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

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31-6 Industrial Communication Systems

31.2.4 Network topology

CAN.network.topology.is.bus.based..Replicated.busses.are.not.referred.in.CAN.standard;.however,.it.is.

possi

ble

.to.imp

lement

.the

.[39

,41].

Over

.the.year

.some.star.topol

ogies

.for.CAN.have.been.prop

osed

.[5,2

6,38].

. e.solu

tion

.pres

ented

in.[26].is.base

.on.a.passi

.star.netw

ork

.topol

ogy

.and.reli

.on.the.use.of.a.cent

ral

.elem

ent,

.the.star.

coup

ler,

.to.conc

entrate

.all.the.inco

ming

.signa

ls.

. e.resu

.of.this.coup

ling

.is.then.broa

dcast

.to.the.

node

. e.solu

tion

.pres

ented

.by.Ruck

.[38].is.base

.on.an.acti

.star.coup

ler

.capa

ble

.of.rece

iving

.the.

inco

ming

.signa

.from.the.node

.bit.by.bit,.impl

ementing

.a.logi

cal

.AND,.and.retr

ansmitting

.the.resu

to.all.node

.None.of.thes

.solu

tions

.is.capa

ble

.of.disc

onnecting

.a.defe

cting

.bran

.and.so,.from.the.

depe

ndability

.poi

.of.vie

.the

.onl

.tole

rate

.spa

tial

.pro

ximity

.fau

lts.

Barranco

.et.al..[5].prop

osed

.a.solu

tion

.base

.in.an.acti

.hub.that.is.able.to.isol

ate

.defe

ctive

.node

.from.

the.netw

ork.

. is.acti

.star.is.comp

liant

.with.CAN.stan

dard

.and.allo

.dete

cting

.a.vari

ety

.of.faul

(stu

ck-at

.node.faul

.shor

ted

.medi

.faul

.medi

.part

ition

.faul

.and.bit-

ipping

.faul

.that.will.caus

the.faul

.node.to.be.isol

ated.

. e.repl

ication

.of.the.acti

.star.has.also.been.consi

dered

.[4]..Rece

ntly,

Silv

.et.al..[41].prop

osed

.a.set.of.comp

onents,

.an.arch

itecture

.and.proto

cols

.to.enab

.the.dyna

mic

.man-

gement

.of.the.topol

ogy

.of.CAN.netw

orks

.made.of.seve

ral

.repl

icated

.buse

.both.to.incr

ease

.the.total.

band

width

.and.to.reco

ngure

.the.netw

ork

.upon.bus.perm

anent

.erro

.In.many.oper

ational

.scen

arios,

the.prop

osed

.solu

tion

.coul

.be.plug

ged

.into.exis

ting

.syst

ems

.to.impr

ove

.its.resil

ience

.to.bus.perm

anent

erro

.wit

hout

.cha

nging

.the.cod

.run

ning

.in.the.nod

es.

31.3 CaN-Based Upper Layer Protocols

Several.CAN-based.higher.layer.protocols.have.emerged.over.the.years..Some.of.them.are.widely.used.

in.indu

stry,

.e.g.

.CANop

.[13].and.Devi

ceNet

.[3],.whil

.othe

.such.as.TCAN.[8],.Flex

CAN

.[35]

.and.

FTT-

CAN

.[2].have.emer

ged

.in.acad

emia

.to.over

come

.some.well

-known

.CAN.limi

tations,

.name

ly,

larg

.and.vari

able

.jitt

er,

.lack.of.cloc

.sync

hronization,

.exi

bility

.limi

tations,

.data.cons

istency

.issu

es,

limi

ted

.erro

.cont

ainment,

.and.limi

ted

.supp

ort

.for.faul

.tole

rance

.[34]

.In.2001

.an.exte

nsion

.of.CAN,.

the.time

-triggered

.CAN.(TTC

AN)

.[25].was.pres

ented

.to.supp

ort

.expl

icit

.time

-triggered

.oper

ation

on.top.of.CAN..Alth

ough

.TTCA

.is.not.stri

ctly

.an.uppe

.laye

.prot

ocol

.base

.on.CAN,.it.is.cons

id-

ered

.in.this.sect

ion

.sinc

.it.oer

.some.serv

ices

.that.can.be.foun

.in.some.CAN-

based

.uppe

.laye

prot

ocols.

Due

.to.spac

.limi

tations,

.this.sect

ion

.pres

ents

.only.an.over

view

.of.a.subs

.of.thes

.CAN-b

ased

proto

cols,

.nam

ely,

.CAN

open,

.Dev

iceNet,

.FTT

-CAN,

.and.TTC

AN.

31.3.1 CaNopen

CANopen,.internationally.standardized.as.CENLEC.EN.50325-4,.is.a.CAN-based.higher.layer.protocol.

and.its.spec

ications

.cove

.appl

ication

.laye

.and.comm

unication

.pro

le,

.a.fram

ework

.for.prog

ram-

mable

.devi

ces,

.reco

mmendations

.for.cabl

.and.conn

ectors,

.and.SI.unit

.and.pre

.repr

esentations.

In.term

.of.con

guration

.capa

bilities,

.CANo

pen

.is.quit

.exi

ble

.and.CANo

pen

.netw

orks

.are.used.in.a.

very.broa

.rang

.of.appl

ication

.eld

.such.as.mach

ine

.cont

rol,

.buil

ding

.autom

ation,

.medi

cal

.devi

ces,

mari

time

.elec

tronics,

.etc..[13]

.CANo

pen

.was.init

ially

.prop

osed

.in.an.EU-E

sprit

.rese

arch

.proje

.and,.

in.1995

.its.spec

ication

.was.hand

.over.to.the.CAN.in.autom

ation

.(CiA

.an.inte

rnational

.grou

.of.

user

.and.man

ufacturers.

. depi

cted

. in. Figu

. 31.2

. the. CANo

pen

. devi

. mode

. can. be. divi

ded

. into. thre

. part

. the.

comm

unication

.inte

rface

.and.prot

ocol

.sow

are,

.the.obje

.dict

ionary,

.and.the.proc

ess

.inte

rface

and.appl

ication

.prog

ram.

. e.comm

unication

.inte

rface

.and.prot

ocol

.sow

are

.prov

ide

.serv

ices

.to.

tran

smit

.and.to.rece

ive

.comm

unication

.obje

cts

.over.the.CAN.bus.. e.obje

.dict

ionary

.desc

ribes

all.data.type

.comm

unication

.obje

cts,

.and.appl

ication

.obje

cts

.used.in.the.devi

ce,

.acti

.as.the.

Controller Area Network 31-7

interface.to.the.application.soware.. e.application.program.provides.the.internal.control.func-

tionality

.as.well.as.the.interface.to.the.process.hardware.interfaces.

.device.prole.is.the.complete.description.of.the.device.application.with.respect.to.the.data.items.

in.the.object.dictionary.. e.object.directory.is.the.central.element.of.every.CANopen.device.describ-

ing

.the.device’s.functionality.. e.object.directory.contains.all.the.parameters.of.a.device.that.can.be.

accessed.via.the.network,.for.example,.device.identiers,.manufacturer.name,.communications.param-

eters,

.and.device.monitoring.. e.device-specic.area.contains.the.connection.to.the.process,.i.e.,.the.

I/O.functionality.and.drivers’.parameters.. e.behavior.in.the.event.of.an.error.can.also.be.congured.

in.the.object.directory..Accordingly,.the.behavior.of.a.device.can.be.adapted.to.the.respective.utilization.

requirements.using.the.object.directory.[13].

CANopen

.uses.standardized.proles,.and.o-the-shelf.devices,.tools,.and.protocol.stacks.are.com-

mercially

.available..In.an.eort.to.promote.reuse.of.application.soware.and.to.potentiate.communica-

tion

.compatibility,.interoperability,.and.interchangeability.of.devices,.CANopen.provides.predened.

application.objects..Also,.manufacturer-specic.functionality.in.devices.can.be.added.to.the.generic.

functionality.described.in.the.proles..CANopen.provides.standardized.communication.objects.for.

real-time.data.(process.data.objects,.PDO),.conguration.data.(service.data.objects,.SDO),.and.special.

functions.(time.stamp,.sync.message,.and.emergency.message).as.well.as.network.management.data.

(boot-up.message,.NMT.message,.and.error.control).

CANopen

.dierentiates.between.two.data.transfer.mechanisms:.fast.exchange.of.short.process.data,.

using.process.data.objects.(PDOs);.and.access.to.the.entries.of.the.object.directory,.that.is.done.via.

service.data.object.(SDO)..PDOs.can.be.asynchronous,.synchronous,.or.on-demand.and.the.transfer.is.

done.without.protocol.overhead.. e.synchronous.transmission.of.messages.is.supported.by.predened.

communication.objects.(sync.message.and.time.stamp.message)..Synchronous.messages.are.transmit-

ted

.with.respect.to.a.predened.synchronization.message,.asynchronous.messages.may.be.transmitted.

at.any.time..SDOs.are.conrmed.data.transfers.that.establish.point-to-point.communication.between.

two.devices,.implementing.services.of.handshaking,.fragmentation,.and.reassembly..SDOs.are.used.for.

parameter.passing,.conguration,.etc.

CANopen

supports

three

types

communication

models:

master/slave,

client/server,

and

producer/

consumer.

. CANopen. networks. also. provide. redundant. transfer. of. safety-oriented. information. in.

dened.time.windows.

App. program, device

Profile implementation

Data types, communication

and application objects

PDOs, SDOs

I/O

Application

Communication

Object-dictionary

CAN bus

FIGURE 31.2 CANopen.device.model.

31-8 Industrial Communication Systems

31.3.2 DeviceNet

DeviceNet,.international.standard.IEC.62026-3,.is.another.CAN-based.higher.layer.protocol,.initially.

pro

posed

.by.All

en-Bradley

.(no

.own

.by.Roc

kwell

.Auto

mation),

.and.is.mos

tly

.ded

icated

.to.ind

ustrial

auto

mation.

.Dev

iceNet

.spe

cications

.inc

lude

.the.app

lication

.lay

.and.dev

ice

.pro

les.

. e.man

agement

of.the.Dev

iceNet

.spe

cication

.was.tra

nsferred

.to.the.Ope

.Dev

iceNet

.Ven

dor

.Ass

ociation

.(OD

VA),

a.no

nprot

.or

ganization

.th

.de

velops

.an

.ma

rkets

.De

viceNet

.[3

.Dev

iceNet

.phy

sical

.lay

.spe

cies

.a.ter

minated

.tru

.and.dro

.lin

.con

guration.

.Com

munication

and.po

wer

.ar

.pr

ovided

.in.th

.tr

unk

.an

.dr

.ca

bles

.by.se

parate

.tw

isted

.pa

.bu

ses.

.to.64.log

ical

.nod

.can.be.con

nected

.to.a.sing

.Dev

iceNet

.net

work,

.usin

.bot

.sea

led

.and.ope

style

.con

nections.

. er

.is.sup

port

.for.str

obed,

.pol

led,

.cyc

lic,

.cha

nge-of-state,

.and.app

lication-triggered

dat

.tra

nsfer.

. e.use

.can.cho

ose

.mas

ter/slave,

.mul

ti-master,

.and.pee

r-to-peer,

.or.a.comb

ination

.con

guration,

.dep

ending

.on.dev

ice

.cap

ability

.and.the.app

lication

.req

uirements.

.Dev

iceNet

.is.bas

.on.the.

Par

.A.of.the.CAN.sta

ndard;

.the

refore,

.it.use

.the.11.bit.ide

ntier.

.Dev

iceNet

.use

.abs

tract

.obje

.mod

els

to.des

cribe

.com

munication

.ser

vices,

.dat

.and.beh

avior.

.A.Dev

iceNet

.nod

.is.bui

.fro

.a.col

lection

.of.

obje

cts

.des

cribing

.the.sys

tem

.beh

avior

.and.gro

uping

.dat

.usin

.vir

tual

.obje

cts,

.as.any.obje

.ori

ented

pro

gramming

.la

nguage

.wo

uld

.[6

DeviceNet

.com

munication

.mod

.de

nes

.an.add

ressing

.sch

eme

.tha

.pro

vides

.acc

ess

.to.obje

cts

.wit

hin

a.de

vice.

. e.ob

ject

.mo

del

.ad

dressing

.in

formation

.in

cludes

•

. Dev

ice address—Referred

.to.as.the.med

.acc

ess

.con

trol

.ide

ntier

.(MA

.ID).is.an.int

eger

.ide

nti-

cation

.val

.assi

gned

.to.eac

.nod

.on.the.net

work.

.A.tes

.exe

cuted

.at.pow

er-up,

.gua

rantees

.the.

uni

queness

.of.th

.va

lue

.on.th

.ne

twork.

•

. Cla

ss identier—Refers

.to.a.set.of.obje

cts

.tha

.rep

resent

.the.sam

.typ

.of.sys

tem

.com

ponent.

 e.cla

.ide

ntier

.is.an.int

eger

.ide

ntication

.val

.assi

gned

.to.eac

.obje

.acc

essible

.fro

.the.

net

work.

•

. Ins

tance identier—Refers

.to.the.act

ual

.rep

resentation

.of.an.obje

.of.a.cla

ss.

. e.ins

tance

.ide

nti-

er

.is.an.int

eger

.ide

ntication

.assi

gned

.to.an.obje

.ins

tance

.tha

.ide

nties

.it.amo

.all.ins

tances

of.th

.sa

.cl

ass

.wi

thin

.a.pa

rticular

.de

vice,

.wi

.a.un

ique

.id

entier

.va

lue.

•

. Att

ribute identier—Attributes

.are. par

ameters

.ass

ociated

.wit

.an. obje

ct.

.Att

ributes

.typ

ically

pro

vide

.som

.typ

.of.sta

tus

.inf

ormation,

.rep

resent

.a.cha

racteristic

.wit

.a.con

gurable

.val

ue,

.or.

con

trol

.the.beh

avior

.of.an.obje

ct.

. e.att

ribute

.ide

ntier

.is.an.int

eger

.ide

ntication

.val

.assi

gned

to.an.ob

ject

.at

tribute.

.Dev

iceNet

.app

lication

.lay

.spe

cies

.how.CAN.ide

ntiers

.are.assi

gned

.and.how.the.dat

.el

is.use

.to.spe

cify

.ser

vices,

.mov

.dat

.and.det

ermine

.its.mea

ning.

.Dev

iceNet

.ado

pts

.the.pro

ducer–

consumer

.mod

el,

.so.the.sou

rce

.dev

ice

.bro

adcast

.the.dat

.to.the.net

work

.wit

.the.pro

per

.ide

ntier.

.All.

dev

ices

.who.nee

.dat

.lis

ten

.for.mes

sages

.and.whe

.dev

ices

.rec

ognize

.the.app

ropriate

.ide

ntier,

.the

con

sume

.th

.da

ta.

 ere

.are.two.typ

.of.mes

sages

.usu

ally

.req

uired

.by.most.aut

omation

.dev

ices:

.I/O.mes

sages

.and.

exp

licit

.mes

sages.

.I/O.mes

sages

.are.for.tim

.cri

tical

.con

trol-oriented

.dat

.and.pro

vide

.ded

icated

.com

mu-

nication

.pat

.bet

ween

.a.pro

ducing

.app

lication

.and.one.or.mor

.con

suming

.app

lications

.and.typ

ically

use.the.high

.pri

ority

.ide

ntiers

.[6]

.Exp

licit

.mes

sages,

.on.the.oth

.han

.pro

vide

.mul

tipurpose

.poi

nt-

to-point

.com

munication

.pat

.bet

ween

.two.dev

ices.

. ey.pro

vide

.the.typ

ical

.req

uest/response-oriented

net

work

.com

munications

.use

.to.per

form

.nod

.con

guration

.and.dia

gnosis

.and.typ

ically

.use.low

pri

ority

.ide

ntiers.

.Fra

gmentation

.ser

vices

.are.als

.pro

vided

.for.mes

sages

.tha

.are.lon

ger

.tha

.8.byt

es.

31.3.3 ttCaN

Time-triggered.communication.on.CAN.(TTCAN).is.an.extension.of.CAN,.introducing.time-triggered.

ope

ration

. bas

. on. a. hig

h-precision

. net

work-wide

. tim

. bas

. The

. are. two. pos

sible

. lev

els

. in.

Controller Area Network 31-9

TTCAN:. level-1. and. level-2.. Level-1. only. provides. time-triggered. operation. using. local. time.

(Cycle_Time)..Level-2.requires.a.hardware.implementation.and.provides.increased.synchroniza-

tion

.quality,.global.time,.and.external.clock.synchronization..As.native.CAN,.TTCAN.is.limited.

to.a.maximum.data.rate.of.1.Mbps,.with.typical.data.efficiency.below.50%..TTCAN.network.topol-

ogy

.is.bus.based..Replicated.busses.are.not.referred.in.TTCAN.standard;.however,.it.is.possible.to.

implement.them.[29].

TTCAN

.adopts.a.Time-Division.Multiple.Access.(TDMA).bus.access.scheme.. e.TDMA.bandwidth.

allocation.scheme.divides.the.timeline.into.time.slots.or.time.windows..Network.nodes.are.assigned.dif-

ferent

.slots.to.access.the.bus.. e.sequence.of.time.slots.allocated.to.nodes.repeats.according.to.a.basic.

cycle..Several.basic.cycles.are.grouped.together.in.a.matrix.cycle..All.basic.cycles.have.the.same.length.

but.can.dier.in.their.structure..When.a.matrix.cycle.nishes,.the.transmission.scheme.starts.over.by.

repeating.the.matrix.cycle.(Figure.31.3).. e.matrix.cycle.denes.a.message.transmission.schedule..

However,.a.TTCAN.node.does.not.need.to.know.the.whole.system.matrix,.it.only.needs.information.of.

the.messages.it.will.send.and.receive.

Since

.TTCAN.is.built.on.top.of.native.CAN,.some.time.windows.may.be.reserved.for.several.event.

messages..In.such.windows,.it.is.possible.that.more.than.one.transmitter.may.compete.for.the.bus.

access.right..During.these.slots,.the.arbitration.mechanism.of.the.CAN.protocol.is.used.to.prioritize.

the.competing.messages.and.to.grant.access.to.the.higher.priority.one..In.this.sense,.the.medium.access.

mechanism.in.TTCAN.can.be.described.as.TDMA.with.Carrier.Sense.Multiple.Access.with.Bitwise.

Arbitration.(CSMA-BA).in.some.predened.time.slots.. is.feature.makes.the.TTCAN.protocol.as.ex-

ile

.as.CAN.during.the.arbitration.windows,.without.compromising.the.overall.system.timeliness,.i.e.,.

the.event-triggered.messages.do.not.interfere.with.the.time-triggered.ones.

.TDMA.cycle.starts.with.the.transmission.of.a.reference.message.from.a.time.master.. e.refer-

ence

.messages.are.regular.CAN.messages.with.a.special.and.known.a.priori.identier.and.are.used.to.

synchronize.and.calibrate.the.time.bases.of.all.nodes.according.to.the.time.master’s.time.base,.providing.

a.global.time.for.the.network.

TTCAN

.level-1.guarantees.the.time-triggered.operation.of.CAN.based.on.the.reference.message.of.a.

time.master..Fault-tolerance.of.that.functionality.is.established.by.redundant.time.masters,.the.so.called.

potential.time.masters..Level-2.establishes.a.globally.synchronized.time.base.and.a.continuous.dri.

correction.among.the.CAN.controllers.

Transmission

columns

Reference

message

Reference

message

Reference

message

Basic

cycle 0

Basic

cycle 2

Basic

cycle 1

Basic

cycle 3

Reference

message

Message

Arbitration

window

Arbitration

window

Arbitration

window

Free

window

Free

window

FIGURE 31.3 TTCAN.system.matrix,.where.several.basic.cycles.build.the.matrix.cycle..(Adapted.from.Führer,T..

et. al., Time. triggered. communication. on. CAN,. In. Proceedings of the Seventh International CAN Conference,.

Amsterdam,.the.Netherlands,.October.2000,.CAN.in.Automation.GmbH.)

31-10 Industrial Communication Systems

 ere.are.three.types.of.time.windows.(Figure.31.3):

•

. Exc

lusive time windows—for

.per

iodic

.mes

sages

.tha

.are.tra

nsmitted

.wit

hout

.com

petition

.for.the.

CAN.bus

.No.oth

.mes

sage

.can.be.sch

eduled

.in.the.sam

.win

dow.

. e.auto

matic

.ret

ransmission,

upo

.er

ror,

.is.no

.al

lowed.

•

. Arb

itrating time windows—for

.eve

nt-triggered

.mes

sages,

.whe

.sev

eral

.eve

nt-triggered

.mes

sages

may.sha

.the.sam

.tim

.win

dow

.and.bus.con

icts

.are.res

olved

.by.the.nat

ive

.CAN.arb

itration.

Two.or.mor

.con

secutive

.arb

itrating

.tim

.win

dows

.can.be.mer

ged.

.Mes

sage

.tra

nsmission

.can.

onl

.be.sta

rted

.if.the

.is.su

cient

.tim

.rem

aining

.for.the.mes

sage

.to.t.in..Auto

matic

.ret

rans-

mission

.of.CA

.me

ssages

.is.di

sabled

.(e

xcept

.fo

.me

rged

.ar

bitrating

.wi

ndows).

•

. Free time windows—reserved.for.future.extensions.of.the.network..A.transmission.schedule.could.

res

erve

.tim

.win

dows

.for.fut

ure

.use

.eit

her

.for.new.nod

.or.to.assi

.exi

sting

.nod

.mor

.ban

width.

.Not

ice

.tha

.a.fre

.tim

.win

dow

.can

not

.be.assi

gned

.to.a.mes

sage

.unl

ess

.it.is.pre

viously

con

verted

.to.ei

ther

.an.ex

clusive

.or.an.ar

bitrating

.ti

.wi

ndow.

Clock

.syn

chronization

.in.a.TTC

.net

work

.is.pro

vided

.by.a.tim

.mas

ter.

. e.tim

.mas

ter

.est

ab-

lishes

.its.own.loc

.tim

.as.glo

bal

.tim

.by.tra

nsmitting

.the.ref

erence

.mes

sage.

.To.com

pensate

.for.sli

ghtly

di

erent

.clo

.dri

s

.in.the.TTC

.nod

.and.to.pro

vide

.a.con

sistent

.vie

.of.the.glo

bal

.tim

.the.nod

per

form

.a.dri



.com

pensation

.ope

ration.

.A.uni

que

.tim

.mas

ter

.wou

.be.a.sing

.poi

.of.fai

lure,

.thu

TTC

.pro

vides

.a.mec

hanism

.for.tim

.mas

ters’

.red

undancy

.and.rep

lacement

.whe

never

.the.cur

rent

time.master.fails.to.send.a.reference.message..In.this.case,.the.CAN.bus.remains.idle.and.any.of.the.

pot

ential

.tim

.mas

ters

.wil

.try.to.tra

nsmit

.a.ref

erence

.mes

sage

.ae

.a.cer

tain

.amo

unt

.of.tim

.In.cas

two.pot

ential

.tim

.mas

ters

.try.to.sen

.a.ref

erence

.mes

sage

.at.the.sam

.tim

.the.nat

ive

.CAN.bit.arb

itra-

tion

.mec

hanism

.ens

ures

.tha

.onl

.the.one.wit

.the.hig

hest

.pri

ority

.win

.Whe

.a.fai

led

.tim

.mas

ter

rec

onnects

.to.the.sys

tem

.wit

.act

ive

.tim

e-triggered

.com

munication,

.it.wai

.unt

.it.is.syn

chronized

.to.

the.ne

twork

.be

fore

.it.ma

.tr

.to.be

come

.ti

.ma

ster

.ag

ain.

.str

ategy

.ado

pted,

.con

cerning

.fau

.con

nement,

.is.ver

.simi

lar

.to.the.one.fol

lowed

.by.nat

ive

CAN

.i.e

.err

.pas

sive

.and.bus

-o.

.Sin

.CAN.fai

lures

.are.alr

eady

.con

sidered

.in.ISO.118

98-1

.sta

dard

.(da

.lin

.lay

er),

.TTC

.con

siders

.mai

nly

.sch

eduling

.err

ors

.(e.

g.,

.abs

ence

.of.a.mes

sage)

.and.eac

TTC

.con

troller

.onl

.pro

vides

.err

.det

ection.

.Act

ive

.fau

.con

nement

.is.le.to.a.hig

her

.lay

.or.to.

the.ap

plication.

TTCAN

.may.wor

.und

.sev

eral

.ope

rational

.mod

es:

.con

guration

.mod

.CAN.com

munication,

.tim

triggered

.com

munication

.or.eve

.syn

chronized

.tim

e-triggered

.com

munication.

.How

ever,

.ope

rating

mod

.may.onl

.cha

nge

.via.con

guration

.mod

. e.sys

tem

.mat

rix

.con

guration

.may.be.rea

.and.wri

ten

.by.the.app

lication

.dur

ing

.ini

tialization,

.but.it.is.loc

ked

.dur

ing

.tim

e-triggered

.com

munication,

.i.e

sch

eduling

.tab

les

.are.loc

ally

.imp

lemented

.in.eve

.nod

.and.mus

.be.con

gured

.bef

ore

.sys

tem

.sta

rt-up.

31.3.4 Ftt-CaN

 e.basis.for.the.exible.time-triggered.communication.on.CAN.(FTT-CAN).protocol.has.been.rst.pre-

sented

.in.[1]

.Bas

ically,

.the.pro

tocol

.make

.use.of.the.dua

l-phase

.ele

mentary

.cyc

.(EC

)

.con

cept

.in.ord

.to.

com

bine

.tim

.and.eve

nt-triggered

.com

munication

.wit

.tem

poral

.iso

lation.

.Mor

eover,

.the.tim

e-triggered

tra

c

.is.sch

eduled

.onl

ine

.and.cen

trally

.in.a.par

ticular

.nod

.cal

led

.mast

er.

. is.fea

ture

.fac

ilitates

.the.

onl

ine

.adm

ission

.con

trol

.of.dyn

amic

.req

uests

.for.per

iodic

.com

munication

.bec

ause

.the.resp

ective

.req

uire-

ments

.are.hel

.cen

trally

.in.jus

.one.loc

.dat

abase.

.Wit

.onl

ine

.adm

ission

.con

trol,

.the.pro

tocol

.sup

ports

the.tim

e-triggered

.tra

c

.in.a.ex

ible

.way

.und

.gua

ranteed

.tim

eliness.

.Fur

thermore,

.the

.is.yet.ano

ther

fea

ture

.that.cle

arly

.dis

tinguishes

.thi

.pro

tocol

.fro

.oth

.pro

posals

.con

cerning

.tim

e-triggered

.com

mu-

nication

.on.CAN.[25

,33],

.that.is,.the.exp

loitation

.of.its.nat

ive

.dis

tributed

.arb

itration

.mec

hanism.

.In.fac

the.pro

tocol

.rel

ies

.on.a.rel

axed

.mast

er–slave

.med

ium

.acc

ess

.con

trol

.in.whi

.the.sam

.mast

.mes

sage

tri

ggers

.the.tra

nsmission

.of.mes

sages

.in.sev

eral

.sla

ves

.sim

ultaneously

.(mas

ter/multislave).

. e.eve

ntual

col

lisions

.bet

ween

.sla

ve’s

.mes

sages

.are.hand

led

.by.the.nat

ive

.dis

tributed

.arb

itration

.of.CAN

Controller Area Network 31-11

 e.protocol.also.takes.advantage.of.the.native.arbitration.to.handle.event-triggered.trac.in.the.

same.way.as.the.original.CAN.protocol.does..Particularly,.there.is.no.need.for.the.master.to.poll.the.

slaves.for.pending.event-triggered.requests..Slaves.with.pending.requests.may.try.to.transmit.immedi-

ately,

.as.in.normal.CAN,.but.just.within.the.respective.phase.of.each.EC.. is.scheme,.similar.to.the.

arbitration.windows.in.TTCAN,.allows.a.very.ecient.combination.of.time.and.event-triggered.trac,.

particularly.resulting.in.low.communication.overhead.and.shorter.response.times.

.FTT-CAN.the.bus.time.is.slotted.in.consecutive.ECs.with.xed.duration..All.nodes.are.synchro-

nized

.at.the.start.of.each.EC.by.the.reception.of.a.particular.message.known.as.EC.trigger.message,.

which.is.sent.by.the.master.node.

Within

.each.EC.the.protocol.denes.two.consecutive.windows,.asynchronous.and.synchronous,.that.

correspond.to.two.separate.phases.(see.Figure.31.4).. e.former.one.is.used.to.convey.event-triggered.

trac,.herein.called.asynchronous.because.the.respective.transmission.requests.can.be.issued.at.any.

instant.. e.latter.one.is.used.to.convey.time-triggered.trac,.herein.called.synchronous.because.its.

transmission.occurs.synchronously.with.the.ECs.. e.synchronous.window.of.the.nth.EC.has.a.dura-

tion

.that.is.set.according.to.the.trac.that.is.scheduled.for.it.. e.schedule.for.each.EC.is.conveyed.by.

the.respective.EC.trigger.message.(see.Figure.31.5)..Since.this.window.is.placed.at.the.end.of.the.EC,.

its.starting.instant.is.variable.and.it.is.also.encoded.in.the.respective.EC.trigger.message.. e.protocol.

allows.establishing.a.maximum.duration.for.the.synchronous.windows.and.correspondingly.a.maxi-

mum

.bandwidth.for.that.type.of.trac..Consequently,.a.minimum.bandwidth.can.be.guaranteed.for.

the.asynchronous.trac.

EC trigger

message

nth EC

TM TM

AMa AMb

AMc α SMa SMb SMc SMd

LTM

LEC

law(n) lsw(n)

Asynchronous

window

Synchronous

window

FIGURE 31.4 e.elementary.cycle.(EC).in.FTT-CAN.

Elementary cycle (EC)

SM1

0 0 0

bit 13 bit 4 bit 2 bit 1

0 0 0 0 0 0 0 0 0 0 01 1 11

…

Trigger message data field (EC schedule)

SM2 SM4 SM13 TM

Synchronous messages

EC trigger

message

FIGURE 31.5 Master/multislave.access.control..Slaves.produce.synchronous.messages.according.to.an.EC.schedule.

conveyed.by.the.trigger.message.

31-12 Industrial Communication Systems

 e.communication.requirements.are.held. in.a. database.located.in.the. master.node,.the.system.

requ

irements

.data

base

.(SRD

B).

. e.SRDB.hold

.the.prop

erties

.of.each.of.the.mess

age

.stre

ams

.to.be.

conv

eyed

.by.the.syst

em,

.both.real

-time

.and.non-

real-time,

.as.well.as.a.set.of.oper

ational

.para

meters

rela

ted

.to.syst

.con

guration

.and.stat

us.

. is.info

rmation

.is.store

.in.a.set.of.thre

.tabl

es:

.the.syn-

hronous

.requ

irements

.tabl

.(SRT

.the.asyn

chronous

.requ

irements

.tabl

.(ART

.and.the.con

guration

and.sta

tus

.rec

ord

.(SC

SR).

Based

.on.the.SRT,.an.onli

.sche

duler

.buil

.the.sync

hronous

.sche

dules

.for.each.EC.. ese.sche

dules

are.then.inse

rted

.in.the.data.area.of.the.resp

ective

.EC.trig

ger

.mess

age

.(see.Figu

.31.5

)

.and.broa

dcast

with.it..Due.to.the.onli

.natu

.of.the.sche

duling

.func

tion,

.chan

ges

.perf

ormed

.in.the.SRT.at.run-

time

will.be.re

ected

.in.the.bus.tra

c

.wit

hin

.a.bou

nded

.del

ay,

.res

ulting

.in.a.ex

ible

.beh

avior.

31.4 CaN Limitations

Judging.by.the.large.installed.base. of.CAN.controllers. in.signicative.application.domains,.system.

desig

ners

.are.sati

sed

.with.CAN.feat

ures;

.howe

ver,

.they.woul

.be.much.happ

ier

.if.CAN.coul

.be.made.

fast

er,

.cove

.long

.dist

ances,

.be.more.dete

rministic

.and.more.depe

ndable

.[34]

.In.fact

.beca

use

.of.its.

limi

ted

.depe

ndability

.feat

ures,

.ther

.is.an.ongo

ing

.deba

.on.whet

her

.the.CAN.proto

col,

.with.prop

enha

ncements,

.can.supp

ort

.safe

ty-critical

.appl

ications

.[21,

27].

.CAN.limi

tations

.and.rece

.deve

lop-

ment

.and.rese

arch

.carr

ied

.out.to.over

come

.them.are.disc

ussed

.in.deta

.in.[34]

. ese.incl

ude

.larg

.and.

vari

able

.jitt

er,

.lack.of.cloc

.sync

hronization,

.exi

bility

.limi

tations,

.data.consi

stency

.issu

es,

.limi

ted

.erro

cont

ainment,

.and.limi

ted

.supp

ort

.for.faul

.toler

ance.

.Over.the.year

.seve

ral

.prop

osals

.cont

ributed

.to.

over

come

.som

.of.CAN.lim

itations

.[4,

8,17,18,37,40].

.spec

ially

.impo

rtant

.limi

tation

.is.rela

ted

.with.inco

nsistent

.comm

unication

.scen

arios

.[36,

40],

.that.

is.one.of.the.stro

ngest

.impa

irments

.to.achi

eve

.high.depe

ndability

.over.CAN.. ese.scen

arios

.are.due.to.

the.proto

col

.spec

ication

.and.may.reve

.them

selves

.both.as.inco

nsistent

.mess

age

.omis

sions

.(IMO

.i.e.,.

some.node

.rece

ive

.a.give

.mess

age

.whil

.othe

.do.not,.and.as.inco

nsistent

.mess

age

.dupl

icates

.(IMD

i.e.,.some.node

.rece

ive

.the.same.mess

age

.seve

ral

.time

.whil

.othe

.rece

ive

.only.once

.Inco

nsistent

comm

unication

.scen

arios

.make.dist

ributed

.cons

ensus

.in.its.die

rent

.form

.e.g.,.membe

rship,

.cloc

sync

hronization,

.consi

stent

.comm

itment

.of.con

guration

.chan

ges,

.or.simpl

.the.consi

stent

.perc

eption

of.asy

nchronous

.eve

nts,

.mor

.di

cult

.to.att

ain.

.prin

ciple,

.and.acco

rding

.to.the.erro

.dete

ction

.and.sign

aling

.capa

bilities

.of.CAN,.any.fram

whic

.suff

ers

.an.erro

.woul

.be.cons

istently

.reje

cted

.by.all.the.node

.of.the.netw

ork.

.Howe

ver,

some.failure.scenarios.have.been.identified.[40].that.can.lead.to.undesirable.symptoms.such.as.

inco

nsistent

. omis

sion

. fail

ures

. and. dupl

icate

. mess

age

. rece

ption.

. The. prob

ability

. of. thos

. erro

scen

arios

.depe

nds

.on.an.impo

rtant

.fact

or,

.the.CAN.bit.erro

.rate.(BER

.Rufi

.et.al.,.pres

ented

[40].an.anal

ysis

.base

.on.the.assu

mption

.that.the.BER.vari

.from.10

−4

,.in.case.of.an.aggressive.

envi

ronment,

.to.10

−6

.in.the.case.of.a.benign.environment..The.results.obtained,.based.on.these.

assu

mptions,

.for.IMO.per.hour.(IMO/

.and.inco

nsistent

.mess

age

.dupl

ications

.per.hour.(IMD/

are.rath

.high.and.woul

.beco

.a.seri

ous

.impa

irment

.for.usin

.CAN.(or.CAN-

based

.prot

ocols)

in.safe

ty-critical

.appl

ications.

Over

.the.year

.seve

ral

.stud

ies

.have.been.cond

ucted

.[10,

30]

.to.asse

.the.wors

t-case

.resp

onse

.time.of.

CAN.mess

ages

.unde

.chan

nel

.erro

rs.

. ese.stud

ies

.used.gene

ric

.erro

.mode

.that.take.into.acco

unt

.the.

natu

.of.the.erro

rs,

.eith

.singl

.bit.erro

.or.burs

.of.erro

rs,

.and.thei

.mini

mum

.inte

r-arrival

.time

Howe

ver,

.no.err

.sta

tistics

.wer

.pro

vided

.to.sup

port

.the.err

.mod

els.

.CAN,.as.well.as.in.CANo

pen

.and.Devi

ceNet,

.the.autom

atic

.mess

age

.retr

ansmission,

.upon.tran

mission

.erro

.is.not.disa

bled.

.Conv

ersely,

.in.TTCA

.the.autom

atic

.mess

age

.retr

ansmission

.of.CAN.is.

disa

bled,

.whil

.in.FTT-

CAN

.it.is.rest

ricted.

.In.TTCA

.and.FTT-

CAN,

.an.erro

.in.a.give

.mess

age

.does.

not.aec

.the.resp

onse

.time.of.othe

rs,

.and.the.erro

.dete

ction

.and.signa

ling

.of.CAN.woul

.norm

ally

ensu

.that.the.erro

.woul

.be.consi

stently

.dete

cted

.by.all.netw

ork

.node

.exce

.in.the.case

.wher

inconsistent.message.delivery.may.occur.

Controller Area Network 31-13

Inconsistent.message.reception.scenarios.are.much.more.frequent.in.the.case.of.TTCAN.than.in.

nati

.CAN.and.the

.fre

quency

.dep

ends

.on.the.BER.of.the.CAN.bus

. e.ass

umptions

.con

cerning

.the.

BER.made.by.Run

.et.al.,.alth

ough

.real

istic

.in.othe

.netw

orks,

.seem

.some

what

.pessi

mistic

.consi

der-

ing

.the.spec

ic

.case.of.CAN.and.spec

ially

.the.char

acteristics

.of.the.CAN.physi

cal

.laye

. is.led.to.the.

desig

.of.an.expe

rimental

.setu

.to.meas

ure

.real.valu

.of.CAN.BER.[20]

.Expe

rimental

.resu

lts

.from.a.

set.of.expe

riments

.show

.that.BER.was.2.6.×.10

−7

.in.an.aggressive.environment.(CAN.network.placed.

near.a.high

-frequency

.arc-

welding

.mach

ine),

.3.1.×.10

−9

.in.a.normal.environment.(CAN.network.at.an.

indu

strial

.prod

uction

.line

.and.3.0.×.10

−11

.at.a.benign.environment.(CAN.network.at.the.university.

labo

ratory).

.Ple

ase

.ref

.to.[17

]

.for.a.det

ailed

.ana

lysis

.and.dis

cussion

.of.the

.iss

ues.

Table

. 31.2. pres

ents

. the. prob

ability

. of. inco

nsistencies

. both. for. CAN. (inc

luding

. CANo

pen

. and.

Devi

ceNet),

.usin

.the.sam

.ass

umptions

.as.in.[40

Observing

.Tabl

.31.2

.one.may.conc

lude

.that.in.nati

.CAN.the.occu

rrence

.of.IMO.has.a.lowe

.prob

ability

.than.prev

iously

.assu

med,

.at.leas

.in.the.envi

ronments

.consi

dered

.in.the.expe

riments.

.In.fact

.it.

is.belo

.the.10

−9

.threshold.usually.accepted.for.safety-critical.applications..However,.the.probability.of.

IMD.(mes

sages

.are.even

tually

.deli

vered

.but.they.coul

.be.out.of.orde

.is.stil

.high.enou

.to.be.take

into.acc

ount.

Concerning

.both. TTCA

. and. FTT-

CAN,

. one. cann

.negl

ect

.IMO. beca

use

. they. are. rath

.fre-

uent.

.Howe

ver,

.it.shou

.be.stre

ssed

.that.this.does.not.mean.that.nati

.CAN.is.more.depe

ndable

.than.

TTCA

.or.FTT-

CAN

.since.nati

.CAN.does.not.boun

.autom

atic

.retr

ansmissions

.that.may.orig

dead

line

.los

ses.

. at.is,.the.pro

bability

.of.IMO.is.low

.in.nat

ive

.CAN

.but.the.pro

bability

.of.a.mes

sage

miss.its.dea

dline

.is.hig

her.

31.4.1 Consequences of Faults at the Node Level

CAN.protocol.does.not.restrict.the.failure semantics.of.the.nodes,.so.that.they.may.fail.in.arbitrary.ways..

Some.of.thes

.fail

ures

.are.autom

atically

.hand

led

.by.CAN’

.nati

.impl

ementation

.of.erro

.dete

ction

capa

bilities

.and.autom

atic

.fail

-silence

.enfo

rcement,

.desc

ribed

.prev

iously,

.lead

ing

.the.erro

neous

.node.

to.a.stat

.the.bus-

o

.stat

.wher

.it.is.unab

.to.inte

rfere

.with.othe

.node

.Howe

ver,

.in.some.spec

ic

situa

tions,

.the

.mec

hanisms

.do.not.ful

.con

tain

.the.err

ors

.wit

hin

.the.nod

es.

Specically,

.a.CAN.node.only.reac

hes

.the.bus-

o

.stat

.(fai

.silen

ce)

.aer.a.rela

tively

.long.peri

.of.

time.(whe

.the.TEC.reac

hes

.255)

.More

over,

.a.CAN.node.runn

ing

.an.erro

neous

.appl

ication

.can.also.

comp

romise

.most.of.the.legi

timate

.tra

.sche

duled

.acco

rding

.to.a.high

.laye

.proto

col

.impl

emented

in.sow

are

.in.a.stan

dard

.CAN.cont

roller,

.simp

.by.acce

ssing

.the.netw

ork

.at.arbi

trary

.poin

.in.time.

(babb

ling idiot

.fail

ure

.mode

.Noti

.that.a.faul

.appl

ication

.runn

ing

.in.a.node.with.a.CAN.cont

roller

may.tran

smit

.high

-priority

.mess

ages

.at.any.time.with

out

.causi

.any.netw

ork

.erro

rs,

.and.cons

equently

the.CAN.cont

roller

.will.neve

.reac

.the.bus-

o

.stat

.An.unco

ntrolled

.appl

ication

.tran

smitting

.at.arbi

trary

.points.in.time.via.a.non-faulty.CAN.controller.is.a.much.severe.situation.than.a.faulty.CAN.control-

ler

.also.tran

smitting

.at.arbi

trary

.poin

.in.time.beca

use,

.in.the.rst.case

.a.non-

faulty

.CAN.cont

roller

has.no.mean

.to.dete

.an.erro

neous

.appl

ication

.tran

smitting

.legi

timate

.tra

.In.the.seco

.case

.the.CAN.

cont

roller

.wou

.ent

.bus

-o

.sta

.ae

.a.whi

.and.the.err

.wou

.be.eve

ntually

.con

ned.

TABLE 31.2 Estimated.Rates.of.IMO.per.Hour.in.CAN,.TTCAN,.and.FTT-CAN

Bit.Error.Rate

CAN

TTCAN

FTT-CAN

Δt.=.5.ms ρ.=.0.5

IMD/h IMO/h IMO/h IMD/h IMO/h

2.6

.×.10

−7

7.59 1.05.×.10

−9

7.59 3.79 3.79

3.1

.×.10

−9

8.93.×.10

−2

1.24.×.10

−11

8.93.×.10

−2

4.46.×.10

−2

4.46.×.10

−2

3.0.×.10

−11

8.75.×.10

−4

1.22.×.10

−13

8.75.×.10

−4

4.37.×.10

−4

4.37.×.10

−4

31-14 Industrial Communication Systems

To. overcome.the.babbling idiot. failure.mode,.special. components.located.between. the.node. and.

the.bus

.the.bus.gua

rdians,

.hav

.bee

.pro

posed

.for.CAN.[9,

19].

.Bus.gua

rdians

.are.usu

ally

.ado

pted

.to.

enf

orce

.fai

l-silence

.beh

avior

.in.nod

.of.a.dis

tributed

.sys

tem

.bec

ause

.the

.are.simp

ler

.tha

.usin

.nod

rep

lication

.and.som

.sor

.of.vot

ing

.mec

hanism.

.Apa

.fro

.the.cos

.iss

ue,

.simp

licity

.is.als

.imp

ortant

bec

ause

.a.sim

pler

.co

mponent

.is.o

.mo

.de

pendable

.th

.a.co

mplex

.on

List of acronyms

CAN. Controller.area.network

TTCAN

. Tim

e-triggered

.CA

FTT-CAN

. Fle

xible-time-triggered

.CA

BER

. Bit.er

ror

.ra

CSMA

. Car

rier

.se

nse

.mu

ltiple

.ac

cess

OSI

. Ope

.sy

stems

.in

terconnection

IMD

. Inc

onsistent

.me

ssage

.du

plicates

IMO

. Inc

onsistent

.me

ssage

.om

issions

EMI

. Ele

ctromagnetic

.in

terference

SOF

. Star

.of.fr

ame

RTR

. Rem

ote

.tr

ansmission

.re

quest

IDE

. Ide

ntier

.ex

tension

DLC

. Dat

.le

ngth

.co

CRC

. Cyc

lic

.re

dundant

.ch

eck

TEC

. Tra

nsmit

.er

ror

.co

unter

REC

. Rec

eive

.er

ror

.co

unter

. Ele

mentary

.cy

cle

references

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