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Abstract

A small computer system interface (SCSI) disk array subsystem has removable disk drive units that can be removed while the subsystem has power applied to it and is operational. In order to prevent malfunctions of other disks in the disk array, and in order to prevent rapid fluctuations in the current in the power circuits of the subsystem, the power and the data and control signals are applied to the inserted disk drive unit in a phased sequence. Initially, the ground of the disk drive unit is connected to the ground of the subsystem along with precharge power which connects the power buses of the disk drive unit to the power buses of the subsystem through resistors so that capacitances connected to the power buses of the disk drive unit are charged gradually through the resistors rather than abruptly. Thereafter, power is applied directly from the power buses of the subsystem to the power buses of the disk drive unit. Finally, the control and data signal connections are made between the subsystem and the disk drive unit. This phased sequence of connections between the subsystem and the disk drive unit is accomplished by providing sets of contacts having differing lengths in a connector on the disk drive unit so that some connections are made before other connections.

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