P. Raja

Fulfilling temporal constraints in fieldbus

Field busses are emerging as a replacement of direct connections from control systems to the process. Many proposals such as FIP, PROFIBUS, CAN, LONWOKS have been made to solve the interconnection problem. Some, like FIP and PROFIBUS, are national standards. However, the temporal requirements, as expressed by bodies like IEC, ISA, IEEE, CENELEC, ISO TCCA, IFC or European projects in the field, are not completely fulfilled by these proposals or not even addressed. In this paper, we first detail the temporal as well as spatial requirements among which consistency plays a key role. We then explain to which extent these requirements are addressed by the different proposals. This analysis shows that none of the proposals entirely fulfils them. We explain how the requirements can be satisfied within the network software, in a simple manner and adequate use of the FIP protocol even if this is not very efficient. Finally, we show the requirements may be satisfied in a simple and efficient manner.<<ETX>>

On the necessary real-time conditions for the producer-distributor-consumer model

The producer-distributor-consumer (PDC) has been created to model the real-time, mainly periodic, traffic of variables in FIP (field instrumentation protocol) fieldbus. One of its features is to provide a buffer for each variable; a buffer on the producer side and one on each consumer. In this paper, with the help of the adapted synchronous model, we derive the necessary condition to avoid buffer overwrites in the PDC model.

Design and implementation of a robust Fieldbus protocol

Fieldbus is a real-time network intended to be employed in the lowest hierarchy level of factory automation. In this paper, the authors give an overview of a typical Fieldbus application. They then describe the flux instrumentation protocol and a modified version of this protocol. They analyse these protocols with respect to two aspects, namely protocol efficiency and protocol robustness.<<ETX>>

Polling algorithms and their properties for Fieldbus networks

Real-time local area networks like Fieldbus have to handle two types of data transfer, synchronous and asynchronous. Synchronous data are generated periodically and have to be transmitted before fixed deadlines. Asynchronous messages are generated by unforeseen events and thus cannot be predicted. The data transfer is handled in two different phases. In this paper the authors study the properties of some scheduling algorithms to see how best the asynchronous messages are scheduled so that they meet their deadlines.<<ETX>>

Modeling and scheduling real-time control systems with relative consistency constraints

Control applications control and monitor an external physical environment. Process control and manufacturing systems are examples of such applications. In such systems the temporal relationship between the external environment and the control system plays an important role. In this paper, the temporal relationship of data within the control system and the external environment, known as data temporal consistency, is considered. First the control system is modeled as a graph of periodic tasks. Then the problem of scheduling a set of periodic tasks on a uniprocessor with the relative consistency as a scheduling constraint is studied and its computational complexity is investigated. A novel aspect of the scheduling problem is with respect to the scheduling constraints. More precisely, it is shown how the relative phasing of a periodic task is constrained by the relative phasings of its predecessor tasks.<<ETX>>

A functional approach for expressing temporal properties of inputs and outputs of real-time applications

Many real-time applications are different from ordinary computer applications because they react to the state or change of state of the external environment within a specified time limit. The authors concentrate on the interface between such applications and the external environment. The interface is represented as real-time objects, on which operations that accomplish correct data flow from the external environment to the application are introduced. The main aim of this work is to characterize the data flow by specifying the semantics of the operations. A functional approach is adopted for specifying and expressing the semantics of the operations.<<ETX>>

Simulating fieldbus applications with DRUGH simulator

Abstract Control applications constitute an important class of systems which use fieldbus networks for communication support. In this paper, we explain the special characteristics of such systems and stress the need for a simulator for such applications. The drugh simulator, developed for this purpose at our institute, is presented. drugh uses the synchronous programming model (extensively used for developing parallel and distributed algorithms), integrates a real-time protocol, and facilitates the development and off-line testing of real-time applications. The paper describes the structure and the functions offered by the simulator. The simulator was developed along with a physical real-time network, Phoebus IIx, and therefore, also served as a testbed for the protocol implementation.

A conceptual framework for distributed control applications

The aim of this paper is to: 1) present a conceptual framework for distributed digital (control) applications which use a real-time local area network (such as fieldbus) for communication support; 2) define various parameters that are required by the framework; and 3) demonstrate the utility of the framework and evaluate certain performance measures with the help of a case study.<<ETX>>

Simulating the fieldbus synchronous model by timed Petri nets

In Raja et al. (1993) the authors presented a synchronous model for periodic fieldbus applications that are distributed over a fieldbus network. The aim of this paper is to formally specify the semantics of the synchronous model. This is done by mapping the rules of the synchronous model to Petri net elements. Some preliminary results showing how the model can be simulated by a timed Petri net are presented. The approach adapted for this purpose consists of distinguishing the various components of the synchronous model and representing them separately as timed Petri net elements. Some properties of the resulting timed Petri net are also briefly discussed.<<ETX>>

An Architecture of a Map Stack for Real Time Applications

Abstract MAP provides communication support for manufacturing applications and factory automation. MMS and Network Management are some applications that depend on MAP for the communication support. These applications use MAP to communicate with manufacturing devices at the cell level. In this paper, we describe the architecture of LITMAP Stack implemented at LIT and discuss its ability to support soft real-time industrial applications. This discussion is based on the scheduling ability of this architecture and the performance of LITMAP Stack.