Introduction

Net Condition Event Systems (NCES) developed by Hanisch and Rausch [6] are a subclass of Condition Event Systems, which were introduced by Sreenivas and Krogh [24]. In the general model of Condition Event Systems, modules described by their input/output behavior are interconnected by means of two signals: (1) piece-wise constant ``condition signals,'' and (2) point-wise non-zero ``event signals.'' Condition signals provide information about discrete states in one module to other modules. Hence, state transitions in other modules can be enabled or disabled by condition signals. Event signals provide information about state transitions in one module. They are used to force state transitions in other modules if these state transitions are enabled. Two representations of Condition Event Systems are known:

1. Boolean Condition/Event Systems where the dynamic behavior of the modules and the signals are represented by Boolean functions and equations [10].
2. Net Condition/Event Systems where the dynamic behavior of the modules is represented by safe (or bounded) Petri Nets. For formal details on NCES we refer the reader to [6], and [22].

Although there are synthesis techniques for both representations of Condition/Event Systems, we refer in the sequel to the Net Condition/Event Systems only.

As mentioned above, NCES are an extension of Petri Nets. The motivation for this extension came from the need to have a modeling framework for the uncontrolled plant that would support modularity. As mentioned earlier, modularity capabilities with (pure) Petri Nets is limited. Once the plant model is obtained, controllers can be synthesized for forbidden state specifications, and a certain class of sequential specifications.