# System Informatics, 2020, # 17

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State identification is the well-known problem in the theory of Finite State Machines (FSM) where homing sequences (HS) are used for the identification of a current FSM state, and this fact is widely used in the area of software testing and verification. For various kinds of FSMs, there exist sufficient and necessary conditions for the existence of preset and adaptive HS and algorithms for their derivation. Nowadays timed aspects become very important for hardware and software systems. In this work, we address the problem of checking the existence and derivation of homing sequences for FSMs with timed guards. The investigation is based on the FSM abstraction of a Timed FSM.

Sequential reactive systems are formal models of programs that interact with the environment by receiving inputs and producing corresponding outputs. Such formal models are widely used in software engineering, computational linguistics, telecommunication, etc. In real life, the behavior of a reactive system depends not only on the flow of input data, but also on the time the input data arrive and the delays that occur when generating responses. To capture these aspects, a timed finite state machine (TFSM) is used as a formal model of a real-time sequential reactive system. However, in most of known previous works, this model was considered in simplified semantics: the responses in the output stream, regardless of their timestamps, follow in the same order in which the corresponding inputs are delivered to the machine. This simplification makes the model easier to analyze and manipulate, but it misses many important aspects of real-time computation. In this paper we study a refined semantics of TFSMs and show how to represent it by means of Labelled Transition Systems. This opens up a possibility to apply traditional formal methods for verifying more subtle properties of real-time reactive behavior which were previously ignored.

Sequential reactive systems such as controllers, device drivers, computer interpreters operate with two data streams and transform input streams of data (control signals, instructions) into output streams of control signals (instructions, data). Finite state transducers are widely used as an adequate formal model for information processing systems of this kind. Since runs of transducers develop over time, temporal logics, obviously, could be used as both simple and expressive formalism for specifying the behavior of sequential reactive systems. However, the conventional applied temporal logics (LTL, CTL) do not suit this purpose well, since their formulae are interpreted over omega-languages, whereas the behavior of transducers are represented by binary relations on infinite sequences, i.e. omega-transductions. To provide temporal logic with the ability to take into account this general feature of the behavior of reactive systems, we introduced new extensions of this logic. Two distinguished features characterize these extension: 1) temporal operators are parameterized by sets of streams (languages) admissible for input, and 2) sets (languages) of expected output streams are used as basic predicates. In the previous series of works we studied the expressive power and the model checking problem for Reg-LTL and Reg-CTL which are such extensions of LTL and CTL where the languages mentioned above are regular ones. We discovered that such an extension of temporal logics increases their expressive capability though retains the decidability of the model checking problem. Our next step in the systematic study of expressive and algorithmic properties of new extensions temporal logics is the analysis of the model checking problem for finite state transducers against Reg-CTL* formulae. In this paper we develop a model checking algorithm for Reg-CTL* and show that this problem is in ExpSpace.

We address the formal verification of the control software of critical systems, i.e., ensuring the absence of design errors in a system with respect to requirements. Control systems are usually based on industrial controllers, also known as Programmable Logic Controllers (PLCs). A specific feature of a PLC is a scan cycle: 1) the inputs are read, 2) the PLC states change, and 3) the outputs are written. Therefore, in order to formally verify PLC, e.g., by model checking, it is necessary to reason both in terms of state transitions within a cycle and in terms of larger state transitions according to the scan-cyclic semantics.
We develop a formalization of PLC as a hyperprocess transition system and an LTL-based temporal logic cycle-LTL for reasoning about PLC.