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The Small Computer Systems Interface (SCSI) has developed into the most versatile interface for communicating between computers and mass storage devices. This is due in a large part to the flexibility built into the standard. The ANSI X3T10 Standards Committee has continued to adapt the SCSI standard to the increased demand for faster and more complex data communications. It has done this while allowing older and slower SCSI implementations to continue to be supported. Logical definitions of how SCSI devices communicate on the bus are exceptionally clean. The physical layer of the bus, however, has continued to pose problems. SCSI extenders, regenerators, converters, and other SCSI bus enhancement devices can be used to alleviate many of those problems.
SCSI And "Other" Devices
SCSI allows for peripherals other than mass storage devices to be used. Other devices such as high-speed scanners, printers, data acquisition systems, video recorders, and any device that incorporates the SCSI interface hardware can be connected to a system, and have the standardized, high-speed communications that SCSI provides. Unfortunately, the cable length limitations of parallel SCSI represent a major problem in configuring systems with these "other" devices. The 1.5 to 6m maximum length of single-ended SCSI and even the maximum of 25m for differential SCSI, in many cases is too short for the above devices to be physically located where they need to be used. In many systems RAID packages, backup devices and other mass storage devices frequently encounter the same cable length problems as the "other" SCSI devices.
SCSI Extenders
SCSI bus extenders solve the problem of placing these devices where they are required. Fibre optic, coaxial, and parallel SCSI extenders separate the two busses into two segments. Each segment has its own set of physical terminations. The extenders do not take an ID on the bus and logically are transparent to the rest of the system. Extenders function in...