SCSI Bus Reference

SCSI stands for “Small Computer System Interface” and is probably the most common high-performance interface for personal computers. It is really a system bus specification, with capacity for many different types of devices, not just mass storage peripherals. In addition to supporting a huge variety of mass storage devices (hard drives, optical, tape, etc.), it also supports other devices like scanners and printers.

The most common mass-storage interface in personal computers, however, remains the ATAPI or IDE interface. Although great strides have been made to improving the performance and increasing variety and versatility, IDE/ATAPI still has many limitations, the most grievous being the resource usage involved: each set of IRQ+I/O resources can support (at most) 2 devices. SCSI, on the other hand, can support 7 devices in the “narrow” setup, and 15 devices in the “wide” configuration. Further, the SCSI bus can be cascaded so that any one of the 15 attached devices is, itself, a host bus adapter with up to 15 devices attached...

Two new attachment interfaces have recently joined the fray, USB (universal serial bus) and  “FireWire” (also known as iLink or IEEE-1394). Both interfaces are serial interfaces (SCSI and ATAPI/IDE are parallel), and are “smart” interfaces that are “supposed” to negotiate identification and resource sharing (unlike SCSI and ATAPI/IDE, where manual identification is typically required). Due to performance limitations, USB (v1) is fine for near-line storage, giving improved connectivity options (hot-swap and automatic identification/enumeration) over other techniques, but it isn’t ready for prime-time use in that capacity. FireWire, on the other hand, has the potential to replace SCSI—in terms of ease-of-use, performance an cost—but that yet remains to be seen, as most high-end vendors don’t yet support it.

That said, the following is a table that lays out the current flavors of SCSI: