Data transfer control system

Claim 1

A data transfer control system for controlling the priority use of a common bus connected to and shared by a plurality of data processors having a priority order that can be changed comprising bus rank memory means, means for transferring to said bus rank memory means priority codes sent from said plurality of data processors, detection means connected to said bus rank memory means to detect the order of priority levels of said priority codes and generate an output priority signal indicative of the priority order of said data processors, means in each data processor for generating common bus request signals, and common bus control means connected to said detection means and said plurality of data processors to receive the common bus request signals generated by the data processors, to select in response to the output priority signal from said detection means the data processor supplying a common bus request signal having the highest priority and to supply a permit signal to the selected data processor, thereby permitting exclusive use of the common bus by said selected data processor, said bus control means including means for stopping data transfer by a first data processor having exclusive use of the common bus and for permitting a second data processor to exclusively use the common bus when the second data processor produces a common bus request signal and has a higher priority order than the first data processor.