A location access controller in Software Integrated ECC200 in Software A location access controller

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A location access controller use none none printer tomake none on none VS.NET accept any p, d w none for none here d p mCard sit(p) = org(d) p dst(d) aut p dom (dap) / then dap(p) := d mCard := mCard \ {d p} mAccept := mAccept {d} end We now nd the physical event of acceptance to a door and the physical lighting of a green light: ACCEPT any d where d mAccept then GRN := GRN {d} end It is interesting to remark on the gap between the logical acceptance of the door (accept event in the software) and the physical acceptance (event ACCEPT of the hardware). This gap evokes a major problem of distributed systems which is that of distinguishing between the intention (software) and the real action (hardware). We now nd the physical event corresponding to the clearing of the door.

Note that the door does not know who is clearing it. PASS any d where d GRN then GRN := GRN \ {d} mP ass := mP ass {d} mAccept := mAccept \ {d} end This physical pass is followed by a logical pass which is almost identical to its version during the previous re nement. The only di erence corresponds to the fact that the.

16.7 Fourth re nement launching of this none none event is now due to the reception of a message. It is to be noted here that the event pass knows who is passing; we are dealing with the person implied in acceptance of the door. Once again we may note a gap between physical detection (PASS event of the hardware) and its logical e ect (pass event of the software):.

pass any d where d mP ass then sit(dap 1 (d)) := dst(d) dap := dap {d} mAckn := mAckn {d} mP ass := mP ass \ {d} end. The event which c none for none onsists in physically blocking the door (from a clock supposedly inside the door which starts up 30 seconds after its acceptance if no one has cleared it in the meantime) is the following. The message of re-blocking is sent to the software..

OFF_GRN any d whe re d GRN then GRN := GRN \ {d} mOf f _grn := mOf f _grn {d} mAccept := mAccept \ {d} end Finally, we can nd a new version of the event o _grn, which gets underway on reception of the previous message.. o _grn any d wher none none e d mOf f _grn then dap := dap {d} mAckn := mAckn {d} mOf f _grn := mOf f _grn \ {d} end. A location access controller The event refuse none for none is slightly modi ed so as to allow a message concerning the lighting of the red light to be sent:. refuse any p, d w here d p mCard ( sit(p) = org(d) p dst(d) aut p dom (dap) ) / then mCard := mCard \ {q p} mRef use := mRef use {q} end. The rst hardware none none event after this corresponds to the reception of the previous message and the e ective lighting up of the red light. The automatic extinction of the red light after 2 seconds corresponds to the following second event which sends a message to the software so as to warn it:. REFUSE any d where d mRef use then RED := RED {d} end OFF_RED any d whe none for none re d RED then RED := RED \ {d} mOf f _red := mOf f _red {d} mRef use := mRef use \ {d} end. The event o _red is slightly modi ed as regards its previous version; it is now set o by the reception of the previous message:. o _red any d where d mOf f _red then mAckn := mAckn {d} mOf f _red := mOf f _red \ {d} end 16.7 Fourth re nement 16.7.5 Synchroniz none for none ation We now obtain the following complete synchronization between the software and the hardware: CARD.

accept ACCEPT refuse REFUSE PASS pass OFF_GRN o _grn ACKN OFF_RED o _red 17 Train system 17.1 Informal int roduction The purpose of this chapter is to show the speci cation and construction of a complete computerized system. The example we are interested in is called a train system.

By this, we mean a system that is practically managed by a train agent, whose role is to control the various trains crossing part of a certain track network situated under his supervision. The computerized system we want to construct is supposed to help the train agent in doing this task. Before entering in the informal description of this system (followed by its formal construction), it might be useful to explain the reason why we think it is important to present such a case study in great detail.

There are at least four reasons which are the following:. (i) This example none none presents an interesting case of quite complex data structures (the track network), whose mathematical properties have to be de ned with great care: we want to show that this is possible. (ii) This example also shows a very interesting case where the reliability of the nal product is absolutely fundamental: several trains have to be able to cross the network safely under the complete automatic guidance of the software product we want to construct. For this reason, it will be important to study the bad incidents that could happen and which we want either to avoid completely or manage safely.

In this chapter, however, we are more concerned by fault prevention than fault tolerance. We shall come back to this in the conclusion. (iii) The software must take account of the external environment that is to be carefully controlled.

As a consequence, the formal modeling we propose here will contain not only a model of the future software we want to construct, but also a detailed model of its environment. Our ultimate goal is to have the software working in perfect synchronization with the external equipment, namely the track circuits, the points (switches), the signals, and also the train drivers. We want to prove 508.

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