Analysis of polling protocols for fieldbus networks

Abstract

Roll-call polling mechanisms are considered for fieldbus networks . Based on a producer/consumer communication model, two different protocols are presented, namely the standardized FIP (Factory Instrumentation Protocol) and a modified FIP protocol that we propos e here. The modified protocol uses redundancy to eliminate local timers used by the FIP protocol . The two protocols are analyzed for efficiency, jitter and robustness . We compare the two protocol s based on implementation and analytical results . Practical aspects concerning : clock precision ; computing time ; and hard real-time constraints are also discussed . The results show that: (1) th e original FIP protocol is fractionally more efficient with respect to its execution time than th e modified one ; (2) the modifications proposed, however, reduce the jitter and improve th e robustness considerably ; (3) the slot time of the original FIP protocol has theoretical upper boun d but the modified protocol has no such limitations . . INTRODUCTIO NControl applications, such as manufacturing systems and process control, constitute an importan t class of real-time applications . Such an application is in constant interaction with an external physical process through field devices (like sensors, actuators, Programmable Logical Controllers , PLCs ). The major activity is to read values (from sensors), calculate new commands (by PLCs ) and send new commands (to actuators), Fig. 1. In general, these three steps are carried ou t periodically in a cyclic manner [2,6] . The response times of these systems are in the order of fe w milliseconds and therefore are generally classified as hard real-time systems [9] .In this paper, attention is focused on the communication support for such systems when the fiel d devices are connected by a real-time local area network . Fieldbus is the standard solution for suc h networks [1,2] . Its purpose is to replace the traditional point-to-point cabling that connect sensor s and actuators with PLCs. FIP (Factory Instrumentation Protocol) [1, 4] is a French nationa l standard for fieldbus. It has also been proposed as an international standard [12] . FIP is basically a polling protocol. A station, called master, controls accesses to the communication medium, and al l other stations connected to the network, called secondary sites, are polled by the master.Two polling mechanisms are generally known in the literature [5] . They are briefly described in th e following :Roll-call polling : The master polls a secondary site and the secondary site transfers messages an d notifies the master at the end of the transmission . Then the master polls the next secondary site i n the polling sequence.Hub polling : The master polls an initial secondary site, the secondary site transfers its messages an d passes on the control to the next site (for example by suffixing a go-ahead field to the end of it s transmission with the next secondary site address) . Thus, the intermediate transfer of control t o the master is avoided .External Environmen t ...

Determining the channel capacity in SCADA systems using polling protocols [power system telecontrol]

“…But it will also depend significantly upon the communications protocol used to communicate between the control center and the remotes [3]. In many control centers, this protocol takes a question-answer form [4] [5]. The control center polls the first remote on the link; if the remote has information to send, it does so, and if not, it sends a null message.…”

Ανάπτυξη Και Υλοποίηση Ολοκληρωμένου Βιομηχανικού Δικτύου Πραγματικού Χρόνου Και Ευφυών Κόμβων

“…Εντούτοις, σε γενικές γραμμές, το αποτέλεσμα αυτών των παραδοχών είναι η μη ακριβής προσέγγιση των συνθηκών πραγματικής λειτουργίας του πρωτοκόλλου. Από την άλλη πλευρά, η εκτίμηση της απόδοσης του πρωτοκόλλου με εξομοίωση είναι ευρέως αποδεκτή, επειδή δίδει πιο πλήρη και ακριβή αποτελέσματα συγκριτικά με τις μαθηματικές μεθόδους και αποτελεί σημείο αναφοράς για το βαθμό της ακρίβειας των μεθόδων αυτών [121][122][123]. Στο κεφάλαιο αυτό γίνεται η εκτίμηση της απόδοσης διαφορετικών τεχνολογιών του δικτύου 8ITBUS.…”