1. A parallel data processing system including a plurality of data processing units comprising: a plurality of trays, only a part of said plurality of trays having outputs operatively connected to said plurality of data processing units and each of said plurality of trays being connected to an adjacent said tray, said part of said plurality of trays being connected to said plurality of data processing units for transmitting and receiving data to and from said plurality of data processing units and said adjacent trays and for storing said data transmitted from said plurality of data processing units and said adjacent trays, and a remaining part of said plurality of trays being not connected to said plurality of data processing units for transmitting and receiving data to and from said adjacent trays and for storing said data transmitted from said adjacent trays; and a clock generator, connected to said plurality of trays and said plurality of data processing units, which generates a clock signal for use in synchronously performing the transfer of said stored data between said plurality of trays and said plurality of data processing units and the data transfer between said part of said plurality of trays and said plurality of data processing units and a data process in said plurality of data processing units.