This part of ISO/IEC 14776 defines the mechanical, electrical, timing, and protocol requirements of the SCSI parallel interface to allow conforming SCSI devices to inter-operate. 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 SCSI devices may be added to the host computers without requiring modifications to generic system hardware. Provision is made for the addition of special features and functions through the use of vendor-specific options. Reserved areas are provided for future standardization. b) To provide compatibility such that conforming SPI-2, SPI-3 devices may interoperate with SPI-5 devices given that the systems engineering is correctly done. Conforming SPI-2, SPI-3, and SPI-5 devices should respond in an acceptable manner to reject SPI-5 protocol extensions. SPI-5 protocol extensions are designed to be permissive of such rejections and thus allow SPI-2 and SPI-3 devices to continue operation without requiring the use of the extensions. The interface protocol includes provision for the connection of multiple SCSI initiator ports (i.e., SCSI devices capable of initiating an I/O process) and multiple SCSI target ports (i.e., SCSI devices capable of responding to a request to perform an I/O process). Distributed arbitration (i.e., bus-contention logic) is built into the architecture of this standard. A default priority system awards interface control to the highest priority SCSI device that is contending for use of the bus and an optional fairness algorithm is defined. This standard defines the physical attributes of an input/output bus for interconnecting computers and peripheral devices. The set of SCSI standards specifies the interfaces, functions, and operations necessary to ensure interoperability between conforming SCSI implementations. This standard is a functional description. Conforming implementations may employ any design technique that does not violate interoperability. This standard has made obsolete single-ended and multimode signaling alternatives. Implementations that use single-ended or multimode signaling alternatives should reference the SCSI Parallel Interface-2 standard (ISO/IEC 14776-112). Figure 1 is intended to show the general structure of SCSI standards. The figure is not intended to imply a relationship such as a hierarchy, protocol stack, or system architecture.