The SCSI parallel interface is a local I/O bus that may be operated over a wide range of transfer rates.The objectives of the SCSI parallel interface are: a) To provide host computers with device independence within a class of devices. Thus,different disk drives, tape drives, printers, optical media drives and other devices may be added to the host computers without requiring modifications to generic system hardware. b) To provide compatibility such that properly conforming SCSI-2 devices may interoperate with SCSI-3 devices given that the systems engineering is correctly done. This international standard defines the mechanical, electrical, timing and protocol requirements of the SCSI parallel interface to allow conforming devices to interoperate. The interface protocol includes provision for the connection of multiple initiators and multiple targets. This standard defines the physical attributes of an input/output bus for interconnecting computers and peripheral devices.