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sensors
process
controller
actuators
Fig. 1. Real-time system
{ requirements,
{ design specications,
{ programs.
Typically, each of these levels comes with its own notation. A problem is thus
how to guarantee correctness across these levels, i.e. how to link these dierent
notations in such a way that
{ the design specications satisfy or correctly rene the requirements,
{ the programs correctly implement the design specication.
This paper is concerned with the following aspects of this problem:
{ concepts : How to dene relationships like \correctly renes" and \correctly
implements" ?
{ methods : How to establish these relationships in practical applications ?
Our answer to the conceptual aspect is the use of a logic-based approach de-
veloped in the ProCoS project [17]. Our answer to the methodological aspect
is not a general one. Rather we suggest that such a method should be tailored
towards particular application areas or technological constraints. In this paper
we consider the area of trac control where Programmable Logic Controllers
are a widespread hardware platform.
The remainder of this paper is organised as follows. Section 2 explains the
general logic-based approach which we pursue. Section 3 introduces a case study
from the area of railway signalling that motivated our method to the design
of real-time systems explained in this paper and that serves to illustrate the
dierent aspects of the method. Section 4 explains Constraint Diagrams as a
graphical notation for formalising real-time requirements. Section 5 introduces
PLC-Automata as a graphical formalisation of the PLC concepts. Section 6 gives
an idea of the progamming notation Structured Text used for PLCs. Section 7
explains details of the Duration Calculus and its role as a semantic basis for Con-
straint Diagrams and PLC-Automata. Section 8 sketches the tool
Moby/plc
supporting the method described so far. Finally, Section 9 draws some conclu-
sion.
 
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