The effects of acyclic traffic on Profibus DP networks

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1 he effects of acyclic traffic on Profibus P networks. Vitturi LAEB-CR, c.so tati Uniti 4, Padova, Italy el: Fax: vitturi@ladseb.pd.cnr.it IERAL REPOR OVEMBER 2000 Abstract Profibus P is a very popular fieldbus included in the IEC International tandard. he features of such a fieldbus make it particularly suitable for cyclic operations, but it also foresees the possibility of performing acyclic activities between masters and slaves. A second (undesired) type of acyclic traffic is that generated by the access to services of the Profibus data link layer protocol (Fieldbus ata Link, FL), by users different from the P protocol. he acyclic traffic can heavily influence the network performances as it introduces a jitter on the cycle time which affects the cyclic operations. In this paper an analytical expression of the Profibus P cycle time is calculated and the effects of both types of acyclic traffic are evaluated. Besides, a technique to eliminate the jitter and to synchronise the network operation is proposed. his solution, which requires only slight changes to the Profibus P protocol, is compatible with the existing applications. 1

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3 1. Introduction Profibus P is a fieldbus designed for use at low level of factory automation systems, where it performs high speed data exchange between process controllers and field devices, such as sensors and actuators, in a master-slave configuration. he standardisation process of Profibus P began with the German national standard I part 3, [1] issued in ubsequently it was included, together with other fieldbuses, in the European tandard E50170 [2] which was updated in 1998 with an extension, named PV1 [3], whose major feature was the introduction of acyclic communication services. Finally, at the beginning of 2000, a complete (and revised) version of Profibus P became part of the IEC international fieldbus standard [4], [5]. he standard specifies mainly three types of devices which can be connected to the network: class 1 masters, class 2 masters and slaves. Class 1 masters are devices usually performing automation and control tasks such as personal computers or programmable controllers, their operation consists typically in polling slave devices and executing control programs. Class 2 masters are configuration devices used mainly for network management tasks. As this paper will not analyse the operation of networks including class 2 masters, in the following the term master will be used intending exclusively class 1 master devices. laves are peripheral devices exchanging input-output octets with the masters. In order to implement the functions specified by the protocol, Profibus P uses the services offered by the data link layer, named Fieldbus ata Link, FL, whose features are reported in [2]. FL specifies a medium access technique based on a token passing procedure which is very close to that standardised by IEEE 802.4, token bus [6]. he data link layer protocol grants a station in possess of the token the right to access the network. he token is circulated among active stations which form a logical ring, but the network can connect also passive stations which never receive the token and consequently can only answer to queries directed to them. he token can be held by a station for a time not greater than the token holding time H, calculated as the difference between the target token rotation time, R (a value set on all the stations which is the upper bound of the token circulation period) and the real token rotation time (the duration of a complete token rotation). 3

4 here are, however, some important differences between FL and IEEE 802.4; in particular FL handles only two levels of message priorities, high and low, while token bus defines four classes of service with a correspondent number of priorities. Moreover, FL, contrarily to token bus, always grants a station that received the token the execution of an high priority message request even if the calculated token holding time is null or negative. A FL active station can act as a Profibus P master if adequately equipped with the software implementing the protocol, but it can also be configured as a P slave; FL passive stations, instead, can only act as P slaves. he data transfer services available at FL level are of connectionless type either confirmed or unconfirmed, similar to those supported by the ype-3 operation of the Logical Link Control (LLC) protocol [7]. Furthermore, FL foresees explicitly the possibility of executing cyclic services which are handled on the basis of a polling list supplied by the user. he operation of a Profibus P network is mainly based on the cyclic polling performed by master devices on the slaves by means of which process data are exchanged periodically; the standard, however, gives also the possibility of performing acyclic actions necessary for handling unpredictable situations such as alarms. his acyclic traffic influences the duration of the network cycle time with the introduction of a jitter which affects the scheduled value for cyclic operation. Moreover, acyclic traffic can also be generated by FL applications which use the protocol services for exchanging data not related to the P applications. he cycle time and the jitter are probably the most important features which have to be taken into consideration when analysing the operation of a fieldbus at device level. In particular, the cycle time has to be kept as short as possible in order to assure the fastest and correct polling of sensor and actuators, while the jitter has to be very low especially for applications where a good synchronisation is needed such as, for instance, motion control applications. his paper analyses in detail the behaviour of Profibus P networks giving particular attention to the effects of the acyclic traffic; moreover, some suggestions for optimising the network operation and reducing the jitter are given. In detail, the paper is organised as follows: section 2 gives a description of the Profibus P protocol, with particular attention to the techniques used for the mapping of protocol functions onto FL services. ection 3 shows the calculation of the cycle time and analyses the two causes that generates jitter: the P acyclic services and the FL traffic. 4

5 ection 4 makes some considerations on how operate correctly a Profibus P network and proposes a modification to the standard which eliminates the jitter and synchronises the network operation with user applications. 2. Profibus P Profibus P is a protocol originally conceived for cyclic data exchange between master and slave devices. In particular, the standard specifies that, when a master is in possess of the token, it executes a polling cycle on the slaves which were been previously initialised by that master. uring a polling cycle, each slave receives from the master its output octets and sends back the input octets. he amount and the type of these octets are specified by means of a configuration message which is sent during initialisation from master to slave before entering the data exchange phase. wo specific global control commands named sync and freeze are available at the masters and allow for simultaneous sending of the outputs and updating of the inputs. uring the cyclic data exchange, each slave, when polled, has the possibility of notifying the presence of a diagnostic message: in this situation the master, before its token holding time expires, will automatically send the diagnostic message read requests to the slaves. he PV1 extension of the protocol gives, to the masters, the possibility of performing acyclic functions on the slaves: in detail, it introduces services for acyclic read and write operations and for the acknowledgement of alarms previously received from the slave with a diagnostic message. As the token does not circulate between slave stations, the acyclic operations can be undertaken only by the masters: they are executed at the end of a polling cycle if there is sufficient token holding time. he Profibus P protocol resident on a master device is composed of two fundamental applications named user-interface and direct data link mapper. he user-interface represents the core of the protocol and is responsible for the correct execution of all operations foreseen by the standard, such as for instance the polling of the slaves and the interface with user applications. he direct data link mapper has the task of mapping the requests coming from the user-interface onto FL services. o this purpose, it is important to point out that, contrarily to what could be expected, the facilities for handling cyclic operations supplied by FL are not used by the P protocol for the cyclic polling of the slaves. he reason is that FL handles the poll lists (used by the cyclic services) exclusively with low priority service requests, while, in order to ensure the complete execution of a polling cycle at each token receipt, these requests must be of high priority. 5

6 Profibus P uses two FL services for implementing its functions: end and Request ata with reply (R) and end ata with o acknowledge (). R is a confirmed connectionless service which allows for the sending of up to 246 octets; the response frame of such a service can also carries up to 246 octets of data. R is used for all the Profibus P functions which involve data transmission between master and slaves, such as cyclic data exchange, diagnostic, parametrisation, acyclic read and write, etc. is a non confirmed connectionless service which is used by the Profibus P protocol in order to issue global control commands from master to slave. he FL services used by the Profibus P protocol can be either of priority high or low depending on the functions they implement. In detail, with refer to the most important functions: the diagnostic reading and the cyclic data exchange are mapped onto R services which have high priority when issuing the request primitive and low priority when receiving the confirm primitive (this latter becomes of high priority when a slave has to signal the presence of diagnostic). Acyclic read and write, alarm acknowledgement, and master-master services use, instead, low priority frames. A very important parameter which has to be taken into consideration when using Profibus P networks is the min_slave_interval: it represents the minimum time which has to elapse between two subsequent queries of the same slave. his parameter is an internal hardware feature of each slave which has to be made available to the masters during the network initialisation phase. For simplicity in the following of this document it will be assumed that the minimum slave intervals have the same value for all the slaves. Fig. 1 reports the procedure followed by a Profibus P master station for handling the message requests to send to FL. 6

7 start min slave interval elapsed YE O wait polling cycle slaves have diagnostic YE O min slave interval elapsed YE execution of diagnostic requests O wait acyclcic message requests YE O min slave interval elapsed YE execution of acyclic message requests O wait master-master requests YE O execution of mastermaster requests Fig. 1: Profibus P master message requests to FL As can be noticed, the min_slave_interval is always checked before every slave query and, if it is not elapsed, a sufficient time must be waited: this is realised sending idle function requests to FL (which are typically mapped onto services with destination address equal to source address). 7

8 he FL protocol of a station handles the service requests coming from the P application in the same way as any other request issued by different FL users, hence, there is the possibility that these latter requests influence the behaviour of a P cycle. In particular, FL services of high priority have the same weight of P polling messages to the slaves and consequently, depending on the order of their arrival, they could be executed before the polling cycle or, worse, they could be interleaved with P slave interrogations. 3. Cycle time 3.1. Analytical calculation he cycle time of a Profibus P network can be expressed analytically by the following formula: C (1) where: P A W FL GAP P is the time employed for polling the slaves; K is the time requested to read the diagnostic from the slaves which signalled the presence of such an information during the polling cycle; A is the time necessary to perform master-slave acyclic activities as foreseen by the PV1 extension of the standard; W is the total amount of time lost in a cycle waiting for the elapsing of the minimum slave intervals; FL is the time necessary for the execution of FL services required from users of the data link layer other than the P application; these users can be either resident on stations running P applications or not. GAP is the time spent for network maintenance activity; K represents the total token transmission time. he times reported in (1) will be now carefully evaluated taking into account that FL messages carrying data (such as the R primitives) are transmitted using the Protocol ata Unit (PU) 8

9 YC (33 bits) tart elimiter Length Length Repeated tart elimiter est. Address ource Address Frame Control ata Un it Frame Check equence End elimiter r. of Octects: up to Fig. 2: Protocol ata Unit used for FL messages carrying data shown in fig. 2. Moreover, it must be considered that, at the physical layer, each octet is coded as an UAR character using 11 bits and that, after the end of a frame transmission, a safety margin of time must be waited by the masters before sending a new frame. A typical value of 50 BI has been used as safety margin (where BI is the time necessary to transmit one bit). he same value has also been used as delay in answering for the slaves, R : this the time which elapses between the arrival of the last bit of a request frame and the sending of the first bit of the response frame. he polling time of the slaves is given by: P (i, j) (2) i 1 K(i) j 1 LV where is the number of Profibus P master stations, K(i) is the number of slaves belonging to the ith master, and LV (i, j) is the time necessary to poll the jth slave of the ith master: this operation is performed by sending a request PU of the type illustrated in fig. 2 carrying the output octets and receiving a response PU of the same type from the slave with the input octets. he times (i, j) are determined by a fixed part (which comprises: the time necessary to transmit all the fields of fig. 2 except the data unit field, the delay in answering of the slaves, and the safety margin added by the masters) and by a variable part directly related to the number of input/output octets. LV (i, j) can LV then be expressed as LV (i, j) L(i, j) where L (i, j) is the number of input/output FIX BY octets exchanged by the ith master with its jth slave, FIX is the time necessary to transmit the fixed part of the PUs and BY is the time necessary to transmit one octet (which is equal to 11 BI ). Assuming M as the number of the slaves present on the network: M K(i), and L O as the total number of input/output octets exchanged: L L(i, j), the formula (2) becomes: O i 1 K(i) j 1 i 1 P M L (3) FIX O BY 9

10 he time requested to read the diagnostic,, depends on the number of slaves from which this information has to be acquired and on its size. he protocol foresees for each slave 6 octets of standard diagnostic information to which can be added up to 238 octets of device specific information. he analytical expression for is: (i, j) (4) i 1 H(i) j 1 IA where H(i) is the number of slaves belonging to the ith master that signalled the presence of diagnostic information, and IA (i, j) is the time necessary to read the diagnostic from the jth slave of the ith master. upposing that, in a given token cycle, in total, slaves have signalled the presence of diagnostic, where H(i), (and M ), the formula (4) becomes: i 1 (5) FIX O BY where (i, j) is the total number of diagnostic octets exchanged. O i 1 H(i) j 1 he range of variation of is comprised between zero, when no slave has signalled the presence MAX of diagnostic, and, a value which can be calculated a priori as the worst case of formula (5). In the same way the time required by P master-slave acyclic services can be expressed as: A G A (6) FIX O where G is the number of acyclic messages and Also BY A O is the total number of octets exchanged. W is subjected to variation and, as can be deduced from fig. 1, it is strictly related to the minimum slave interval, MI. his latter time is particularly important at high network transmission speeds, i.e. when the cycle times are very short. For instance, a MI of 200 µs (a typical value), at a speed of 12 Mbit/s corresponds to 2400 BI which is approximately the time necessary to poll a slave with 200 input/output octets (supposing a slave delay in answering of 50 BI ), while at a speed of 500 kbit/s the same time is equivalent to 100 BI. FL cannot be a priori estimated as it depends on the number of FL users and on the network traffic they generate. However, as it will be shown in the following, the effects of such a kind of traffic can be very dangerous. GAP is the time employed by a station in possess of the token to check its address range for addition or removal of stations. his operation is cyclic with a periodicity set by the network 10

11 administrator and it consists in sending to the addressed station a frame named request FL status and then waiting for the response frame. he time employed with the frame structure given in [2] is equal to 215 BI. For a Profibus P master station, it is convenient to execute such a check at each token receipt: in this way the cycle time will be slightly incremented, but there will be not jitter generated by GAP. his solution will be adopted for all the examples reported in this paper. K, the total token transmission time, can be expressed by: K ( ) (7) FL O where 66 O BI is the time requested for a single token transmission and FL of FL active stations present on the network which do not implement P applications tandard cycle time is the number he ideal operation of a Profibus P network is encountered when there is no acyclic traffic of any type. In such a case the cycle time is constant and determined by the time requested for cyclic activities and by the minimum slave interval. More in detail, defining the standard cycle time as the time necessary to perform cyclic activities: If C (8) C P MI GAP K, then the cycle time of the network is equal to C because a new polling cycle can start immediately after the end of the previous one. It is evident that such a situation is mostly verified in networks operating at low transmission speeds, with an high number of nodes, and with a large amount of input/output octets to be exchanged. Conversely, if, before beginning a new polling cycle it is necessary to wait the elapsing C MI of the minimum slave interval, then the cycle time is equal to MI. When there is not acyclic traffic, also the token holding time of each station is a constant, given by: H (9) R C In order to satisfy the requests coming from the P applications, every master station must have sufficient token holding time to poll its slaves, that is, it must be: H max (i) (10) i 1 P References [1] eutsches Institut fuer ormung: Profibus-P tandard 11

12 ranslation of the German ational tandard I part 3. Beuth Verlag GmbH Burggrafenstraße, 6, -100 Berlin, 30 Germany [2] European Committee for Electrotechnical tandardization General Purpose Field Communication ystem volume 2, Profibus, E50170/2, ec [3] Profibus utzerorganisation e.v.: Profibus-P extensions to E50170 Version 1.0, ecember Haid-und-eu-traße 7, Karlsruhe, Germany. [4] International Electrotechnical Commission, IEC : igital data communications for measurement and control Fieldbus for use in industrial control systems part 5: Application Layer service definition, communication model type 3 specification January 2000 [5] International Electrotechnical Commission, IEC : igital data communications for measurement and control Fieldbus for use in industrial control systems part 6: Application Layer protocol specification, type 3 January 2000 [6] International tandard Organisation, IO: oken Bus Access Method IO I , [7] International tandard Organisation, IO: Logical Link Control IO I , [8] European Committee for Electrotechnical tandardization General Purpose Field Communication ystem volume 3, WorldFIP, E50170/3, ec [9] International Electrotechnical Commission: igital data communications for measurement and control Fieldbus for use in industrial control systems part 3: ata Link service definitions, communication model type 1 January [10] International Electrotechnical Commission: igital data communications for measurement and control Fieldbus for use in industrial control systems part 4: ata Link protocol specification, type 1 January

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