We propose a hybrid method for synthesis of hierarchical structured Petri nets. In a top-down manner, we decompose a system into a set of subsystems at each level of abstraction, each of these is specified as a blackbox Petri net that has multiple inputs and outputs. We stipulate that each subsystem satisfies the following I/O constraints: (1) At any instance of time, at most one of the inputs can be activated; and (2) If one input is activated, then the subsystem must consume the input and produce exactly one output within a finite length of time.

We give a stepwise refinement procedure which starts from the initial high-level abstraction of the system and expands an internal place of a blackbox Petri net into a more detailed subnet at each step. By enforcing the I/O constraints of each subsystem in each intermediate abstraction, our refinement maintains the sequencing of transitions prescribed by the initial abstraction of the system. Next, for the bottom-up synthesis, we present interconnection rules for sequential, parallel, and loop structures and prove that each rule maintains the I/O constraints.

Thus, by incorporating these interconnection rules into our refinement formulation, our approach can be regarded as a hybrid Petri net synthesis technique that employs both top-down and bottom-up methods. The major advantage of the method is that the modeling details can be introduced incrementally and naturally, while the important logical properties of the resulting Petri net are guaranteed.