This paper defined a kind of communication protocol between the RIP card and the printhead circuit for the inkjet printing system. Protocol project was chosen in the protocol design, for which Petri net was used. After defining the message, the protocol model was established based on colored Petri net with inhibitor arcs. The model has priority and allow different message to transmit on the same data bus subsequently. Though analyzing and simulating in CPN Tools, the model was proved to be able to realize the function.