.des

tina-

tion

.add

ress

.Ethe

rnet

.el

.thu

.mul

tiplying

.the.lat

ency

.for.a.fac

tor

.of.at.lea

.3..For.stor

e-and-forward

swi

tches,

.the.lat

ency

.tim

.dep

ends

.on.the.mes

sage

.size.and.can.be.ver

.rel

evant

.(up.to.120.μs.per.swi

tch,

if.ma

ximum

.siz

.Eth

ernet

.me

ssages

.ar

.em

ployed).

39.7.3 Cycle time

 e.cycle.time.is.a.factor.of.paramount.importance.since.EPL.applications.(for.example,.those.concerned.

wit

.mot

ion

.con

trol

.iss

ues)

.may.req

uire

.ver

.sho

.cyc

.tim

.(we

.bel

.1.ms,.as.dis

cussed

.in.[14

]).

. e.cyc

tim

.of.EPL.may.be.tri

vially

.cal

culated

.as.the.sum.of.the.per

iods

.sho

.in.Fig

ure

.39.

. T T T T T

C st is ac id

= + + + . (39.3)

where

.the.add

ends

.in.Equ

ation

.39.

.repr

esent

.the.dur

ations

.of.the.star

.isoc

hronous,

.asy

nchronous,

.and.

idl

.peri

ods,

.res

pectively.

.How

ever,

.the.EPL.cycl

.tim

.is.most

.det

ermined

.by.the.dur

ation

.of.the.iso

chronous

.peri

.whic

.in.its.tur

.is.a.dir

ect

.fun

ction

.of.the.num

ber

.of.que

ries

.to.the.con

gured

.CNs

39-10 Industrial Communication Systems

Indeed,.the.size.of.each.communication.slot.depends.on.the.amount.of.data.transmitted.as.well.as.on.the.

net

work

.topo

logy

.and.lat

ency

.ind

uced

.by.the.net

work

.com

ponents,

.whic

.the

refore

.als

.in

uence

.the.

cycl

.tim

.Mor

eover,

.the.ado

ption

.of.the.mul

tiplexed

.com

munication

.clas

.(ei

ther

.alo

.or.in.con

junction

wit

.the.con

tinuous

.one

)

.all

ows

.to.red

uce

.the.max

imum

.num

ber

.of.slo

.in.eac

.cycl

.and.con

sequently

the.cycl

.tim

.as.wel

Finally,

.it.is.also.imp

ortant

.to.rem

ember

.tha

.the.CNs.hav

.also.to.per

form

.some.pro

tocol-related

tas

.lik

.for.exa

mple,

.SoC.pro

cessing

.or.the.mai

ntenance

.of.the.EPL.cyc

.sta

.mac

hine.

. us

.the.tim

req

uired

.to.han

dle

.the.di

erent

.nod

.act

ivities

.also.has.to.be.con

sidered.

.In.thi

.con

text,

.the.EPL.pro

tocol

pro

vides

.a.(li

mited)

.lev

.of.con

trol

.sin

.it.all

ows

.the.MN.to.con

trol

.the.pol

.ord

.per

mitting,

.for.exa

ple,

.to.avo

.pol

ling

.the.CNs.wit

.hig

hest

.SoC.lat

ency

.imm

ediately

.ae

.the.send

ing

.of.the.SoC.fra

me.

39.7.4 acyclic traffic

According.to.the.EPL.specication,.the.asynchronous.period.aims.at.the.transfer.of.non.time-critical.

dat

.typ

ically

.rel

ated

.wit

.the.net

work

.man

agement

.and.sup

ervision

.(AS

.mes

sages)

.as.wel

.as.wit

non

-EPL

.tra

c

.(e.

g.,

.TCP.or.UDP

/IP

.mes

sages).

.Des

pite

.not.bei

.add

ressed

.in.the.EPL.spe

cication,

the.use.of.asy

nchronous

.ser

vices

.for.car

rying

.inf

requent

.rea

l-time

.dat

.e.g

.ala

rms,

.cou

.als

.bri

ec

iency

.ga

ins

.pr

ovided

.th

.su

itable

.la

tency

.va

lues

.ca

.be.gu

aranteed.

.lat

ency

.ass

ociated

.wit

.the.asy

nchronous

.tra

c

.is.a.rel

evant

.per

formance

.gu

.sin

.it.is.

imp

ortant

.to.hav

.acc

eptable

.res

ponse

.tim

.in.the.net

work

.man

agement

.ser

vices

.as.wel

.as.on.the.use

r-level

app

lications

.su

pported

.on.IP.tr

ac.

.lat

ency

.exp

erienced

.by.asy

nchronous

.req

uests

.dep

ends

.on.the.cyc

.dur

ation,

.on.the.per

iod

.of.

the.rea

l-time

.dat

.ass

ociated

.wit

.the.CN,.and.on.the.par

ticular

.sch

eduling

.alg

orithm

.use

.by.the.MN..

 e.cyc

.dur

ation

.de

nes

.the.numb

.of.req

uests

.tha

.can.be.han

dled

.by.tim

.uni

. e.per

iod

.of.the.

rea

l-time

.dat

.ass

ociated

.wit

.the.CN.con

straints

.the.sign

aling

.lat

ency

.sinc

.CNs.not

ify

.the.MN.abo

the.exi

stence

.of.req

uests

.via.PRe

.mes

sages.

. us

.in.the.wor

st-case

.situ

ation,

.the.CN.may.hav

.to.wai

one.ent

ire

.pol

.per

iod

.pri

.to.be.abl

.to.inf

orm

.the.MN.of.the.exi

stence

.of.req

uests.

.Fin

ally,

.the.lat

ency

als

.dep

ends

.on.the.sch

eduling

.dec

isions

.at.the.MN.. e.use

.can.in

uence

.the

.dec

isions

.by.assi

gning

hig

her

.pr

iorities

.to.th

.mo

.ti

me-sensitive

.as

ynchronous

.me

ssages.

.an.exa

mple,

.an.EPL.net

work

.in.whi

.N.di

erent

.asy

nchronous

.req

uests

.occ

.at.the.sam

.tim

at.one.or.mor

.CNs.can.be.con

sidered.

. e.CNs.can.onl

.not

ify

.tha

.eve

.to.the.MN.in.the.fol

lowing

cyc

le,

.and.the.MN.sha

.sch

edule

.at.mos

.one.req

uest

.in.eac

.fol

lowing

.cyc

le,

.thu

.req

uiring

.a.tot

.of.

N.+.2.cycles.to.handle.all.the.requests..It.has.to.be.noticed.that.the.number.of.cycles.necessary.to.process.

the.pen

ding

.req

uests

.doe

.not.dep

end

.on.the.cyc

.dur

ation;

.thu

.dec

reasing

.the.cyc

.tim

.(if.pos

sible)

lea

.to.sh

orter

.re

sponse

.ti

mes.

.the.oth

.han

.the.lat

ency

.of.acy

clic

.mes

sages

.cou

.be.red

uced,

.all

owing

.mul

tiple

.ASn

.tra

ns-

missions

.in.the.sam

.cyc

le.

. is.is.act

ually

.add

ressed

.by.the.lat

est

.ver

sion

.of.the.EPL.spe

cication

.[6]

whi

.mak

.suc

.an.opp

ortunity

.pos

sible,

.pro

vided

.tha

.the

.is.eno

ugh

.rem

aining

.tim

.in.the.cyc

le.

Ano

ther

.pos

sible

.way.of.imp

roving

.the.res

ponse

.tim

.for.asy

nchronous

.tim

e-sensitive

.mes

sages

.wou

con

sist

.in.re

serving

.pa

.of.th

.as

ynchronous

.ph

ase

.ex

clusively

.fo

.al

arms,

.as.di

scussed

.in.[1

3].

references

. 1.. Bernecker. &. Rainer. Industrie-Elektronik. GmbH,. Eggelsberg,. Austria. [Online]:. http://www.br-

automation.com

. 2.. Eth

ernet

.POWERLINK.St

andardization

.Gr

oup.

.[Onlin

e]:

.ht

tp://www.ethernet-powerlink.org

. 3.. IXXA

.Aut

omation

.GmbH:.POWERLINK.inter

face

.boa

.for.PCI.bus.system

.PL-IB.200/PCI,.

Tec

hnical

.descr

iption.

.[Onlin

e]:

.ht

tp://www.ixxat.com/powerlink_pci_interface_en.html

. 4.. Ope

nPOWERLINK:

.Ope

.sou

rce

.sol

ution

.for.POW

ERLINK

.MN.and.CN..[On

line]:

.www

.sourceforge.

net/projects/openPOWERLINK