Salvatore Monforte

Evaluating worst case response time in mono and multi-master profibus DP

Profibus DP networks are often used as the communication infrastructure for supporting distributed computer-controlled applications, which, very often, impose strict real-time requirements. The aim of this paper is to present a novel methodology for evaluating the worst-case message response time in Profibus DP networks where high and low priority information flows coexist.

Designing Real-Time Systems Based on Mono-Master Profibus-DP Networks

Abstract Profibus networks are widely used as the communication infrastructure for supporting distributed computer-controlled applications. Most of the times, these applications impose strict real-time requirements. Profibus-DP has gradually become the preferred Profibus application profile. It is usually implemented as a mono-master Profibus network, and is optimised for speed and efficiency. The aim of this paper is to analyse the real-time behaviour of this class of Profibus networks. Importantly, we develop a new methodology for evaluating the worst-case message response time in systems where high-priority and cyclic low-priority Profibus traffic coexist. The proposed analysis constitutes a powerful tool to guarantee prior to runtime the real-time behaviour of a distributed computer-controlled system based on a Profibus network, where the real¬time traffic is supported either by high-priority or by cyclic poll Profibus messages.

MULTI-MASTER PROFIBUS DP MODELLING AND WORST CASE ANALYSIS-BASED EVALUATION

This paper provides an analysis of the real-time behaviour of the multi- master Profibus DP network. The analysis is based on the evaluation of the worst- case message response time and the results obtained are compared with those present in literature, pointing out its capability to perform a more accurate evaluation of the performance of the Profibus network. Copyright

Multicycle Polling Scheduling Algorithms for FieldBus Networks

The paper deals with the scheduling of periodic information flow in a FieldBus environment. The scheduling problem is defined from an analytical point of view, giving a brief survey of the most well-known solutions. One of these is called multicycle polling scheduling, which is based on the hypothesis that all the production periods of the periodic processes to be scheduled are harmonic. Although in some process control or manufacturing scenarios, this hypothesis may be acceptable, there are many real industrial processes to which it cannot be applied. The aim of the paper is to make a contribution towards solving the scheduling problem. It essentially concerns extension of the theory on which multicycle polling scheduling is based to a much more realistic and general scenario, where the periods of all the processes to be scheduled have arbitrary values. The authors present a new formulation of multicycle polling scheduling, called extended multicycle polling scheduling, and demonstrate that it comprises the scenario currently considered in the literature. Two algorithmic solutions for extended multicycle polling scheduling are then proposed, giving a computational complexity analysis which will highlight the capability of the algorithmic scheduling solutions to be performed on-line. The paper concludes by comparing the multicycle polling scheduling approach known in literature and the one presented in the paper. Comparison is performed by evaluating the use of available bandwidth to serve both periodic and asynchronous traffic in the two approaches.

Fault tolerance in Interbus-S standard

Abstract The paper deals with the fault tolerance in the Interbus-S, one of the most used FieldBus communication systems. Industrial environments are often featured by critical processes to be controlled. For this reason, the use of a FieldBus to convey information on which these processes are based, requires the presence of fault tolerance mechanisms able to recover a fault in the communication system on its occurrence. In the paper, the authors will highlight the vulnerability of the Interbus-S to fault occurrence in a particular communication node, the Master, foreseen by the Interbus-S standard to distribute the available bandwidth to the control devices connected to the communication system. Finally, the authors will present a protocol extension that allows the whole communication system to continue working after the occurrence of a fault in the Master. The main feature of the proposal is its full compatibility with the Interbus-S standard, as it will be shown in the paper.

Proposing and evaluating allocation algorithms in a grid environment

Distributed computing addresses workload challenges by aggregating and allocating computing resources to provide for unlimited processing power. In the last ten years, it has grown from a concept that would enable organizations to simply balance workloads across heterogeneous computer resources, to an ubiquitous solution that has been embraced by some of the worlds leading organizations across multiple industry sectors. And while distributed computing harnesses the full potential of existing computer resources by effectively matching the supply of processing cycles with the demand created by applications, even more importantly it has paved the way for grid computing - a more powerful, yet global approach to resource sharing. The aim of this paper is to propose new allocation algorithms for workload management in a computing grid environment. A simulation tool is used to validate and estimate performance of these algorithms. The paper is organised as follows. After a brief introduction about grid environment and on the DataGrid Project in Section 1, Section 2 presents the proposal for novel allocation policies. Finally, Section 3 provides for a performance evaluation of the algorithms proposed, comparing their performances it with those offered by the actual solution adopted in the DataGrid Project.

A fault recovery model in IEC/ISA FieldBus communication protocol

The aim of the paper is to propose a fault recovery model to be adopted in the IEC/ISA FieldBus communication protocol. The authors present the results of their research, which highlights that some types of fault cannot be recovered by the IEC/LSA FieldBus protocol, whereas others are recovered but the time required is at times excessive. The paper then suggests some innovative recovery mechanisms which, along with those the protocol currently offers, are able to guarantee a high degree of reliability. The proposed mechanisms are compatible with the actual FieldBus communication protocol as they could be included in the system and network management layer, which is not yet totally defined. The paper concludes by presenting an evaluation of the fault recovery mechanisms proposed, demonstrating their effectiveness.

Enhancing reliability in IEC/ISA fieldbus

Abstract The aim of the paper is to discuss and evaluate the impact of the occurrence of faults on the IEC/ISA FieldBus protocol. This communication system was conceived to be used mainly in process control environment, where robustness to the occurrence of faults is an important requirement. After a survey of the FieldBus protocols main features, certain types of fault are defined, the occurrence of which may be critical for the communication system. The paper then presents the results of research conducted by the authors to see whether the current protocol is capable of recovering these types of fault. These results show that some types of fault cannot be recovered by the IEC/ISA FieldBus protocol. For this reason, a fault recovery model is presented. It is made up of functional blocks and procedures to be adopted at the System and Network Management Layer of the FieldBus communication stack. The paper concludes by presenting an evaluation of the fault recovery mechanisms proposed, demonstrating their effectiveness. This will be done from two different points of view. For each kind of fault that cannot be recovered by the current fault recovery mechanisms, it will be demonstrated that the proposed fault recovery model is able to recover the fault. This is not enough to evaluate the fault recovery mechanisms, but it is very important to see the effects of the occurrence of a fault on the control processes using the FieldBus communication system. Some of these effects may be the loss of critical information, errors in the reconstruction of analog sampled signals, and so on. An efficient fault recovery model must limit the effects of faults on the control processes as far as possible. The proposed fault recovery model will be evaluated in the paper according to this aim.

Adding fault tolerance mechanisms to Interbus-S

Field bus technology is now a reality in industrial environments. There are many field bus systems commercially available, and each is suitable for particular kinds of applications. In this scenario the Interbus-S system is playing a leading role, due to the efficiency of its protocol. However, a drawback of this communication system is the centralisation of the mono-master arbitration scheme. The presence of a single device to co-ordinate communication activities makes the Interbus-S protocol vulnerable to fault occurrences in the master. Maintaining full compatibility with the existing standard, the authors have defined a protocol extension which allows the whole communication system to continue working after the occurrence of a fault in the master node.