Abstract: A selection system for an information processing system wherein a unit, such as a magnetic tape unit, is shared by a plurality of control units. At least first and second signal lines are connected between each control unit at a driver circuit and a receiver circuit pair. The two signal lines connected between each control unit correspond to a single shared unit, and each control unit provides a driver circuit and a receiver circuit connected so as to transmit and receive signals carried on each line. Each line is connected to a separate driver circuit and receiver circuit at each control units. A block signal is transmitted from one of the control units through a first signal line to the other control units to prevent them from starting their operation for accessing the shared unit, and selection signals are transmitted from possible plurality of started control units through the second signal line at different timings to thereby determine priority of the started control units for accessing the shared unit.