Abstract: We present in this paper a methodology how to use 'Parallel Automaton' to set up the requirements to specify and to execute small Computer Based Systems (CBS). A 'Parallel Automaton' is an extended form of the Mealy Machine. It handles a finite set of events (or variable conditions or clock conditions) which can occur in parallel, and performs a finite set of actions which can be done in parallel. In the 'Parallel Automaton' there is no concept of "global state" as in the Mealy Machine. Instead, to each action and each event, is associated a "private state" representing their occurrence in the application. Nevertheless, the number of event/sanctions private states is also finite. This single notation ('Parallel Automaton' with Private States) can be used to describe in the same way requirements and specifications. More than that, these two descriptions can be connected. The aims of the application can be described using a 'Parallel Automaton', as a black-box with initial inputs and final outputs. This 'Parallel Automaton' can then be refined and enhanced with intermediate conditions and actions to obtain detailed requirements. By successive refinements and enhancements, a sufficiently detailed executable specification can be derived. We present this methodology through a simple CBS example, for the requirements and the specifications using the 'Parallel Automaton' notation. We then give an architecture of a Virtual Machine that we have built to execute such a 'Parallel Automaton' on a network.