Real-Time Fieldbus Communications Using Profibus Networks

Transcription

1 Real-Time Fieldbus Commuicatios Usig Profibus Networs Eduardo Tovar (Member) Departmet of Computer Egieerig, ISEP, Poltechic Istitute of Porto Rua de São Tomé, 4200 Porto, Portugal Tel.: , Fax: Fracisco Vasques (o-member) Departmet of Mechaical Egieerig, FEUP, Uiversit of Porto Rua dos Bragas, 4099 Porto Codex, Portugal Abstract This paper provides a comprehesive stud o how to use Profibus fieldbus etwors to support real-time idustrial commuicatios, that is, o how to esure the trasmissio of real-time messages withi a maximum boud time. Profibus is based o a simplified Timed Toe (TT) protocol, which is a well-proved solutio for real-time commuicatio sstems. However, Profibus differs with respect to the TT protocol, thus prevetig the applicatio of the usual TT protocol real-time aalsis. I fact, real-time solutios for etwors based o the TT protocol rel o the possibilit of allocatig specific badwidth for the real-time traffic. This meas that a miimum amout of time is alwas available, at each toe visit, to trasmit real-time messages. Coversel, with the Profibus protocol, i the worst-case, ol oe real-time message is processed per toe visit. We propose two approaches to guaratee the real-time behaviour of the Profibus protocol: a Ucostraied Low Priorit Traffic profile ad a Costraied Low Priorit Traffic profile. The proposed aalsis will show that the first profile is a suitable approach for more resposive sstems (tighter deadlies), whilst the secod allows for icreased o-real-time traffic throughput. Kewords Field buses, Protocols, Real-Time Sstems I. INTRODUCTION A recet tred i distributed process cotrol sstems is to itercoect the distributed elemets b meas of a multi-poit broadcast etwor, istead of usig the traditioal poit-topoit lis. Withi idustrial commuicatio sstems, fieldbus etwors are speciall iteded for the itercoectio of process cotrollers, sesors ad actuators, at the lower levels of the factor automatio hierarch (Fig. ). These hierarchical levels have dissimilar message flows, i terms of required respose times, amout of iformatio to be trasferred, required reliabilit ad message rates (how frequetl messages must be trasferred) []. - -

2 It is ow that time costraits are more striget as we go dow i the automatio hierarch. I the cotext of this paper, we cosider time costraits or deadlies as the maximum allowable time spa betwee a message trasfer request ad its trasmissio o the bus. I fact, the total message ccle dela results from multiple factors, such as the access ad queuig delas, the trasmissio time (frame legth / trasmissio rate) ad the protocol processig time. As we are dealig with real-time commuicatio across a shared trasmissio medium, we focus our aalsis o the access dela ad queuig dela factors. These factors deped o the Medium Access Cotrol (MAC) mechaism used b the fieldbus etwor. Plaig Level Bus ccle-time >> 000 ms Factor Level Bus ccle-time < 000 ms Cell Level Bus ccle-time < 00 ms Field Level Bus ccle-time < 0 ms CNC I/O PC AREA CONTROLLER Higher Level Networs PC Valve PLC Fieldbus Networs Drive DCS HOST Cell or Fieldbus Networs Trasmitter Field Device Figure - Commuicatio Networs withi the Factor Automatio Hierarch Differet approaches for the MAC mechaism have bee adopted b fieldbus commuicatio sstems. As sigificat examples, the Timed Toe protocol i Profibus [2], the cetralised pollig i FIP [2] ad the Carrier Sese Multiple Access with Collisio Avoidace (CSMA/CA) i CAN [3], ca be metioed. Recetl, several studies o the abilit of fieldbus etwors to cope with real-time requiremets have bee preseted, such as those b Tidell et al. [4] o CAN, Pedro ad Burs [5] ad Vasques [6] o FIP ad fiall Tovar ad Vasques [7-8] o Profibus. I this paper we address the Profibus MAC abilit to schedule messages accordig to their real-time requiremets, i order to support real-time distributed applicatios. This wor is a step forward i the pre-ru-time schedulabilit aalsis of Profibus etwors [7-8]

3 II. ESSENTIALS ON THE PROFIBUS PROTOCOL A. Geeral Characteristics Profibus was recetl cosidered as oe of the fieldbus solutios of the Geeral-Purpose Fieldbus Commuicatio Sstem Europea Stadard, the EN 5070 [2]. The Profibus MAC mechaism is based o a toe passig procedure used b master statios to grat the bus access to each oe of them, ad a master-slave procedure used b master statios to commuicate with slave statios. The Profibus toe passig procedure uses a simplified versio of the Timed Toe protocol [9]. These MAC mechaisms are implemeted at the laer 2 of the OSI referece model, which, i Profibus, is called Fieldbus Data Li (FDL). I additio to cotrollig the bus access ad the toe ccle time (a feature that will be later explaied i sectio II.B), the FDL is also resposible for the provisio of data trasmissio services for the FDL user (e.g., the applicatio laer). Profibus supports four data trasmissio services: Sed Data with No acowledge (SDN); Sed Data with Acowledge (SDA); Request Data with Repl (RDR) ad Sed ad Request Data (SRD). The SDN is a uacowledged service used for broadcasts from a master statio to all other statios o the bus. Coversel, all other trasmissio services are based o a real dual relatioship betwee the iitiator (master statio holdig the toe) ad the respoder (slave or master statio ot holdig the toe). A importat characteristic of these services is that the are immediatel aswered, with a respose or a acowledgemet. This feature, also called "immediate-respose", is particularl importat for the real-time bus operatio. I additio to these services, idustrial applicatios ofte require the use of cclical trasmissio methods. A cetrall cotrolled pollig method (cclical pollig) ma be used to sca simple field devices, such as sesors or actuators. Profibus eables a poll list to be created at the FDL laer, allowig the executio of cclical pollig services based o RDR ad SDR services. A importat Profibus cocept is the message ccle. A message ccle cosists of a master's actio frame (request or sed/request frame) ad the associated respoder's acowledgemet or respose frame. User data ma be trasmitted i the actio frame or i the respose frame. All the statios, except the toe holder (iitiator), shall i geeral moitor all requests. The acowledgemet or respose must arrive withi a predefied time, the slot time, otherwise the iitiator repeats the request. At the etwor set-up phase, the maximum umber of retries, before a commuicatio error report, must be defied i all master statios

4 The Profibus real-time aalsis preseted i this paper is based o the owledge of the message ccle duratio. This duratio must iclude the time eeded to trasmit the actio frame ad receive the related respose, ad also the time eeded to perform the allowed umber of message retries. B. Behaviour of the Medium Access Cotrol (MAC) Oe of the mai fuctios of the Profibus MAC is the cotrol of the toe ccle time. After receivig the toe, the measuremet of the toe rotatio time begis. This measuremet expires at the ext toe arrival ad results i the real toe rotatio time (T RR ). A target toe rotatio time (T TR ) must be defied i a Profibus etwor. The value of this parameter is commo to all masters, ad is used as follows. Whe a statio receives the toe, the toe holdig time (T TH ) timer is give the value correspodig to the differece, if positive, betwee T TR ad T RR. Profibus defies two categories of messages: high priorit ad low priorit. These two categories of messages use two idepedet outgoig queues. If at the arrival, the toe is delaed, that is, the real rotatio time (T RR ) was greater tha the target rotatio time (T TR ), the master statio ma execute, at most, oe high priorit message ccle. Otherwise, the master statio ma execute high priorit message ccles while T TH > 0. T TH is alwas tested at the begiig of the message ccle executio. This meas that oce a message ccle is started it is alwas completed, icludig a required retries, eve if T TH expires durig the executio. We deote this occurrece as a T TH overru. The low priorit message ccles are executed if there are o high priorit messages pedig, ad while T TH > 0 (also evaluated at the start of the message ccle executio, thus leadig to a possible overru of T TH ). Apart from distiguishig high ad low priorit message ccles, the Profibus MAC differetiates three subtpes of low priorit message ccles: poll list, o-cclic low priorit (applicatio laer ad remote maagemet services) ad Gap List message ccles. The Gap is the address rage betwee two cosecutive master addresses, ad each master must periodicall chec the Gap addresses to support damic chages i the logical rig. After all high priorit messages have bee carried out, poll list message ccles are started. If the poll ccle is completed withi T TH, the requested low priorit o-cclical messages are the carried out, ad a ew poll ccle starts at the ext toe arrival with available T TH. If a poll ccle taes several toe visits, the poll list is processed i segmets, without isertig requested low priorit o-cclical messages. Low priorit o-cclical message ccles are carried out ol at the ed of a complete poll ccle. At most oe Gap address is checed per toe visit, if there is still available T TH, ad there are o pedig messages

5 Fig. 2 sthesises the Profibus MAC message hadlig procedures, where pl_le stads for the legth of the poll list. Toe Receipt Reset ad Release T RR (up) T TH T TR - T RR Release T TH (dow) T TH < 0? high priorit message? high priorit message? m = pl_le? process high priorit message process high priorit message m = m = m + T TH < 0? poll Poll_List member T TH < 0? m = pl_le? gap update low priorit message? process low priorit message T TH < 0? Toe Pass Figure 2 - Profibus MAC Message Hadlig Procedures III. BASIC CONCEPTS OF THE PROFIBUS MAC TIMING ANALYSIS A. Networ ad Message Models We cosider a bus topolog cotaiig master statios. A special frame (the toe) circulates aroud the logical rig formed b the masters. We deote the logical rig latec (toe wal time, icludig ode latec dela, media propagatio dela, etc.) as τ. Message ccles geerated i the sstem at ru-time ma be put ito a high priorit outgoig queue (high priorit messages) or ito a low priorit outgoig queue (low priorit messages). We - 5 -

6 defie h (α) high priorit message streams i each master α. A message stream correspods to a sequece of message ccles related with, for istace, the readig of a specific process variable. We deote the β th (β =, 2, h (α) ) high priorit stream associated to a master α as Sh (α) β. A high priorit message stream Sh (α) β is characterised as Sh (α) β = (Ch (α) β, Dh (α) β ). Ch (α) β is the maximum amout of time required to perform a high priorit message ccle (as defied i IIA) of stream β i master α. Dh (α) β is the message's relative deadlie, which is the maximum amout of time that ma elapse betwee the message trasfer request ad its trasmissio o the bus. We cosider that, i the worst-case, the deadlie ca be see as the miimum iter-arrival time betwee two cosecutive message requests i the same stream. As we do ot mea to guaratee a deadlies for the low priorit traffic, we assume ol oe low priorit message stream per master α, which gathers all the o-high priorit traffic issued b that master. Thus, we characterise a low priorit message stream as Sl (α) = (Cl (α), lp (α) ). Cl (α) is the maximum amout of time required to perform a low priorit message ccle i master α. lp (α) is the maximum umber of o-high priorit message ccles that a master α is allowed to perform at each toe visit, which is a meaigful parameter to costrai the low priorit traffic. The followig stream otatio will the be used: β ( Chβ Dhβ ) α ( α ) ( Cl, lp ) ( α ) ( α ) ( α ) Sh =, ( α ) ( ) Sl = (2) B. Real-Time Characteristics of the Profibus Protocol Compared to the Timed Toe protocol [9], the mai differece of the Profibus toe passig cosists i the absece of schroous badwidth allocatio (H i ). I the Timed Toe protocol this is a relevat statio parameter, sice it specifies the amout of time a statio has to trasfer its real-time traffic. I Profibus, if a master receives a late toe (T RR was greater tha T TR ) ol oe high priorit message ma be trasmitted. As a cosequece, i Profibus, low priorit traffic ma drasticall affect the high priorit traffic capabilities. I fact, if the low priorit traffic is ot costraied whe a master receives a earl toe (T RR smaller tha T TR ), the master ma use all the available time (T TH = T TR T RR ) to process low priorit traffic, delaig the toe rotatio. I this case, the subsequet masters ma be limited to ol oe high priorit message trasmissio whe holdig the toe. Figure 3 illustrates this situatio

7 I Profibus, the absece of schroous badwidth allocatio prevets the use of the traditioal Timed Toe real-time aalsis. I fact, real-time solutios for etwors based o the Timed Toe protocol, such as [0,] (for FDDI etwors) or [2] (for IEEE toe bus), rel o the possibilit of allocatig specific badwidth for the real-time traffic. T TR Master Master 2 Master 3 Toe Arrival Message Ccle with T TH > 0 Toe Trasmissio Message Ccle with T TH < 0 Figure 3 - Example of Profibus Toe Use C. Approach Used i the Profibus Timig Aalsis The major cotributio of this paper is to prove that, despite the absece of schroous badwidth allocatio, it is possible to guaratee the real-time behaviour of the high priorit traffic with the Profibus protocol. We propose the two followig approaches: A Ucostraied Low Priorit Traffic profile, where real-time traffic requiremets are satisfied eve whe ol oe high priorit message is trasmitted per toe visit, idepedetl of the low priorit traffic load; A Costraied Low Priorit Traffic profile where, b cotrollig the umber of low priorit message trasfers, all pedig real-time traffic is trasmitted at each toe visit. The proposed aalsis will demostrate that the first profile is a suitable approach for more resposive sstems (tighter deadlies), whilst the secod allows for a icreased o-real-time traffic throughput. IV. SETTING T TR FOR THE UNCONSTRAINED LOW PRIORITY TRAFFIC PROFILE I this profile, we cosider the followig assumptio as the basis for the aalsis: the trasmissio of oe high priorit message per toe arrival is alwas guarateed, idepedetl of the low priorit traffic load. Uder this assumptio, the queuig of outgoig messages will ver much impact the realtime properties of the sstem. We will cosider i our aalsis both priorit based queuig ad - 7 -

8 First-I-First-Out (FIFO) queuig. This last is the oe ormall used i Profibus implemetatios. Our aalsis is based o the defiitio of a deadlie costrait ad, usig that deadlie costrait, the defiitio of a upper boud for the T TR parameter. I this cotext, the deadlie costrait is defied as the coditio that must be satisfied i order to guaratee that all real-time messages are trasmitted before their deadlies, i.e., before the ed of the miimum iter arrival time betwee two cosecutive message arrivals. A. Deadlie Costrait with FIFO Outgoig Queues Betwee two cosecutive toe visits, several messages from distict streams Sh (α) β ma be put ito the FIFO outgoig queue. It is a premise that the differet messages i the FIFO outgoig queue belog to distict message streams, otherwise a deadlie was missed. I a worst-case sceario, assumig that each master will be able to trasmit ol oe message per toe visit, if ϕ messages are to be trasmitted, this trasmissio will ol be completed after ϕ toe visits to that master. The critical case is whe the message with the shortest value for Dh (α) β becomes the last oe i the FIFO outgoig queue. Fig. 4 illustrates this sceario. Maximum Waitig Time for Mh Mh 4, Mh 3, Mh 2, Mh Master Shortest Deadlie Ch 4 Ch 3 Ch 2 Ch Master 2 Master 3 Master 4 T TR - τ T TR Figure 4 - Example of a Maximum Waitig Time i the FIFO Outgoig Queue I this sceario, whe master seds the toe to master 2, messages from all the h streams arrive at the FIFO outgoig queue i the followig order: Mh 4, Mh 3, Mh 2, Mh. Cosiderig that the message deadlies are as follows: Dh 4 > Dh 3 > Dh 2 > Dh, the most striget message (Mh ) ma wait util the 4 th toe visit to be trasmitted

9 We deote the Maximum Waitig time for a message at the master as T () MW. We also deote the maximum boud of the toe iter-arrival time at a master as T ccle. As a cosequece, i a master with h () high priorit message streams, T () MW will be: MW T = h T ccle A deadlie costrait ca the be formulated as follows: mi Dhi TMW, master i.. h ( ) = That is, real-time message deadlies are guarateed if all the message deadlies i all masters are greater tha or equal to T () MW. Combiig (3) ad (4) we ca re-write the deadlie costrait as: (3) (4) Dh h mi i ccle, master i.. h ( ) = T (5) B. Evaluatio of T ccle I order to appl the deadlie costrait (5), we eed to estimate T ccle, which is the upper boud for T () RR (i.e., the maximum elapsed time betwee two cosecutive toe arrivals to the master ). The real toe rotatio time (T () RR ) will be smaller tha T TR, except if oe or more masters i the logical rig cause the toe to be late. The mai cause for the toe lateess is the overru of the T () TH. The toe lateess ma be worseed if the followig masters, havig received a late toe, still trasmit, each oe, oe high priorit message. () Cosiderig the worst-case sceario i which a master overrus its T TH ad the followig masters util master - (modulo ) receive ad use a late toe, the maximum toe lateess (T del ) ca be defied as: ( ) T del = C M where C M () stads for the logest message ccle associated to each master : Thus, T ccle ca the be defied as: C = = max max Ch i i=.. h, Cl M ccle TR del (7) (8) T = T + T (9) To illustrate the T ccle evaluatio, assume the followig sceario: after a toe ccle without message trasmissios, master receives the toe (T () TH = T TR - τ, sice T () RR = τ). This master ca actuall hold the toe durig T () TH plus the duratio of its logest message. I this situatio - 9 -

10 all the followig masters i the logical rig will receive a late toe, thus ol beig able to process, at most, oe high priorit message ccle. Figure 5 illustrates this sceario. T ccle = T TR + C i + Ch i (2) + Ch i (3) Master T TH = T TR - τ C i Master 2 Ch i (2) Master 3 τ/3 Ch i (3) Figure 5 - Toe Iter-arrival Time A more accurate defiitio for T ccle ca be foud i [3], where factors such as the differet message ccle legths ad the relative positio of each master i the logical rig are tae ito cosideratio for the defiitio of T ccle ], which ma have a differet value i each master. C. Settig T TR with FIFO Outgoig Queues The deadlie costrait (5) ca ow be re-writte as: mi ( Dh ) i=.. h i h ( TTR + Tdel ), master which meas that for guarateeig real-time traffic, the T TR parameter is bouded as follows, 0 T TR mi mi Dh i.. h h i= T This expressio imposes a upper boud value for T TR. The lower boud for T TR is 0 as, b defiitio, T TR ca ot be egative. From (), a egative result for T TR would mea that the etwor high priorit traffic would ot be schedulable. It ca be see that the upper boud for T TR decreases as deadlies get tighter or the umber of high priorit streams per master (h () ) icreases. The maximum toe lateess (T del ) is also a factor to cosider, sice it ver much affects the T TR parameter. D. Usig Priorit Outgoig Queues The major drawbac of FIFO outgoig queues to support the high priorit message streams, is the priorit iversio problem, i.e., the message with the most striget deadlie ca be the del (0) () - 0 -

11 last oe i the FIFO queue ad thus it will be delaed util all the other high priorit pedig messages are trasmitted. This drawbac ca be overcome if priorit queuig is used istead of FIFO queuig. We propose the use of a deadlie based priorit algorithm to sort the outgoig queue, i order to guaratee that the first message to be set is the oe with the earliest deadlie. This ca be cosidered as a FDL implemetatio issue, provided that time iformatio is supplied to the FDL. E. Deadlie Costrait with Outgoig Priorit Queues Profibus high priorit message streams (Sh i () ) are characterised b their relative deadlies (Dh i () ) ad their time legth (Ch i () ). If we assume the worst-case iter-arrival time, these message streams ca be see as periodic message streams. It ca be show that for periodic tass, there is a feasible schedule if ad ol if there is a feasible schedule for the LCM (least commo multiple) of the periods [4]. Moreover, it ca be show that if tass share a commo request time, a schedulabilit sufficiet coditio is that tass are schedulable for the logest of the periods [5]. Thus, cosiderig the aalog betwee tass ad messages, it is sufficiet to aalse the schedulabilit of the high priorit message streams for a time spa (T spa () ) correspodig to: T spa ( i=.. h ) { Dh } = max i (2) Cosiderig also that, i the worst-case, the toe will visit master each T ccle (9), the umber of toe visits ( ) N tvisits durig T () spa is give b: N tvisits T = T spa ccle I (3), the mius oe results from the worst-case sceario where message ccles are passed to the outgoig queue just after the toe beig released, ad the floor fuctio results from T spa () ot beig a multiple of T ccle. Figure 6 illustrates these coditios. Cotrar to the FIFO outgoig queue case, it ca be show that if the umber of message requests durig T spa () is bouded to N ( ) tvisits (3), the orderig the outgoig queue usig the earliest deadlie schedulig algorithm [5] is a sufficiet coditio to guaratee message deadlies. I fact, defiig the cocept of toe use rate as the percetage of toe visits used to trasmit high priorit messages, aalogies with the case of tas schedulig i a moo-processor eviromet ca be made. - -

12 Toe arrival max{dh i () } Master First toe visit just before messages have bee passed to the outgoig buffer T ccle () Thus the floor fuctio Figure 6 - Toe Visits to Master withi T spa I [5] Liu ad Lalad showed that for a give set of m tass, the earliest deadlie schedulig algorithm is feasible if ad ol if: m i= C T i i (4) where C i represets the processor ruig time of tas i ad T i represets the periodicit of tas i. I other words, the set of tass is schedulable if the processor utilisatio factor (fractio of processor time used to process tass) is less tha 00%. Cosiderig that the toe use rate must be smaller tha 00%, the umber of high priorit message requests arisig durig the logest deadlie (or period, as we assume the deadlie equal to the worst-case iter-arrival time) iterval, must be smaller tha N ca the be formulated as follows: ( h ) i= T Dhi spa N tvisits, master ( ) tvisits. A deadlie costrait Figure 7 shows a example of a master with 4 high priorit streams ad illustrates the schedulig, usig both priorit ad FIFO outgoig queues. (5) Deadlie = 2 x Tccle Deadlie = 4 x Tccle Deadlie = 8 x Tccle Deadlie = 8 x Tccle Max{Dh} Tccle Deadlie Miss Requested Toe Visit Trasmitted Low Priorit Queue Hi Last FIFO Queue First Figure 7 - Comparative Schedulig Example - 2 -

13 F. Maximum Boud for T TR with Outgoig Priorit Queues Combiig (3) ad (5) we have, h i= T Dhi spa ( i) Tspa TTR + T del, master which allows the set of the T TR for guarateeig the real-time message deadlies. (6) G. Some Cosideratios about the Ucostraied Low Priorit Traffic Profile With this profile, as the aalsis is based o a worst-case sceario of oe high priorit message ccle per toe arrival, we propose a toe ccle time as small as possible, that is, a short T TR. Therefore, as each master will have a more frequet opportuit to trasfer its high priorit messages, this profile seems a suitable approach for resposive sstems, with few high priorit message streams with striget deadlies. Comparig the use of FIFO ad priorit based outgoig queues, the mai differece is that i the former a more urget message ma have to wait several toe arrivals, whilst i the latter, such message would be the first oe to be trasmitted. V. SETTING T TR FOR THE CONSTRAINED LOW PRIORITY TRAFFIC PROFILE I this profile we cosider the followig assumptio as the basis for the aalsis: at each toe arrival, a master must be able to execute all pedig high priorit message ccles. Uder this assumptio, low priorit traffic must be costraied at each master, i order to guaratee the real-time properties of the sstem. A. Defiitio of the Deadlie Costrait Cosiderig that all real-time messages are to be set at each toe visit, the deadlie costrait ca be defied as: mi Dhi ( i=.. h ) T ccle, master Furthermore, as each master must be able to trasmit both all high priorit traffic ad all allowed low priorit traffic, a upper boud for the real toe rotatio time ca be defied as follows: where ( Cl ) Tccle h = i= Ch ( lp Cl ) + τ i + = lp correspods to the low priorit allowed traffic for master, i.e., the maximum amout of low priorit traffic which ca be trasmitted at each toe visit. The deadlie costrait ca the be re-writte as follows: (7) (8) - 3 -

14 ( lp Cl ) + τ h mi Dhi Chi + (9) i, = i= = B. Settig the T TR Parameter T TR must be set i order to guaratee that, at the toe arrival, there will be alwas eough time to execute all the pedig high priorit traffic. This meas that, at the toe arrival, T TH = T TR - T RR must be sufficiet to trasmit all high priorit traffic, i.e.: T TR Tccle+ max =.. sice T ccle is the upper boud of T RR. Figure 8 illustrates a sceario with 3 masters, where h =2, h (2) =2, h (3) =2, lp =3, lp (2) = ad lp (3) =4. Combiig (8) ad (20), a lower boud for T TR is give b: h ( ) h ( ) TTR Chi + ( lp Cl ) + τ + max Chi (2) =.. = i= = i= Expressios (9) ad (2) are the basis for settig the T TR parameter for the Costraied Low Priorit Traffic profile. It ca be see that there is o upper boud for T TR sice, as there is a upper boud for T RR (20), expressio (9) is a sufficiet coditio to guaratee high priorit message deadlies. h i= Ch Miimum T TR i (20) Maximum Ccle Time Master Ch Ch 2 Cl Cl Cl Ch Ch 2 Master 2 Ch (2) Ch 2 (2) Cl (2) Master 3 Ch (3) Ch 2 (3) Cl (3) Cl (3) Cl (3) Cl (3) Figure 8 - Ccle Time Example for the Costraied Low Priorit Profile C. Some Cosideratios about the Costraied Low Priorit Traffic Profile I this profile, as each master must be able to execute all its pedig real-time traffic at each toe visit, the toe ccle time will be much loger tha i the Ucostraied Low Priorit Traffic profile case. Therefore, as each master will eed to wait more time to trasmit its high priorit messages, the supported message deadlies are ecessaril looser tha those supported b the Ucostraied Low Priorit Traffic profile

15 However, with the Costraied Low Priorit Traffic profile, there is more available time trasmit low priorit traffic, which allows for a icreased o-real-time traffic throughput. VI. IMPLEMENTATION ISSUES I this sectio, we give some guidelies cocerig implemetatio issues about the Costraied Low Priorit Traffic profile. We propose two differet alteratives:. The first is based o the implemetatio of a low priorit message couter at the MAC level, iteded to cotrol the umber of trasferred low priorit messages per toe visit. 2. The secod is based o the applicatio level cotrol of low priorit services, such as applicatio laer o-cclical low priorit services ad remote maagemet services. A. Cotrollig Low Priorit Traffic at the MAC Level We defie at each master the maximum umber of low priorit messages to be trasferred (lp () ), per toe visit. The low priorit traffic is the cotrolled b meas of a low priorit messages couter (lp_c). Figure 9 illustrates such implemetatio (refer to figure 2 for the traditioal Profibus implemetatio). B. Cotrollig Low Priorit Traffic at the Applicatio Laer Level Cocerig the traffic geerated explicitl b the user, it is ot advisable to use the Live List maagemet service. The Live List service requests the FDL status of all statios (masters ad slaves). It will geerate multiple frames i the etwor. If, i the worst-case, ever master statio requests a Live List, expressio (8) would the be as follows: T ccle h = i= Ch ( lp Cl ) + + i + = + s τ C (22) where C live stads for a request status message ccle legth ad + s correspods to the aggregate umber of master ad slave statios. This would lead to a much loger T ccle. Cocerig low priorit o-cclical services, the applicatio process must be able to accept a T ccle parameter, i order to cotrol the umber of low-priorit messages (lp () ) geerated i the master. Expressio (8) must cosider the ifluece of the Gap updatig (see II.B). We deote C gap as the legth of a Gap maiteace message ccle. Expressio (23) icludes the ifluece i T ccle resultig from the Gap maiteace, cosiderig that, i the worst-case, each master executes oe Gap maiteace message ccle i each toe visit. = live - 5 -

16 Toe Receipt Reset ad Release T RR (up) T TH T TR - T RR Release T TH (dow) T TH < 0? high priorit message? process high priorit message high priorit message? process high priorit message pl_c=0 m = pl_le? m = m + m = T TH < 0? poll Poll_List member lp_c=lp_c+ lp_c=lp? T TH < 0? m = pl_le? lp_c=lp? gap update low priorit message? process low priorit message lp_c=lp_c+ lp_c=lp? T TH < 0? Toe Pass Figure 9 - Implemetatio of a Low Priorit Message Couter at the MAC Level I order to support Poll List, a additioal term must be added to (8). The user must be careful usig Poll Lists, sice Poll List messages are FDL triggered. Thus, the whole list legth must be cosidered for the evaluatio of T ccle : T ccle h = i= Ch ( lp Cl ) + + C gap + i + = where C poll () stads for the master Poll List legth = poll τ C (23)

17 VII. NUMERICAL EXAMPLE We cosider a sceario with 6 master statios with differet timig requiremets (table ). Master Master 2 Master 3 Master 4 Master 5 Master 6 () Dh () Dh 2 () Dh 3 50ms (Dmi) 90ms 20ms 60ms 60ms 80ms 00ms 80ms 30ms 200ms 00ms 80ms ms 0ms 40ms 00ms 00ms C 2ms 2ms 2ms 2ms 2ms 2ms lp () Table - Numerical Example For simplificatio, we assume that the maximum message legth is, i all cases, equal to 2ms. Usig a Mbps etwor, ad if request ad respose frames total 400 bits, the frame duratio is 400µs. Cosiderig 260µs for commuicatio stac ad propagatio dela, each message ccle will tae 660µs. Cofigurig each master to support up to 2 message replies, we get the 2ms figure for the total legth of the message ccle. We also assume that τ = 0,ms. I order to assess the resposiveess of each proposed profile, the miimum supported relative deadlie of message stream Sh will be evaluated. A. Ucostraied Low Priorit Traffic Profile for the FIFO Queue Case From (7) T del = 6 2 = 2 ms. I this case, T TR upper boud () is imposed b master 4 (or 60 master 5), with T TR 2 = 8 ms. / = 4 3 Usig (0), the miimum relative deadlie for Sh is either D mi/t TR = 8 = 2 20 = 40 ms for T TR = 8ms, or D mi/t TR = 0 = 2 2 = 24 ms for T TR = 0ms (i this case it would ot be possible to trasmit low priorit messages). B. Ucostraied Low Priorit Traffic Profile for the Priorit Queue Case Usig(6), we have: TTR TTR TTR / = = 3ms 40 2 = 23ms / = = 20.5 ms / = T TR = / = 4 T TR T TR Therefore, the upper boud for T TR is 3 ms. 2.3 ms 00 2 = 3ms / = = 3ms / =

18 A value for Dmi ca also be evaluated usig (6). The miimum relative deadlie for Sh is either D mi/t TR = 3 = 00/ 2.99 = 33.33ms for T TR = 3ms, or D mi/t TR = 0 = 00/ 6.99 = 4.3 ms for T TR = 0 ms (o trasmissio of low priorit messages). C. Costraied Low Priorit Traffic Profile From (2) we ca evaluate the lower boud for T TR : T TR = 76.ms From (9), we ca determie the value for Dmi: D mi/lp = = 70.ms This last result meas that the message stream set show i table is ot schedulable usig the Costraied Low Priorit profile. I fact, with this profile, the most striget deadlie must be at least 70. ms, larger tha both Dh ad Dh (4). VIII. CONCLUSIONS The major cotributio of this paper is to prove that is possible to guaratee real-time commuicatio behaviour usig Profibus fieldbus etwors, thus allowig its use to support distributed computer-cotrolled sstems. I order to achieve real-time commuicatio behaviour usig Profibus etwors, we propose two distict approaches: a Ucostraied Low Priorit Traffic profile ad a Costraied Low Priorit Traffic profile. For both profiles we derive deadlie costraits, which, if verified, guaratee real-time messages deadlies. We also propose the implemetatio of a deadlie based priorit mechaism for the high priorit outgoig queue, which sigificatl improves the real-time characteristics of the Ucostraied Low Priorit Traffic profile. We discuss two optioal implemetatio schemes for costraiig the low priorit traffic i the Costraied Low Priorit Traffic profile. The additio of a low priorit message ccle couter to the Profibus MAC is, for the authors, a more elegat approach to implemet this profile. ACKNOWLEDGEMENTS This wor was partiall supported b FLAD uder the project SISTER 47/97 ad b ISEP uder the project REMETER. The authors would also lie to acowledge Artur Cardoso for the useful commets o a previous versio of this paper

19 REFERENCES [] J.-D. Decotigie ad P. Pleievaux, A Surve o Idustrial Commuicatio Networs, i Aales des Télécommuicatios, ivited paper, Vol. 48, No. 9-0, pp , 993. [2] EN 5070, "Geeral Purpose Field Commuicatio Sstem", Europea Stadard, CENELEC, Jul 996. [3] SAE J583, "Cotroller Area Networ (CAN), a I-Vehicle Serial Commuicatio Protocol", SAE Hadboo, Vol. II, 992. [4] K. Tidell, H. Hasso ad A. Welligs, Aalsig Real-Time Commuicatios: Cotroller Area Networ (CAN), i Proceedigs of the IEEE Real Time Sstems Smposium (RTSS 94), pp , S.Jua, Puerto Rico, December 994. [5] P. Pedro ad A. Burs, Worst Case Respose Time Aalsis of Hard Real-Time Sporadic Traffic i FIP Networs, i Proceedigs of 9 th Euromicro Worshop o Real-time Sstems, Toledo, Spai, Jue 997. [6] F. Vasques, Sur L Itégratio de Mécaismes d Ordoacemet et de Commuicatio das la Sous-Couche MAC de Réseaux Locaux Temps-Réel, PhD Thesis, available as techical Report LAAS Nº 96229, Toulouse, Frace, Jue 996. [7] E. Tovar ad F. Vasques, Guarateeig Real-Time Message Deadlies i Profibus Networs, i proceedigs of the 0 th Euromicro Worshop o Real-time Sstems, Berli, Germa, Jue 998. [8] E. Tovar ad F. Vasques, Settig Target Rotatio Time i Profibus Based Real-Time Distributed Applicatios, submitted to the 5 th IFAC Worshop o Distributed Computer Cotrol Sstems (DDCS'98), Como, Ital, September 998. [9] R. M. Grow, A Timed Toe Protocol for Local Area Networs, i Proceedigs of Electro 82, Toe Access Protocols, Paper 7/3, Ma 982. [0] G. Agrawal, B. Che, W. Zhao ad S. Davari, Guarateeig Schroous Message Deadlie with the Timed Toe Medium Access Cotrol Protocol, IEEE Trasactios o Computers, Vol. 43, No. 3, pp , March 994. [] Q. Zheg ad K. G. Shi, Schroous Badwidth Allocatio i FDDI Networs, IEEE Trasactios o Parallel ad Distributed Sstems, Vol. 6, No. 2, pp , December 995. [2] P. Motuschi, L. Cimiiera ad A Valezao, Time Characteristics of IEE802.4 Toe Bus Protocol, IEE Proceedigs, Vol. 39, No., pp. 8-87, Jauar 992. [3] E. Tovar ad F. Vasques, Ccle Time Properties of the PROFIBUS Timed Toe Protocol, submitted to IEE Proceedigs Software, August 998. [4] E. Lawler ad C. Martel, Schedulig Periodicall Occurrig Tass o Multiple Processors, Iformatio Processig Letters, 2, Februar 98. [5] C. Liu ad J. Lalad, Schedulig Algorithms for Multiprogrammig i Hard-Real-Time Eviromet, Joural of the ACM, Vol. 20, No., pp.46-6, Jauar