Hardware architecture for hierarchical control of large Petri net

We propose a multiprocessor controlled by Petri net, which is applied to a sequence controller for factory automation. Systems for factory automation can be described using Petri nets. But it is difficult to describe a large system with the Petri net as one graph. Thus, we divide a large Petri net into a main-net and many smaller sub-nets hierarchically, in a way similar to a main program and subroutines in ordinary programming languages. Operations for "sub-net call" and "sub-net return" are proposed and introduced to the architecture of a Petri net controller. "Call-place" is introduced to call a sub-net. To execute high speed operations for "sub-net call" and "sub-net return", we propose a hardware unit, called History Unit. The main function of History Unit is implemented by a memory accessible in two dimension, row-wise and column-wise, like an associative memory. History Unit can realize simpler and much faster "sub-net operations".<
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