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

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34-6 Industrial Communication Systems

Moreover,.some.spare.time.is.available.in.each.microcycle.for.the.transmission.of.asynchronous.

tra

. is.will.allo

.the.BA.to.consi

der

.addi

tional

.requ

ests

.for.asyn

chronous

.tra

.and.allo

cate

.them.

into.a.suit

able

.time.slot

.If.no.requ

est

.is.rece

ived,

.the.BA.will.tran

smit

.padd

ing

.iden

tier

.(i.e

.iden

tiers

that.do.not.refe

.to.any.stat

ion)

.unti

.the.end.of.the.elem

entary

.cycl

.in.orde

.to.indi

cate

.to.the.othe

stat

ions

.tha

.it.is.sti

.act

ive.

Looking

.at.Figu

.34.5

.we.can.obse

rve

.that.the.sche

dule

.is.repe

ated

.aer.a.numb

.of.micr

ocycles

(12.in.the.exam

ple

.cons

idered).

. is.inte

rval

.is.call

.macr

ocycle.

34.3.3 transmission of asynchronous traffic

Not.all.the.information.required.by.a.distributed.application.can.be.distributed.by.the.BA.through.the.

cycl

.scan.tabl

.In.fact

.asyn

chronous

.tra

.can.be.prod

uced

.by.both.spor

adic

.vari

ables

.(e.g

.alar

signa

ls)

.and.mess

ages

.(e.g

.con

guration

.les

.In.thes

.case

.as.this.tra

.cann

.be.fore

seen

.a.prio

it.cann

.be.sche

duled.

. e.solu

tion

.adopt

.in.Worl

dFip

.is.to.link.this.kind.of.tra

.to.cycl

.tra

whic

.inst

ead

.can.be.sche

duled.

.Both.asyn

chronous

.vari

ables

.and.mess

ages

.are.tran

smitted

.by.using.

simil

.app

roaches,

.but.wit

.two.imp

ortant

.di

erences:

•

. Vari

ables

.are.shor

ter

.than.mess

ages,

.so.it.is.easie

.for.the.BA.to.nd.a.free.spac

.in.the.scan.tabl

to.all

ocate

.the

•

. Vari

ables

.nev

.req

uire

.to.be.con

rmed,

.whe

reas

.mes

sages

.can.be.con

rmed.

.tra

nsmission

.of.asy

nchronous

.tra

c

.is.per

formed

.in.thr

.ste

ps:

. 1..In.the.rst.step

.the.prod

ucer

.comm

unicates

.to.the.BA.its.need.for.addi

tional

.band

width,

.sett

ing

a.el

.insi

.an.RP_

DAT

.fra

me.

. 2.. e.BA.ask

.the.pro

ducer

.the.det

ailed

.lis

.(am

ount)

.of.the.asy

nchronous

.tra

c.

. 3.. e.BA.send

.the.tran

smission

.auth

orization

.to.the.prod

ucer,

.acco

rding

.to.the.spac

.avai

lable

insid

.its.sch

eduling

.tab

le.

34.4 application Layer

In.the.AL,.a.set.of.abstract.objects.called.application.objects.are.visible.. e.totality.of.these.objects.

make

.up.a.virt

ual

.data

base

.that.repr

esents

.the.dist

ributed

.data

base.

. ese.objec

.can.be.mani

pulated

thro

ugh

.some.spec

ialized

.serv

ices

.that.are.suit

able

.for.peri

odic/aperiodic

.data.tran

sfer

.in.indu

strial

appl

ications.

Services

.pro

vided

.by.the.AL.can.be.cla

ssied

.into.thr

.gro

ups:

•

. Bus.arb

itrator

.app

lication

.ser

vices

.(AB

AS)

•

. Manu

facturing

.per

iodical/aperiodical

.ser

vices

.(MP

•

. Subs

.of.the.ser

vices

.tha

.are.pre

sent

.in.the.Man

ufacturing

.Mes

saging

.Spec

ication

.(su

bMMS)

.main.modu

.is.MPS.prov

iding

.supp

ort

.to.peri

odic

.tra

.that.repr

esents

.the.most.comm

.traf

c

.in.Wor

ldFIP.

. is.mod

ule

.pro

vides

.the.use

.wit

.the.fol

lowing

.ser

vices:

•

. Loca

.rea

d/write

.ser

vices

•

. Remo

.rea

d/write

.ser

vices

•

. Vari

able

.tra

nsmission/reception

•

. Info

rmation

.con

cerning

.the.fre

shness

.of.the.var

iables

.con

sumed

•

. Info

rmation

.con

cerning

.the.spa

tial

.and.tem

poral

.con

sistence

.of.the.var

iables

.con

sumed

.conc

epts

.of.fres

hness

.and.spat

ial/temporal

.consi

stence

.are.of.grea

.impo

rtance

.in.Worl

dFIP

.and.

are.ass

ociated

.to.the.abi

lity

.of.the.pro

tocol

.to.pro

vide

.a.tim

ely

.com

munication.

WorldFip 34-7

Information.timeliness.is.one.main.feature.of.WorldFip.protocol,.and.to.this.aim.when.a.user.

receives.a.variable.he.or.she.can.also.receive.information.regarding.its.freshness.. is.is.obtained.

through. a. Boolean. ag. that. can. be. bound. to. each. variable. to. be. consumed. by. the. application.

processes.

Figure

.34.6.shows.how.the.timeliness.information.is.processed.and.transferred.from.the.pro-

ducer

.process.to.the.consumer..A.producer.process.(in.the.le.side.of.Figure.34.6).uses.a.write.

service.to.store.the.variable.value.inside.a.suitable.buer,.and.a.refreshment.status.check.process.

evaluates.the.timeliness.of.this.value.with.reference.to.a.theoretical refresh period.(TRP)..Each.time.

a.new.value.is.stored.in.the.buer,.the.status.ag.is.set.“true”.and.a.timer.is.started.with.a.duration.

equal.to.the.TRP..If.the.timer.expires.before.the.variable.is.refreshed.with.a.new.sample,.the.status.

ag.is.reset.to.“false.”.In.this.way,.when.the.consumer.process.receives.the.variable,.it.can.know.if.

its.value.has.been.correctly.refreshed.or.not.

34.5 timing Properties of WorldFIP Networks

As.stated.earlier,.timeliness.is.one.fundamental.property.that.strongly.features.the.WorldFIP.protocol.

and.dierentiates.it.from.other.eldbuses..In.this.section,.we.will.analyze.and.evaluate.some.important.

timing.properties.of.WorldFIP.with.reference.to.the.construction.of.the.scan.table.and.the.transmission.

of.asynchronous.trac.

34.5.1 Concept of Producer/Distributor/Consumer

As.stated.earlier,.in.WorldFIP,.the.exchange.of.identied.variables.services.are.based.on.a.producer/

distributor/consumer

.(PDC).model,.which.relates.producers.and.consumers.within.the.distributed.

system..In.this.model,.for.each.process.variable.there.is.one,.and.only.one.producer,.and.several.

consumers..In.order.to.manage.transactions.associated.with.a.single.variable,.a.unique.identier.

is.associated.with.each.variable.. e.WorldFIP.DLL.is.made.up.of.a.set.of.produced.and.consumed.

buers,.which.can.be.locally.accessed.(through.AL.services).or.remotely.accessed.(through.network.

services).

.AL.provides.two.basic.services.to.access.the.DLL.buers:.L _PUT.req,.to.write.a.value.in.a.local.

produced.buer,.and.L_GET.req,.to.obtain.a.value.from.the.local.consumed.buer..None.of.these.

services.generate.activity.on.the.bus..Produced.and.consumed.buers.can.be.also.remotely.accessed.

through.a.network.transfer.service.(also.known.as.buer transfer).. e.BA.broadcasts.a.question.frame.

ID_DAT,.which.includes.the.identier.of.a.specic.variable.. e.DLL.of.the.station.that.has.the.corre-

sponding

.produced.buer.responds.using.a.response.frame.RP_DAT.. e.DLL.of.the.station.that.con-

tains

.the.produced.buer.then.sends.an.indication.to.the.AL.(L_SENT.ind).. e.DLL.of.the.station(s).

that.has.the.consumed.buers.accepts.the.value.contained.in.the.RP_DAT,.overwriting.the.previous.

value.and.notifying.the.local.AL.with.a.L_RECEIVED.ind.

Write service

Layer 7

Layer 2

V S V S

Producer

Consumer

Read service

Variable

status

Layer 1

FIGURE 34.6 Transfer.of.a.variable.from.the.producer.to.the.consumer.

34-8 Industrial Communication Systems

34.5.2 Buffer transfer timings

A.buer.transfer.implies.the.transmission.of.a.pair.of.frames:.ID _DAT,.followed.by.a.RP_DAT..We.denote.

this.sequ

ence

.as.an.eleme

ntary transaction.

. e.dura

tion

.of.this.tran

saction

.equa

.the.time.need

.to.

tran

smit

.the.ID_D

.fram

.plus.the.time.need

.to.tran

smit

.the.RP_D

.fram

.plus.twic

.the.turn

around

.time.(t

).. e.turnaround.time.is.the.time.elapsed.between.any.two.consecutive.frames.

Every

.tran

smitted

.fram

.is.enca

psulated

.with.cont

rol

.info

rmation

.from.the.PHL..Speci

cally,

.an.

ID_D

.fram

.has.alwa

.8.byte

.(cor

responding

.to.a.2.byte

.iden

tier

.plus.6.byte

.of.cont

rol

.info

rma-

tion),

.wher

eas

.a.RP_D

.fram

.has.also.6.byte

.of.cont

rol

.info

rmation

.plus.up.to.128.byte

.of.usef

data

. e.dur

ation

.of.a.mes

sage

.tra

nsaction

.is

. C

len len

bps

( )

+ ×

id_dat rp_dat

2 .

(34.

where

bps

.sta

nds

.for.the.net

work

.dat

.rat

len(<frame>)

.is.the.len

gth,

.in.bit

.of.fra

.<fr

ame>

For

.inst

ance,

.assu

.that.all.vari

ables

.have.a.data.eld.with.4.byte

.(res

ulting

.in.ID_D

AT:

.8.byte

.and.

RP _D

AT:

.10.byte

s).

.If.t

.=.20.μs.and.the.network.data.rate.is.2.5.Mbps.then,.the.duration.of.an.elemen-

tary

.tra

nsaction

.is.(64.+.80)

/2.5

.+.2.×.20.=.97.

.μs.(Eq

uation

.34.

1).

34.5.3 Bus arbitrator table

In.WorldFIP.networks,.the.bus arbitrator table.regulates.the.scheduling.of.all.buer.transfers.. e.BAT.

impo

ses

.the.timin

.of.the.peri

odic

.bue

.tran

sfers

.and.also.regu

lates

.the.aper

iodic

.bue

.tran

sfers.

.In.

Sect

ion

.34.3

.2,

.we.have.disc

ussed

.the.impo

rtance

.of.the.BAT.as.an.indi

spensable

.tool.for.the.sche

duling

.of.

peri

odic

.tran

smissions.

.In.this.sect

ion,

.we.will.deal.with.this.in.dept

.and.will.disc

uss

.the.prob

lem

.of.how.

to.dyna

mically

.orga

nize

.the.BAT.when.new.requ

ests

.for.asyn

chronous

.tran

smission

.are.rece

ived

.by.the.BA..

Assu

.here.a.Worl

dFIP

.syst

.wher

.six.vari

ables

.are.to.be.peri

odically

.scann

ed,

.with.scan.rate

.as.show

in.Tabl

.34.2

. e.Worl

dFIP

.BAT.must.be.set.in.orde

.to.cope.with.thes

.timin

.requ

irements.

.Two.impo

tant

.para

meters

.are.asso

ciated

.with.a.Worl

dFIP

.BAT:.the.micro

cycle

.(ele

mentary

.cycl

.and.the.macr

ocycle.

 e.micro

cycle

.impo

ses

.the.maxi

mum

.rate.at.which.the.BA.perf

orms

.a.set.of.scan

.Usua

lly,

.the.micro

cycle

is.set.equa

.to.highe

.comm

.fact

.(HCF

)

.of.the.requ

ired

.scan.peri

odicities.

.Usin

.this.rule

.and.for.the.

exam

ple

.show

.in.Tabl

.34.2

.the.valu

.for.the.micro

cycle

.is.set.to.1.ms..A.poss

ible

.sche

dule

.for.all.the.peri

odic

.scan

.can.be.as.illu

strated

.in.Figu

.34.7

.wher

.we.cons

ider

.C.=.97.6.μs.for.each.elem

entary

.tran

saction.

.is.easy.to.depi

.that

.for.exam

ple,

.the.sequ

ence

.of.micr

ocycles

.repe

ats

.each.12.micr

ocycles.

. is.

sequ

ence

.of.micr

ocycles

.is.said.to.be.a.macr

ocycle

.and.its.leng

.is.give

.by.the.lowe

.comm

.mult

iple

(LCM

)

.of.the.sca

.per

iodicities.

.HCF

/LCM

.app

roach

.for.bui

lding

.a.Wor

ldFIP

.BAT.has.the.fol

lowing

.pro

perties:

. 1.. e.sca

nning

.per

iods

.of.the.var

iables

.are.mul

tiples

.of.the.mic

rocycle.

. 2.. e.vari

ables

.are.not.scan

ned

.at.exac

tly

.regu

lar

.inte

rvals.

.For.the.give

.exam

ple,

.only.vari

ables

.A.

and.B.are.scan

ned

.exac

tly

.in.the.same.“slo

t”

.with

.the.micr

ocycle.

.All.othe

.vari

ables

.sue

.from.

a.slig

.comm

unication

.jitt

er.

.For.inst

ance,

.conc

erning

.vari

able

.F,.the.inte

rval

.betw

een

.micr

cycles

.1.and.7.is.5.8

.ms,.whe

reas

.bet

ween

.mic

rocycles

.7.and.13.it.is.6.1

.ms.

TABLE 34.2 Example.Set.of.Periodic.

Bue

.Tra

nsfers

Identier A B C D E F

Periodicity

.(ms) 1

2 3 4 4 6

WorldFip 34-9

. 3.. The.length.of.the.macrocycle.can.induce.a.memory.size.problem,.since.the.table.parameters.

must.be.stored.in.the.BA..For.instance,.if.the.scanning.periodicities.of.variables.E.and.F.were,.

respectively,.5.and.7.ms,.the.length.of.the.macrocycle.would.be.420.microcycles.instead.of.

just12.microcycles.

Both

.the.communication.jitter.and.memory.size.problems.have.been.addressed.in.the.literature..In.[1],.

the.authors.discuss.dierent.methodologies.to.reduce.the.BAT.size,.without.penalising.the.communica-

tion

.jitter.. e.idea.is.very.simple,.and.it.basically.consists.on.reducing.some.of.the.scan.periodicities.in.

order.to.have.a.harmonic.pattern.. e.problem.of.table.size.has.also.been.addressed.in.other.works.[2,3],.

however,.in.a.dierent.perspective..In.the.referred.work,.the.authors.discuss.an.online.scheduler.(instead.

of.storing.the.schedule.in.the.BA’s.memory),.which.is.not.directly.applicable.to.the.WorldFIP.case.

. is. also. worth. mentioning. that. Figure. 34.7. represents. a. macrocycle. composed. of.synchronous.

microcycles,.that.is,.for.the.specic.example,.each.microcycle.starts.exactly.1.ms.aer.the.previous.one..

Within.a.microcycle,.the.spare.time.can.be.used.by.the.BA.to.process.aperiodic.requests.for.buer.

transfers,.message.transfers,.and.padding.identiers..A.WorldFIP.BA.can.also.manage.asynchronous.

microcycles,.not.transmit.padding.identiers..In.such.case,.a.new.microcycle.starts.as.soon.as.the.peri-

odic

.trac.is.performed.and.there.are.no.pending.aperiodic.buer.transfers.or.message.transfers..Initial.

periodicities.are.not.respected.since.identiers.may.be.more.frequently.scanned.

34.5.4 WorldFIP aperiodic Buffer transfers

 e.BA.handles.aperiodic.buer.transfers.only.aer.processing.the.periodic.trac.in.a.microcycle.. e.

portion.of.the.microcycle.reserved.for.the.periodic.buer.exchanges.is.denoted.as.the.periodic window.of.

the.microcycle.. e.time.le.aer.the.periodic.window.until.the.end.of.the.microcycle.is.denoted.as.the.

aperiodic window.of.the.microcycle.. e.aperiodic.buer.transfers.take.place.in.three.stages:

. 1.. When.processing.the.BAT.schedule,.the.BA.broadcasts.an.ID_DAT.frame.concerning.a.periodic.

variable,.say.identier.X.. e.producer.of.variable.X.responds.with.a.RP_DAT.and.sets.an.ape-

riodicrequest.bit.in.the.control.eld.of.its.response.frame.. e.BA.stores.variable.X.in.a.queue.of.

requests.for.variable.transfers..Two.priority.levels.can.be.set.when.the.request.for.aperiodic.transfer.

is.made:.urgent.or.normal.. e.BA.has.two.queues,.one.for.each.priority.level.

. 2.. In.the.aperiodic.window,.the.BA.uses.an.identication.request.frame.(ID_RQ).to.ask.the.pro-

ducerof.the.identier.X.to.transmit.its.list.of.pending.aperiodic.requests.. e.producer.of.X.

responds.with.a.RP_RQ.frame.(list.of.identiers).. is.list.of.identiers.is.placed.in.another.BA’s.

queue,.the.ongoing aperiodic queue.

Macrocycle

Microcycle

1 2 3 4 5 6 7 8 9 10 11 12 13

T (ms)

RP_DAT(32 µs)

(20 µs)

ID_DAT(25.6 µs)

FIGURE 34.7 Schedule.for.the.periodic.transfers.of.Table.34.2.

34-10 Industrial Communication Systems

. 3..Finally,.the.BA.processes.requests.for.aperiodic.transfers.that.are.stored.in.its.ongoing.aperiodic.

queue..For.each.transfer,.the.BA.uses.the.same.mechanism.as.the.used.for.the.periodic.buer.

transfers.(ID _DAT.followed.by.RP_DAT).

.is.important.to. note.that.a.station.can. only. request.aperiodic.transfers.using.responses.to. peri-

odic

. variables. that. it. produces. and. that. are. congured. in. the. BAT.. For. the. sake. of. simplicity,. in.

the. following. sections. we. consider. that. all. aperiodic. requests. concern. the. use. of. the. AL. service.

L_FREE_UPDATE.req(ID_*,Urgent),.thus,.only.the.urgent.queues.(both.at.the.requesting.station.and.

at.the.BA).are.considered..Finally,.it.is.important.to.stress.that.the.urgent.queue.in.the.BA.is.only.processed.

if,.and.only.if,.the.BA’s.ongoing.aperiodic.queue.is.empty,.as.detailed.in.Figure.34.8..Also,.it.is.important.to.

note.that.at.the.end.of.a.microcycle,.a.transaction.is.processed.if,.and.only.if,.there.is.still.time.to.complete.it.

34.5.5 Setting the WorldFIP Bat: rate Monotonic approach

34.5.5.1 Model for the Periodic Buffer transfers

Assume

.a.system.with.np.periodic.variables.(Vp

,.i.=.1,.…,.np)..Each.periodic.variable.is.character-

ized

.as

. Vp Tp Cp

i i i

( )

, . (34.2)

where

.corresponds.to.the.periodicity.of.Vp

.(assume.a.multiple.of.1.ms)

. is. the. length. of. the. transaction. corresponding. the. buer. transfer. of. Vp

. (as. given. by.

Equation34.1)

34.5.5.2

Building the WorldFIP Bat (rM approach)

For

.the.periodic.trac,.end-to-end.communication.deadlines.can.be.guaranteed.by.an.a.priori.test.since.

the.BAT.implements.a.static.schedule.of.the.periodic.variables..In.this.section,.we.provide.tools.and.

analysis.for.setting.the.WorldFIP.BAT.according.to.a.rate.monotonic.(RM).approach.[4].

Start of aperiodic

window

Aperiodic ongoing

queue empty?

Time left to process

buer exchange?

Requests in the

urgent queue?

Time left to process

buer exchange?

Yes

End of aperiodic

window

Time left to process

padding frame?

Yes

ID_DAT(Aperiodic)

RP_DAT(Aperiodic)

PAD

ID_RQ

RP_RQ

FIGURE 34.8 Sequence.of.transactions.during.the.aperiodic.window.

WorldFip 34-11

Considering.the.WorldFIP.characteristics,.the.BAT.can.be.built.as.follows:

. 1..From.the.var

iable

.wit

.the.sho

rtest

.per

iod

.til

.the.var

iable

.wit

.the.lon

gest

.per

iod

. a.. If.the.load.in.each.cycl

.plus.the.vari

able’s

.leng

.(bu

.tran

sfer

.leng

th)

.is.stil

.shor

ter

.than.

the.valu

.of.the.micr

ocycle,

.then.sche

dule

.a.scan.for.that.vari

able

.in.each.of.the.micr

ocycles

(wit

hin

.the.mac

rocycle)

.mul

tiple

.of.the.per

iod

.of.the.var

iable.

. b.. If.the.load.in.some.of.the.micr

ocycles

.does.not.allo

.sche

duling

.a.scan.for.that.vari

able,

sche

dule

.it.in.a.subs

equent

.micr

ocycle

.up.to.the.micr

ocycle

.in.whic

.a.new.scan.for.that.vari

able

.sho

uld

.be.mad

.If.thi

.is.not.pos

sible,

.the.var

iable

.set.is.not.sch

eduled.

For

.the.exam

ple

.of.Tabl

.34.2.(Cp

.=.0.0976.ms,.∀

).and.considering.the.RM.algorithm,.the.BAT.

beco

mes

.as.repr

esented

.in.Tabl

.34.3

.wher

.bat[i,.j].is.a.tabl

.of.bool

eans

.with.i.rang

ing

.from.1.up.to.np,.

and.j.ran

ging

.fro

.1.up.to.N.(nu

mber

.of.mic

rocycles

.in.a.mac

rocycle).

Below,

.we.desc

ribe

.an.algo

rithm

.for.buil

ding

.the.BAT.using.the.RM.algo

rithm.

.In.the.algo

rithm,

.the.

vecto

.load[

·]

.is.used.to.store.the.load.in.each.micr

ocycle

.as.the.tra

.is.sche

duled.

.It.also.assu

mes

.that.

the.arra

.Vp[,].is.orde

red

.from.the.vari

able

.with.the.shor

test

.peri

.(Vp[1,

])

.to.the.vari

able

.with.the.lon-

est

.peri

.(Vp[np,]

. e.prop

osed

.RM-ba

sed

.algo

rithm

.alre

ady

.give

.an.indi

cation

.whet

her

.all.tra

is.sch

edulable

.or.not.(li

.22)

------------------------------------------------------------------------

- Rate Monotonic for Building the BAT

------------------------------------------------------------------------

function rm_bat;

input: np /* number of periodic variables */

Vp[i,j]/* array containing the periodicity

/* and length of the variables */

μCy /* value of the microcycle */

N /* number of microcycles */

output:

bat[i,cycle] /* i ranging from 1 to np */

/* cycle ranging from 1 to N */

begin

1: for i = 1 to np do

2: cycle = 1;

3: repeat

4: if load[cycle] + Vp[i,2] <= μCy then

5: bat[i,cycle] = 1;

6: load[cycle] = load[cycle] + 1;

7: cycle = cycle + (Vp[i,1] div μCy)

TABLE 34.3 BAT.(Using.RM).for.Example.of.Table.34.1

Microcycle

1 2 3 4 5 6 7 8 9 10 11 12

bat

.[A, cy

cle] 1 1 1 1 1 1 1 1 1 1 1 1

bat [B, cycle] 1 0 1 0 1 0 1 0 1 0 1 0

bat [C, cycle] 1 0 0 1 0 0 1 0 0 1 0 0

bat [D, cycle] 1 0 0 0 1 0 0 0 1 0 0 0

bat [E, cycle] 1 0 0 0 1 0 0 0 1 0 0 0

bat [F, cycle] 1 0 0 0 0 0 1 0 0 0 0 0

34-12 Industrial Communication Systems

8: else;

9: cycle1 = cycle1 + 1;

10: ctrl = FALSE;

11: repeat

12: if load[cycle1] + Vp[i,2] <=

μCy then

13: ctrl = TRUE

14: end if;

15: until (ctrl = TRUE) or (cycle1 >=

(cycle + (Vp[i,1] div μCy)));

16: if cycle1 >= (cycle + (Vp[i,1]

div μCy)

) then

17: bat[

i,cycle1] = 1;

18: load[cycle1] = load[cycle1] + 1;

19: cycle = cycle + (Vp[i,1] div μCy)

20: else

21: /* MARK Vpi NOT SCHEDULABLE with

RM Algorithm */

22: cycle = cycle + (Vp[i,1] div μCy)

23: end if

24: end if

25:until cycle > N

26:end for

return bat;

------------------------------------------------------------------------

Note.that.by.using.the.RM.algorithm.some.of.the.variables.with.longer.periods.can.be.scheduled.in.

subs

equent

.micr

ocycles,

.thus.indu

cing

.an.incr

eased

.comm

unication

.jitt

er.

.For.exam

ple,

.if.the.netw

ork

data.rate.is.1.Mbps.inst

ead

.of.2.5.Mbps.(Cp

.=.(64.+.80)/1.+.2.×.20.=.184.μs),.a.microcycle.is.only.able.to.

sche

dule

.up.to.ve.per

iodic

.bu

.tra

nsfers

.(Ta

ble

.34.

.Fig

ure

.34.

9).

34.5.5.3

Sett

ing the WorldFIP B

at: Earl

iest Deadline

appro

ach

Assume

.now.the.set.exa

mple

.sho

.in.Tab

.34.

.in.whi

.all.Cp.=.0.3

.ms.

Wit

.the.RM.appr

oach,

.the.rst.requ

est

.for.Vp

.would.miss.the.deadline..In.this.case,.the.macrocycle.

is.six.micr

ocycles

.(Tab

.34.6

.In.fact

.ther

.is.no.empty.slot.for.a.vari

able

.in.the.rst.thre

.micr

ocycles.

When.comp

ared

.to.the.RM.sche

duling,

.one.of.the.know

.adva

ntages

.of.the.earl

iest

.dead

line

.rst.(EDF

)

sche

duling

.is.tha

.it.all

ows

.a.bet

ter

.uti

lization

.[4]

TABLE 34.4 BAT.(RM).for.Modied.Example.of.Table.34.1

Microcycle

1 2 3 4 5 6 7 8 9 10 11 12

bat

.[A,.cyc

le] 1 1 1 1 1 1 1 1 1 1 1 1

bat

.[B,.cyc

le] 1 0 1 0 1 0 1 0 1 0 1 0

bat

.[C,.cyc

le] 1 0 0 1 0 0 1 0 0 1 0 0

bat

.[D,.cyc

le] 1 0 0 0 1 0 0 0 1 0 0 0

bat

.[E,.cyc

le] 1 0 0 0 1 0 0 0 1 0 0 0

bat

.[F,.cyc

le] 0 1 0 0 0 0 1 0 0 0 0 0

WorldFip 34-13

34.5.5.4 Building the WorldFIP Bat (EDF approach)

With

.the.EDF.Approach,.variables.are.scheduled.according.to.the.earliest.deadline.(in.terms.of.micro-

cycle).

.If.several.variables.have.the.same.deadline,.priority.is.given.to.the.one.with.the.earliest.request.

Below,

.we.describe.of.an.algorithm.for.building.the.BAT.using.the.EDF.approach.. e.array.disp[,].is.

used.to.store.in.disp[i,.1].if.there.is.a.pending.request.for.variable.i,.in.disp[i,.2].the.deadline.(multiple.of.

the.microcycle).and.in.disp[i,.3].the.microcycle.at.which.the.request.is.made..Note.that.for.algorithmic.

convenience,.the.rst.requests.(for.all.the.variables).appear.in.cycle.=.0,.and.from.those,.some.will.be.

scheduled.in.the.rst.microcycle.(cycle.+.1).

-------------------------------------------------------------------------

- Earliest Deadline First for Building the BAT

-------------------------------------------------------------------------

function edf_bat;

input: np

/* number of periodic variables */

Vp[i,j]/* array containing the periodicity

/* and length of the variables */

μCy /* value of the microcycle */

/* number of microcycles */

1 2

Microcycle

Not enough time to process

the transaction

Macrocycle

3 4 5 6 7 8 9 10 11 12

t (ms)

FIGURE 34.9 Schedule.(RM).for.the.example.of.Table.34.4.

TABLE 34.5 Set.of.Periodic.Buer.Transfers.

(Cp.=.0.30.ms)

Identier A B C D E F

Periodicity

.(ms) 1 2 2 3 3 3

TABLE 34.6 BAT.(Using.RM).

forExample.of.Table.34.4

Microcycle

1 2 3 4 5 6

bat

.[A, cycle] 1 1 1 1 1 1

bat [B, cycle] 1 0 1 0 1 0

bat [C, cycle] 1 0 1 0 1 0

bat [D, cycle] 0 1 0 1 0 0

bat [E, cycle] 0 1 0 1 0 0

bat [F, cycle] X 0 0 0 0 1

34-14 Industrial Communication Systems

output:

bat[i,cycle] /* i ranging from 1 to np */

/* cycle ranging from 1 to N */

begin

1: cycle = 0;

2: repeat

3: /* determine requests at each microcycle */

4: for i = 1 to np do

5: if cycle mod Vp[i,1] = 0 then

6: disp[i,1] = 1;

7: disp[i,2] = Vp[i,1] + cycle;

8: disp[i,3] = cycle

9: else

10: /* MARK Vpi NOT SCHEDULABLE */

11: end if

12: end for;

13:

14:/* schedule vars in current microcycle */

15: load_full = FALSE;

16: repeat

17: no_rq = TRUE;

18: earliest = MAXINT;

19: var_chosen = 0; /* no var. chosen */

20:

21: /* chose the earliest deadline from

pending requests */

22: for i = 1 to np do

23: if disp[i,1] = 1 then

24: no_rq = FALSE;

25: if disp[i,2] <= earliest then

26: if var_chosen = 0 then

27: earliest = disp[i,2];

28: var_chosen = i

29: else

30: /* decide earliest req. */

31: if disp[i,3] <

disp[var_chosen,3] then

32: earliest = disp[i,2];

33: var_chosen = i

34: end if

35: end if

36: end if

37: end if

38: end for;

39:

40: /* Verify Load in Current Microcycle */

41: if load[cycle + 1] + Vp[i,2] <= μCy then

42: bat[var_chosen, cycle + 1] = 1

43: load[cycle + 1] = load[cycle + 1]

+ Vp[var_chosen,2];

44: disp[var_chosen,1] = 0;

45: disp[var_chosen,2] = 0;

46: disp[var_chosen,3] = 0;

WorldFip 34-15

47: else

48: load_full = TRUE

49: end if;

50: until (load_full = TRUE) or (no_rq =TRUE)

51: cycle = cycle + 1;

52:until cycle = N;

return bat;

-------------------------------------------------------------------------

Using.the.EDF.approach,.the.BAT.concerning.the.variables.set.represented.in.Table.34.5.becomes.

as.show

.in.Tabl

.34.7

. is.set.is.now.sche

dulable,

.whic

.was.not.possi

ble

.with.the.RM.appr

oach

(Tab

.34.6

.Howe

ver,

.the.comm

unication

.jitt

.is.incr

eased,

.since.some.of.the.bue

.tran

sfers

.with.

more.stri

ngent

.dead

lines

.are.occa

sionally

.“del

ayed”

.by.bue

.tran

sfers

.with.less.stri

ngent

.dead

lines.

.For.

inst

ance,

.vari

able

.Vp

.is.scheduled.for.the.fourth.microcycle,.whereas.with.the.RM.approach.would.be.

sche

duled

.for.the.thi

.mic

rocycle.

34.5.5.5

respo

nse

time analy

sis for the

aperi

odic

traff

.den

.the.wors

t-case

.resp

onse

.time.for.an.aper

iodic

.tran

sfer

.as.the.time.inte

rval

.betw

een

.plac

ing,

.at.time.inst

ant

,.the.L _FREE _UPDATE.req(ID_Va

, urgent).in.the.local.urgent.queue.

and.the.comp

letion

.of.the.bue

.tran

sfer

.conc

erning

.the.aper

iodic

.vari

able

.Va

.in.a.BA’s.aperiodic.

wind

ow.

. e.resp

onse

.time.asso

ciated

.to.an.aper

iodic

.bue

.tran

sfer

.inclu

des

.the.foll

owing

.thre

comp

onents:

. 1.. e.time.elap

sed

.betw

een

.and.the.time.instant.when.the.requesting.station.is.able.to.indicate.

the.BA.(via.RP_D

AT,

.with.the.requ

est

.bit.set).that.ther

.is.an.aper

iodic

.tran

sfer

.requ

est

.pend

ing.

We.de

.thi

.tim

.int

erval

.as.the.dea

.int

erval

.of.a.pro

ducer

.sta

tion.

. 2.. e.time.that.the.requ

est

.indi

cation

.stay

.in.the.BA’s.urge

.queu

.till.the.rela

ted

.ID_R

Q/RP_RQ

pair.of.fra

mes

.is.pro

cessed

.in.an.ape

riodic

.win

dow.

. 3.. And. the.time. the.bue

.exch

ange

.requ

est

.for. vari

able

.Va

.stays.in.the.BA’s. ongoing.aperi-

odic

.queu

.till.the.rela

ted

.ID_D

AT_Ap/RP_DAT

.pair.of.fram

.is.proc

essed

.in.an.aper

iodic

wind

ow.

34.5.5.6

Uppe

r Bound for the Dead Interval

.uppe

.boun

.for.the.dead.inte

rval

.in.a.stat

ion

.k.is.rela

ted

.to.the.smal

lest

.scan

ning

.peri

.of.a.pro-

uced

.vari

able

.in.that.stat

ion.

.It.is.impo

rtant

.to.note.that.a.peri

odic

.vari

able

.(Vp

).is.not.polled.at.regular.

TABLE 34.7 BAT.(using.EDF).

forex

ample

.of.Tab

.34.

Microcycle

1 2 3 4 5 6

bat

.[A, cy

cle] 1 1 1 1 1 1

bat [B, cycle] 1 0 1 0 1 0

bat [C, cycle] 1 0 0 1 1 0

bat [D, cycle] 0 1 0 1 0 0

bat [E, cycle] 0 1 0 0 0 1

bat [F, cycle] 0 0 1 0 0 1