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

Подождите немного. Документ загружается.

34-16 Industrial Communication Systems

intervals.since.there.is.a.communication.jitter.inherent.to.the.BAT.setting.. erefore,.the.upper.bound.

for.the.dea

.int

erval

.in.a.sta

tion

.k.is

j Vp j j j

Vp k

Tp J Cp Vp Tp Tp

= + + =

{ }

, : minwith

produced in

(34.

where

.is.the.maximum.communication.jitter.of.Vp

.foll

owing

.assu

mption

.is.inhe

rent

.to.Equa

tion

.34.3

.a.loca

.aper

iodic

.requ

est

.is.only.proc

essed

(set

ting

.the.requ

est

.bit.in.the.RP _ D

.fram

.if.it.arri

ves

.befo

.the.star

.of.the.rela

ted

.ID _ D

AT.

Henc

.the.ter

.Cp

.is.included.in.Equation.34.3.

34.5.5.7

aperi

odic Busy Interval

.wors

t-case

.resp

onse

.time.for.an.aper

iodic

.vari

able

.tran

sfer

.occu

.if,.when.the.requ

est

.is.plac

.in.

the.BA’s.urge

.queu

.(σ

.aer.t

),.the.queue.already.has.requests.for.all.the.other.aperiodic.variables.in.

the.net

work.

.We.con

sider

.tha

. 1..For.each.aper

iodic

.vari

able,

.a.requ

est

.for.iden

tication

.must.be.made

.and.thus.the.netw

ork

.load.

is.max

imized

. 2.. And.that.thos

.requ

ests

.will.begi

.to.cont

end

.for.the.medi

.acce

.at.the.star

.of.the.macr

cycle:

.cri

tical instant.

.also.consi

der

.that.all.aper

iodic

.tra

.has.a.mini

mum

.inte

r-arrival

.time.betw

een

.requ

ests,

.whic

is.grea

ter

.than.its.wors

t-case

.resp

onse

.time

. ere

fore,

.no.othe

.aper

iodic

.requ

est

.appe

ars

.befo

.the.

comp

letion

.of.a.prev

ious

.one..Henc

.the.maxi

mum

.numbe

.of.aper

iodic

.requ

ests

.pend

ing

.in.the.BA.is.

na,.na.bein

.the.numbe

.of.aper

iodic

.requ

ests

.(di

erent

.stat

ion

.can.requ

ire

.an.aper

iodic

.bue

.tran

sfer

of.a.sam

.var

iable)

.tha

.can.be.mad

.in.the.net

work.

.den

.the.time.span.betw

een

.the.crit

ical

.inst

ant

.and.the. end.of.the.proc

essing

.of.aper

iodic

requ

ests

.that.are.pend

ing

.at.the.crit

ical

.inst

ant

.as.an.aper

iodic busy interval

.(ABI

)

.since.all.aper

iodic

wind

ows

.wit

hin

.the.mic

rocycles

.are.use

.to.pro

cess

.ape

riodic

.tra

c.

.is.also.clea

.that.to.proc

ess

.all.thos

.na.requ

ests,

.the.aper

iodic

.wind

ows

.will.perfo

.alte

rnately

sequ

ences

.of.(ID _R

Q/RP_RQ)

.and.(ID_D

AT/RP_DAT),

.as.the.BA.give

.prio

rity

.to.the.ongoi

.ape-

iodic

.queu

.(see.Figur

.34.2

.If.all.the.aper

iodic

.vari

ables

.have.a.simi

lar

.leng

th,

.Ca*.may.be.den

as.the.maxi

mum

.leng

.of.all.the.(ID_R

Q/RP_RQ)

.and.(ID_D

AT/RP_DAT)

.tran

sactions

.concer

ing

.the.aperiodic.trac.. erefore,.the.number.of.transactions.to.be.processed.during.the.ABI.is.

2×.na,.corr

esponding

.to.the.set.of.ID_R

Q/RP_RQ

.tran

sactions

.and.the.set.of.ID _D

AT/RP_DAT

tran

sactions.

With

.thes

.assu

mptions,

.the.anal

ysis

.for.the.wors

t-case

.resp

onse

.time.for.the.aper

iodic

.tra

.is.as.foll

ows.

34.5.5.8

Wors

t-Case

respo

nse

time

.wors

t-case

.resp

onse

.time.of.aper

iodic

.tran

sfers

.is.the.func

tion

.of.the.netw

ork

.peri

odic

.load.duri

the.ABI.since.it.boun

.the.aper

iodic

.wind

ows

.leng

th.

. e.leng

.of.the.aper

iodic

.wind

.in.the.lth.

cycl

.(l.=.1,.…,.N,.N.+.1,.…).may.be.eva

luated

.as.fol

lows:

aw l Cy bat i l Cp

( ) ( [ , ] )= − ×

∑

(34.

 erefore,

.the.numb

.of.ape

riodic

.tra

nsactions

.tha

.t.in.the.lth.ape

riodic

.win

dow

.is

nap l

aw l

( )













(34.

WorldFip 34-17

 e.number.of.microcycles.(N′).in.an.ABI.is.then

′

= = ∧ ≥ ×

′

∑

N nap l na

min{ }, ( )Ψ Ψ Ψwith 2

(34.6)

that

.is,.the.minimum.number.of.microcycles.within.which.the.number.of.available.“slots”.(each.“slot”.

with.the.length.of.Ca*).is.at.least.2.×.na.

Knowing

.N′,.the.length.of.the.aperiodic.busy.interval.(len _ abi).may.be.evaluated.as.follows:

len abi N Cy bat i N Cp na nap l

_ ( ) ( [ , ] ) ( )=

′

− × + × + × −











′

−

∑

1 2



∑

(34.7)

where

i=1,…,np

(bat[i,.N].×.Cp

).gives.the.length.of.the.periodic.window.in.microcycle.N′

. ×. na. −. Σ

l=1,…,N′−1

nap(l).). ×. Ca*. gives. the. length. of.the. aperiodic. window,. with. respect. to. the.

aperiodic.busy.interval,.also.in.microcycle.N′

 erefore,

.the.worst-case.response.time.for.an.aperiodic.transfer.requested.at.station.k.is

Ra len abi

k k

= +σ _

(34.8)

and

.the.maximum.admissible.interval.between.consecutive.aperiodic.requests.of.an.aperiodic.variable.

in.a.station.k.is

MIT Va Ra len abi

k k k

( ) _≥ = +σ

(34.9)

34.5.5.9

Example of aperiodic traffic Scheduling

Assume

.that.a.system.congured.for.six.periodic.variables.(Table.34.1).must.also.support.nine.aperiodic.

buer.exchanges..Assume.also.that.the.length.of.each.Vp

,.∀

,.is.Cp

.=.Cp.=.0.0976.ms,.and.that.for.spo-

radic

.trac.Ca*.=.0.1.ms..If.the.BAT.is.implemented.as.shown.in.Table.34.2,.transactions.concerning.the.

nine.aperiodic.variables.are.scheduled.as.shown.in.Figure.34.10.

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

Time (ms)

Length of the ASI

Microcycle

Macrocycle

ID DAT, RP DAT transactor length, aperiodic trac

Periodic elementary transactor length periodic trac

ID_RP, RP_RQ transactor length, aperiodic trac

1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9

FIGURE 34.10 Example.of.aperiodic.trac.schedule.

34-18 Industrial Communication Systems

references

. 1.. Kim,.Y.,.Jeong,.S.,.and.Kown,.W..A.pre-run-time.scheduling.method.for.distributed.real-time.sys-

tems

. in. a. FIP. envir

onment.

. Con

trol Engineering Practice,

. 6,. 103–109,. 1998,. Perga

mon/Elsevier

Science

. 2.. Kuma

ran,

.S..and.Deco

tignie,

.J.-D..Mul

ticycle

.opera

tions

.in.a.eld.bus:.App

lication

.lay

.implic

ations.

Pro

ceedings of IEEE Annual Conference of Industrial Electronics Society (IECON’89),

.Phil

adelphia,

PA,.1989,.pp

.531–536.

. 3.. Raj

.P..and.Nou

bir,

.G..Sta

tic

.and.dyn

amic

.pol

ling

.mec

hanisms

.for.eldb

.netw

orks.

.Oper

ating

Systems Review,

.27(3),.34–45,.1993.

. 4.. Liu

.L..and.J..Lay

land.

.Sch

eduling

.algo

rithms

.for.mul

tiprogramming

.in.a.har

.rea

l-time

.envir

on-

ment.

.Jo

urnal of the ACM,

.20(1),.46–61,.1973.

35-1

35.1 Introduction

 is.chapter.describes.the.foundation.eldbus.(FF).protocol,.proposed.by.the.Fieldbus.Foundation.

orga

nization

.and.stan

dardized

.as.IEC.norm.[IEC

00a,IEC00b,VP02,PF03,PN09].

. is.proto

col,

.whic

has.a.stro

.inu

ence

.from.the.Worl

.FIP.proto

col

.and.is.main

.used.in.proc

ess

.autom

ation

.appl

ica-

tions

.[BE0

1],

.has.as.its.main.char

acteristic

.a.full.dist

ribution

.of.cont

rol

.func

tions,

.whic

.can.run.on.

proc

essing

.unit

.embed

ded

.on.sens

ors

.and.actu

ators.

.In.the.open

.nonp

roprietary

.Foun

dation

.arch

tecture,

.each.eld.devi

.has.its.own.“int

elligence”

.and.comm

unicates

.via.an.all-

digital,

.seri

al,

.two-

way

comm

unications

.syst

em.

.For.that.purp

ose,

.stan

dardized

.func

tion

.bloc

ks,

.such.as.PID.cont

rol,

.anal

and.digi

tal

.inpu

.and.outp

uts,

.etc.,.are.avai

lable.

. is.sect

ion

.give

.an.over

view

.on.the.physi

cal

.laye

(Man

chester-encoded

.bus-

powered

.comm

unication),

.the.comm

unication

.stac

.capa

bilities,

.and.on.the.

“use

.lay

er”

.(on.whi

.fun

ction

.blo

cks

.are.int

erconnected

.and.con

trol

.str

ategies

.are.de

ned).

35.2 Foundation Fieldbus Overview

Every.FF.eld.device.is.capable.of.executing.those.functionalities.encapsulated.in.function.blocks.that.are.

the.basi

.elem

ents

.for.prog

ramming

.FF.appl

ications

.and.execu

ting

.cont

rol

.and.auto

mation

.algo

rithms,

such.as.PID.cont

rol,

.arit

hmetic

.oper

ations,

.anal

.and.digi

tal

.inpu

t/output,

.etc..Anot

her

.majo

.char

acter-

istic

.of.the.FF.prot

ocol

.is.its.advan

ced

.diag

nostic

.capa

bility,

.whic

.den

.not.only.the.heal

.of.the.devi

but.now.also.in.a.stan

dard

.form

.that.can.be.easi

.inte

grated

.into.asset

-management

.syst

ems.

.orde

.to.guar

antee

.that.mess

ages

.exch

anged

.betw

een

.func

tion

.bloc

.resid

ing

.on.die

rent

.devi

ces

are.succe

ssfully

.tran

sferred

.to.thei

.dest

inations

.with

.preci

.and.boun

ded

.temp

oral

.inte

rvals,

.FF.reli

Foundation Fieldbus

35.1. Introduction.....................................................................................35-1

35.2

. Foun

dation

.Fie

ldbus

.Ove

rview.....................................................35-1

35.3

. Topo

logy............................................................................................35-2

35.4

. Driv

ers

.(DD

.EDD

.and.FDT

/DTM)..........................................35-3

35.5

. Cabl

es.................................................................................................35-3

35.6

. Segm

ent

.Desi

gn................................................................................35-4

35.7

. FFPS

—Fieldbus

.Pow

.Sup

plies....................................................35-5

35.8

. Inst

allation

.of.Seg

ment

.in.Saf

.Are

as...........................................35-6

35.9

. Inst

allation

.of.Seg

ments

.in.Cla

ssied

.Are

as...............................35-7

FISCO—Fieldbus.Intrinsically.Safe.COncept. •. High-Energy.

Trunk–Fieldbus.Barrier.Solution

35.10.Project.Documentation...................................................................35-8

35.11

.Inst

allations

.and.Com

missioning.................................................35-8

35.12

.Main

tenance...................................................................................35-10

References...................................................................................................35-10

Carlos Eduardo

Pereira

Federal UniversityofRio

Gran

do Sul

Augusto Pereira

Pepperl-Fuchs

Ian Verhappen

Industrial Automation

Netw

orks

Inc

35-2 Industrial Communication Systems

on.schedule.timetables.. ese.can.be.generated.online.or.o-line.and.are.used.by.the.link.active.sched-

uler

.(LAS

. e.LAS.mana

ges

.all.mess

age

.tran

sfers

.betw

een

.die

rent

.eld.devi

ces

.on.a.bus.and.it.is.also.

in.char

.of.star

ting

.the.exec

ution

.of.func

tion

.bloc

.in.each.devi

ce.

.Due.to.its.impo

rtance,

.in.case.of.

fail

ure

.of.the.devi

.runn

ing

.the.LAS,.anot

her

.prev

iously

.assig

ned

.devi

.will.autom

atically

.assu

resp

onsibility

.for.mess

age

.sche

duling.

. e.LAS.is.an.impo

rtant

.conc

ept

.towar

.achi

eving

.a.dete

rmin-

istic

.real

-time

.beha

vior

.and.over

coming

.comm

.dec

iencies

.of.eld

bus

.tech

nologies,

.in.rela

tion

.to.

timi

.con

straints,

.hig

h-processing

.loa

.and.com

munication

.sch

edulability

.[FA

00].

Fieldbus

.netw

ork

.tra

.is.peri

odic

.and.a.singl

.inte

raction

.of.a.sche

dule

.in.a.link.is.call

.a.macr

cycle.

.Duri

.the.sche

duled

.comm

unications

.port

ion

.of.the.macr

ocycle,

.all.the.data.rela

ted

.to.proc

ess

cont

rol

.stra

tegies

.are.tran

smitted.

.One.part.of.the.macr

ocycle

.is.used.for.peri

odic

.data.(cyc

lical

.comm

nications)

.publ

ishing

.(ext

ernal

.link

.and.cont

rol

.data

)

.and.anot

her

.part.is.used.for.acyc

lic

.back

ground

acti

vities

.(sup

ervision

.and.netw

ork

.mana

gement

.mess

ages).

.Peri

odic

.data.are.consi

dered

.high.prio

ity

.and.thei

.timi

.requ

irements

.must.be.resp

ected

.duri

.the.exec

ution

.of.the.cont

rol

.appl

ication.

Asyn

chronous

.req

uests

.(bo

.ini

tiated

.by.dev

ices

.or.by.ext

ernal

.use

rs)

.hav

.low

.pri

ority

.and.are.onl

tran

smitted

.betw

een

.the.tran

smissions

.of.peri

odic

.data.if.and.only.if.they.do.not.dela

.the.peri

odic

comm

unications.

.Usua

lly,

.the.comp

lete

.supe

rvision

.info

rmation

.may.requ

ire

.seve

ral

.macr

ocycles

.to.

be.comm

unicated

.from.the.eld.devi

ces

.to.the.supe

rvision

.work

stations.

. is.kind.of.info

rmation

.is.

nonp

eriodic.

.LAS.sele

cts

.betw

een

.peri

odic

.data.and.acyc

lic

.mess

ages

.using.two.type

.of.token

.Peri

odic

.data.

are.requested.using.message.frames.named.compel.data.(CD),.which.are.sent.to.the.target.publisher.

node

.(sch

eduled

.token.to.peri

odic

.data

)

.acco

rding

.to.the.sche

dule

.time

table.

.Once.the.publ

isher

.node.

rece

ives

.the.CD,.it.plac

.its.data.on.the.bus.using.a.data.tran

sfer

.(DT).data

gram

.and.any.devi

ces

.on.the.

netw

ork

.can.the

.rea

.the.mes

sage

.(pu

blish–subscriber

.pat

tern).

.port

ions

.of.the.comm

unications

.cycl

.occu

pied

.by.neit

her

.the.tran

smission

.of.CD.nor.DT,.the.

LAS.send

.an.unsc

heduled

.token.call

.pass.token.(PT).to.one.of.thos

.devi

ces

.incl

uded

.in.a.live.list.

(a.list.with.all.devi

ces

.that.are.curr

ently

.conn

ected

.to.the.loca

.link

.Each.unsc

heduled

.token.has.a.

boun

ded

.inte

rval,

.duri

.whic

.the.rece

iving

.node.can.use.the.bus.to.tran

smit

.a.pend

ing

.mess

age

.unti

no.furt

her

.nonp

eriodic

.mess

ages

.exis

.or.the.inte

rval

.expi

res.

. is.boun

ded

.inte

rval

.prec

ludes

.that.the.

nonp

eriodic

.mess

age

.exce

eds

.the.gap.befo

.the.next.sche

dule

.tran

smission.

.When.no.wait

ing

.nonp

riodic

.mess

age

.or.unsc

heduled

.inte

rval

.expi

res,

.the.rece

iving

.devi

.send

.the.token.back.to.the.LAS.by.

mean

.of.a.ret

urn

.toke

.(RT

.FF.appl

ication

.may.incl

ude

.seve

ral

.proc

ess

.cont

rol

.stra

tegies.

.One.stan

d-alone

.func

tion

.bloc

.or.

many.func

tion

.bloc

.link

.to.othe

.func

tion

.bloc

.are.used.to.impl

ement

.each.proc

ess

.cont

rol

.stra

egy

.desig

ned

.by.the.user

. ese.func

tion

.bloc

.consi

.of.a.set.of.data.and.algo

rithms

.that.are.inte

nded

to.prov

ide

.a.give

.func

tionality

.to.the.appl

ication.

. ey.may.repr

esent

.a.cont

rol

.or.I/O.func

tion

.bloc

Exam

ples

.are.PID.cont

rollers,

.inpu

.and.outp

.bloc

ks,

.tran

sducer

.bloc

ks,

.etc..Curr

ently,

.ther

.are.com-

ercially

.avai

lable

.devi

ces

.that.can.cont

ain

.up.to.19.func

tion

.bloc

ks.

.Each.spec

ic

.devi

.runs.the.func

tion

.bloc

.at.a.spec

ic

.spee

.depe

nding

.on.its.inte

rnal

.comp

uting

.reso

urces

.(pro

cessor,

.memo

ries,

etc.

.Cons

equently,

.the.func

tionality

.prov

ided

.by.a.give

.func

tion

.bloc

.may.have.die

rent

.exec

ution

time

.depe

nding

.on.each.devi

.in.whic

.it.is.exec

uted.

. is.aspe

.shou

.be.take

.into.acco

unt

.by.the.

sche

duler

.by.rea

ding

.thi

.inf

ormation

.fro

.the.DD.le

35.3 topology

 e.FF.specication.denes.two.levels.of.networks:.H1,.network.for.control.with.31.25.kbps;.and.HSE,.data.

high

way

.with.100.Mbps

.H1.is.a.Manc

hester

.bus–

powered

.and.is.usua

lly

.appl

ied

.at.the.eld.leve

.and.can.

also.used.in.ex-a

pplications

.with.safe

.barr

iers.

. e.orig

inal

.speci

cation

.incl

udes

.a.high

.speed.seria

comm

unications

.prot

ocol

.that.was.call

.H2.but.it.has.neve

.been.deve

loped.

. e.high

-speed

.Ethe

rnet

(HSE

)

.bus.is.adop

ted

.at.high

.leve

ls,

.allo

wing

.an.easi

.conn

ection

.to.Ethe

rnet-based

.cont

rol

.netw

orks.

Foundation Fieldbus 35-3

Four.device.classes.are.specied.for.HSE.networks:.(1).host.devices.(HD).are.PCs.or.controller.devices.with.

an.Ethe

rnet

.port

.(2).a.link.devi

.(LD).allo

.an.conn

ection

.from.an.Ethe

rnet

.netw

ork

.to.mult

iple

.H1.

segme

nts,

.(3).fore

ign

.I/O.gate

ways

.are.comp

onents

.for.inte

grating

.thir

d-party

.eld

buses,

.and.(4).Ethe

rnet

devi

ces

.(ED).that.allo

.a.direc

.inte

gration

.to.Ethe

rnet

.netw

orks.

35.4 Drivers (DD, EDDL, and FDt/DtM)

 e.majority.of.today’s.“smart”.analog.instruments.use.EDDL.(electronic.device.description.language),.

oen.shor

tened

.to.DD.(dev

ice

.desc

ription)

.in.conv

ersation.

.EDDL.is.the.base.tech

nology

.unde

rlying

the.HART

.Pro

bus

.PA,.and.FF.proto

cols

.by.den

ing

.a.signi

cant

.part.of.thei

.user.laye

.DD.les.for.

all.appr

oved

.FF.devi

ces

.are.acce

ssible

.from.the.Fiel

dbus

.Foun

dation

.Web.site..Each.devi

.has.a.uniq

set.of.thre

.or.four.les.in.the.zip.le.asso

ciated

.with.the.devi

ce.

.All.of.thes

.les.exce

.the.CFF,.or.

capa

bilities

.le

.whi

.is.in.pla

.ASC

.are.bin

ary

.le

.orig

inal

.EDDL.spec

ication

.as.den

.by.IEC.6180

.Part.2.cont

ains

.litt

.supp

ort

.for.grap

hics;

thus

.the.inte

rface

.and.degr

.of.inte

gration

.betw

een

.eld.devi

ces

.and.host

.was.not.only.limi

ted,

.but.

the.leve

.of.inte

gration

.and.info

rmation

.that.coul

.be.pres

ented

.to.a.user.vari

.betw

een

.each.inst

alla-

tion

.dep

ending

.on.the.hos

.bei

.use

One

.tech

nology

.that.was.deve

loped

.to.atte

mpt

.to.over

come

.thes

.limi

tations

.was.FDT/

DTM

.(el

devi

.tool/

device-type

.mana

ger).

. e.FDT/

DTM

.tech

nology

.uses.EDDL.as.the.basis.for.its.den

itions,

so.that.devi

.manu

facturers

.can.den

.the.“loo

.and.feel

”

.of.how.the.user.acce

sses

.the.devi

.info

mation

.for.othe

.than.proc

ess

.vari

able

.(PV).and.rela

ted

.signa

ls,

.such.as.thos

.used.to.con

gure

.and.

cali

brate

.the.dev

ice.

35.5 Cables

 e.Fieldbus.Foundation.uses.a.pair.of.wires.to.supply.the.DC.voltage.and.the.communication.signal.of.

the.prot

ocol.

. is.cabl

.is.norm

ally

.shie

lded

.to.mini

mize

.noise.on.the.segme

nt.

.As.show

.in.Tabl

.35.1

.the.

eld

bus

.stan

dard

.den

.four.poss

ible

.cabl

.type

. e.most.comm

only

.used.cabl

.is.type.“A.”.When.type.

“A”.cabl

.is.used,.netw

orks,

.or.segme

nts

.as.they.are.comm

only

.call

ed,

.with.tota

.comb

ined

.leng

.(sum.of.

the.trun

.plus.all.spur

.of.up.to.1900.m.are.poss

ible

.with

out

.the.use.of.repe

aters.

. e.speci

cation

.does

howe

ver,

.allo

.for.up.to.four.repe

aters

.to.be.used.on.a.sing

.segme

nt.

.the.desig

ner

.choo

ses

.anot

her

.cabl

.type.than.type.“A,”.the.avai

lable

.segm

ent

.leng

.will.be.redu

ced.

For.exam

ple,

.if.a.type.“B”.cabl

.(twi

sted

.pair.with.over

all

.shie

ld)

.were.to.be.used

.the.total.cabl

.budg

avai

lable

.is.redu

ced

.to.1200.m..Expe

rience

.has.show

.that.Ohm’

.law.is.oen.the.dete

rmining

.facto

.in.

the.limi

tation

.on.over

all

.leng

th,

.so.an.eec

tive

.way.to.be.able.to.incr

ease

.the.leng

.of.a.netw

ork

.to.thes

maxi

mums

.is.to.use.a.lar

ger

.dia

meter

.cab

.wit

.les

.resi

stance.

.IEC.6115

8-2

.spec

ies

.in.Sect

ion

.11.7

.that.“ e.cabl

.used.for.test

ing

.Fiel

dbus

.devi

ces

.with.

a.31.2

.kbps.volt

age-mode

.MAU.for.conf

ormance

.to.the.requ

irements

.of.this.part.of.IEC.6115

.shal

be. a. singl

.twis

ted-pair

.cabl

.with. over

all

.shie

.meet

ing

.the. foll

owing

.mini

mum

.requ

irements

.at.

25.deg.C….subs

ection

.d).maxi

mum

.d.c..resis

tance

.(per.cond

uctor)

.=.24.Ohm/

km.”

. en.late

.in.the.

TABLE 35.1 Foundation.Fieldbus.Cables

Type Description Size Maximum.Length

A Shielded,

.twi

sted-pair #18

.AWG.(0.8.mm

) 1900.m.(6232..)

B Multiple,

.twi

sted-pair

.wit

.shield

#22

.AWG.(0.32.mm

) 1200.m.(3963..)

C Multiple,

.twi

sted-pair

.wit

hout

.shield

#26

.AWG.(0.13.mm

) 400.m.(1312..)

D Multi-core,

.w/o.twi

sted

.pair

andha

ving

.overa

.shield

#16.AWG.(0.1.25.mm

) 200.m.(656..)

35-4 Industrial Communication Systems

specication,.B.1.Cable.description.and.specications,.“ e.preferred.Fieldbus.cable.is.specied.in.11.7.2.

for.conformance.testing,.and.it.is.referred.to.as.type.‘A’.Fieldbus.cable.”. erefore,.it.is.possible.to,.as.we.

just.described,.use.larger.diameter.cable.and.remain.in.compliance.with.the.standards.

.reduce.the.possible.confusion.in.this.area,.the.Fieldbus.Foundation.has.recently.released.specica-

tion

.FF-844.“H1.Cable.Test.Specication.”.When.manufacturers.submit.their.cable.for.testing.against.this.

standard,.they.can.obtain.an.FF.“check.mark”.for.compliance.and.this.will.provide.the.end.user.a.high.

level.of.condence.that.when.properly.installed,.this.cable.will.result.in.reliable.eldbus.communications..

 e.FF-841.standard.supports.the.use.of.single.and.multi-pair.cables.for.eldbus.H1.communications.

35.6 Segment Design

One.of.the.unique.features.of.the.FF.technology.is.that.it.allows.“control.in.the.eld.”.However,.to.do.

so,.just.like.was.the.case.with.pneumatic.control,.all.the.elements.of.the.control.loop.must.reside.on.the.

same.segment.or.cable..Because.the.analog.input.(AI).and.analog.output.(AO).devices.must.be.on.

the.same.cable.for.“control.in.the.eld,”.even.if.you.are.not.planning.to.implement.this.feature.from.the.

beginning.of.the.project,.the.segments.should.be.designed.so.that.you.can.do.so.in.the.future.without.

needing.to.physically.rearrange.your.network.

When

.starting.the.segment.design.component.of.a.FF.project,.you.should.have.on.hand.the.following.

minimum.information:

•

. Plant.layout—for.the.approximate.position.of.the.instruments.and.the.routing.of.the.cable.trays.

in.3D,.to.determine.approximate.cable.lengths.

•

. Process.and.instrumentation.diagram.(P&ID)—for.the.assignment.of.appropriate.eld.devices.to.

the.same.segment.

•

. Area.classication.drawing—because.it.has.an.impact.on.the.type.of.power.conditioners.that.can.

be.selected.and.used.

Other

.decisions.that.you.will.need.to.make.prior.to.starting.the.design.include

•

. Maximum.number.of.points.per.segment

•

. Protection.method.to.be.used..Will.it.be.intrinsic.safety.(IS).or.its.equivalent,.explosion.proof.or.

other.means

•

. Number.of.spares.for.future.expansion

•

. Basic.network.design.(tree/chicken.foot).or.some.other.arrangement

Figure

.35.1.shows.the.tree.or.chicken.foot.arrangement,.which.is.the.most.widely.used.design.basis.

for.eldbus.networks.

.conguration.has.a.“trunk”.dened.as.the.wire.pair.between.the.two.terminators,.with.spurs.to.

each.of.the.individual.eld.devices.

Single twisted pair

Junction box

PC, PLC, or DCS

FIGURE 35.1 Foundation.eldbus.network.topology.

Foundation Fieldbus 35-5

Many.designs.also.endeavor.to.take.into.account.the.criticality.of.the.individual.eld.devices,.so.

that.in.the.unlikely.event.of.a.communications.failure,.the.impact.to.the.control.system.is.minimized..

It.is.recommended.that.you.follow.similar.rules.as.are.being.used.to.manage.this.risk.to.determine.the.

assignment.of.devices.to.I/O.cards.today..By.using.these.guidelines,.the.impact.on.a.single.component.

failure.on.a.design.will.be.similar.or.less.than.for.an.analog.control.loop,.especially.if.you.have.“control.

in.the.eld.”.With.control.in.the.eld,.as.long.as.the.eld.devices.have.power,.it.is.possible.for.them.to.

continue.to.control.at.the.last.set.point,.though.there.will.be.“loss.of.view”.by.the.DCS.and.process.

operator..Fieldbus.is.also.designed.to.“fail.gracefully”.with.the.additional.options.on.loss.of.contact.with.

the.host.of.holding.last.position,.or.moving.to.a.mechanical.fail.position.

.following.formula.is.used.to.determine.the.voltage.available.at.any.device.or.node.(V

).in.the.

eldbus.network.

V V I I I I R

d p d HH SC Dmin

= − + + −

( )













∑

where

.is.the.voltage.at.power.supply

.is.the.current.required.to.power.device

.is.the.current.required.to.power.handheld.unit(s),.typically.10.mA

.is.the.current.load.on.segment.when.short.circuit.is.activated

Dmin

.is.the.current.demand.of.device.on.network.with.minimum.current.requirement

.is.the.resistance.of.cable

35.7 FFPS—Fieldbus Power Supplies

FF.H1.networks.require.eldbus.power.supplies.(FBPS)/power.conditioners.because.the.eldbus.signal.

itself.is.normally.between.0.75.and.1.V.(approximately.9.mA).peak.to.peak.. is.Manchester-encoded.

signal.changes.every.16.or.32.ms,.so.a.traditional.24.V.bulk.power.supply.would.be.working.full.time.to.

absorb.this.signal.to.maintain.a.steady.DC.voltage..FBPS/conditioners.typically.source.their.power.from.

a.24.V.DC.bulk.power.supply,.as.shown.in.Figure.35.2.

FBPS

.or.power.conditioners.must.comply.with.Fieldbus.Foundation.standard.FF-831.“eldbus.power.

supply.test.specication.”.Aer.a.FBPS.has.passed.the.tests.outlined.in.this.specication,.they.will.have.

the.eldbus.“check.mark”.conrming.their.suitability.for.use.on.H1.networks.

Fieldbus power supply/conditioner

CPU 2

CPU 1

FF I/O

24 V DC

12–28 V

80–500 mA

FIGURE 35.2 Foundation.eldbus.power.supply/conditioner.

35-6 Industrial Communication Systems

 e.majority.of.today’s.FBPS.have.galvanic.isolation.between.the.eldbus.communication.module.

to.the.host.and.the.eld.segment.where.the.instruments.are.installed.as.this.prevents.disturbances.and.

noises.from.the.eld.or.host.system.and.the.bulk.power.supply,.propagating.from.one.system.to.another;.

thus,.minimizing.the.risk.of.damage.to.the.host.I/O.card.

FBPS

.are.designed.for.a.range.of.conditions,.and.as.a.result,.the.power.conditioner.may.have.

voltage.output.between.12.and.28.V.and.80–500.mA.. e.predominant.factor.aecting.the.available.

power.range.and.associated.voltage/current.available.is.the.area.classication.into.which.it.is.sourced.

if.you.wish.to.have.live.working.on.the.full.system,.partial.system.(spurs.only),.or.use.gas.testing.and.

similar.restrictions.at.all.times.when.doing.maintenance.

35.8 Installation of Segment in Safe areas

 e.eldbus.specications.state.that.a.maximum.of.32.devices.can.be.installed.on.a.single.H1.network;.

however,.doing.so.would.result.in.a.very.long.macrocycle.and.higher.risk.than.most.end.users.would.be.

willing.to.accept.should.something.happen.to.communications.on.that.port..Consequently,.and.in.part.

because.many.host.systems.reach.a.parameter/port.limit.at.this.level,.most.designs.use.a.maximum.of.

16.loop.powered.devices.per.segment.

Many

.projects.therefore.start.their.design.basis.with.the.maximum.number.of.devices.per.network.

of.12.devices.per.segment..Many.designs.have.a.corollary.design.rule.that.of.these.12.devices,.no.more.

than.four.control.loops.(valves).should.be.used.

Because

.eldbus.is.wired.in.parallel,.it.is.strongly.recommended.that.the.spurs.have.short-circuit.

protection.for.all.spur.connections.. is.is.so.that.a.short.circuit.on.a.spur.or.in.an.instrument.will.

not.cause.a.shutdown.of.the.entire.segment..Fortunately,.almost.all.eld.device.couplers.have.this.

capability.

Many

.projects.also.develop.a.number.of.“standard”.eldbus.enclosure.congurations.with.a.xed.

number.of.spurs.and.home.run.cables.per.junction.box.. is.standardization.makes.the.project.design.

easier,.simplies.the.installation,.and.optimizes.the.number.of.items.that.need.to.be.carried.in.stock..

One.other.benet.is.that.many.eldbus.device.coupler.manufacturers.now.also.oer.a.standard.range.

of.“precongured”.junction.boxes,.so.that.all.the.project.needs.to.do.is.mount.the.enclosure,.which.has.

been.specied.and.designed.for.the.environment.where.it.is.to.be.installed.and.run.the.cables.to.the.

appropriate.terminations.. e.drawing.here.(Figure.35.3).shows.the.minimum.size.junction.box.specied,.

which.is.a.four-spur.conguration.

Monitoring PC or DCS

Junction box

Single twisted pair

FIGURE 35.3 Fieldbus.foundation.installation.in.the.eld.

Foundation Fieldbus 35-7

35.9 Installation of Segments in Classied areas

Because.FF.requires.that.the.device.be.connected.to.a.network.to.perform.maintenance,.the.majority.

of.eldbus.installations.with.equipment.in.classied.areas.are.using.the.IS.technology..IS.technology.

allows.“live.working,”.meaning.all.eld.work.can.be.done.“hot,”.with.the.network.energized.and.com-

munications

.with.the.host.at.all.times..“Live.working”.means.you.can.work.without.continuous.gas.

testing.and.is.therefore.the.safest.way.to.protect.your.plant..An.alternative.to.IS,.or.its.equivalents.as.

described.below,.is.to.use.explosion-proof.technology.

One

.advantage.of.eldbus.technology.and.digital.communications,.however,.is.that.a.signicant.por-

tion

.of.the.work.that.in.the.past.had.to.be.done.at.the.device.for.analog.systems.(i.e.,.range.change).can.

now.be.completed.over.the.network.without.actually.having.to.go.to.the.physical.device.in.the.eld.

.technology.is.based.in.the.reduction.of.the.energy.to.the.eld.to.levels.below.the.ignition.curves.

shown.in.Figure.35.4,.plus.a.safety.margin..For.analog.circuits,.this.is.typically.done.using.IS.barriers;.

however,.in.the.case.of.FF,.this.circuitry.is.either.incorporated.in.the.eldbus.power.conditioner.or.in.

the.eldbus.barrier.mounted.in.the.eld.junction.box.

.typical.FFPS.delivers.approximately.24–28.V.DC.and.currents.of.up.to.360.mA.power.to.the.eld.

segment.. is.is.well.above.all.the.explosion.limit.curves.shown.on.Figure.35.4..FF.oers.two.additional.

technologies.other.than.an.IS.power.supply.that.is.limited.to.approximately.80–110.mA.at.around.12.V.

to.address.this.problem..One.solution.is.with.IS-equivalent.power.supplies.and.the.other.is.to.reduce.the.

available.energy.level.in.the.eld.using.the.above-mentioned.eldbus.barrier.technology.

35.9.1 FISCO—Fieldbus Intrinsically Safe COncept

Unlike.the.IS.entity.concept,.which.is.derived.from.theoretical.calculations,.the.FISCO.is.based.on.

actual.eld.trials.and.experiments.by.the.Physikalisch-Technische.Bundesaanstalt.(PTB).research.cen-

ter

.in.Germany..Because.this.IEC-60079-15.standard.is.based.on.experimentation,.all.installations.must.

operate.within.the.maximum.limits.within.which.the.experiments.were.conducted.. ese.limits.are.that.

the.total.cable.length.of.the.system.is.limited.to.a.maximum.of.1000.m.in.IIC/Groups.A,.B.gases.and.

1900.m.in.IIB/Groups.C,.D.(limited.by.FF-831.and.therefore.identical.to.any.fieldbus.system)..The.

maximum.spur.length.for.any.FISCO.installation.is.60.m.per.spur.

Because

.the.FISCO.power.supply.must.operate.at.voltages.below.the.ignition.curve,.FISCO.systems.

are.not.able.to.connect.to.trunk.and.cable.combinations.that.are.as.long.as.non-IS.systems..Despite.this,.

FISCO.is.able.to.support.trunk.lengths.greater.than.500.m.with.full.system.current.loads..It.should.be.

500

Power supply characteristics

400

Short circuit current (mA)

300

200

100

12 14 16

Open circuit voltage (V)

IIB non-incendive

IIB intrinsically safe

IIC intrinsically safe

IIC non-incendive

FIGURE 35.4 Power.supply.characteristics.