The Small Computer System Interface, SCSI (pronounced “scuzzy”), is a bus standard that allows a number of different devices to communicate data over a parallel bus. Depending on the SCSI variation, the parallel bus can be 8 bits, 16 bits, or 32 bits wide. SCSI first became widely estab- lished in the Apple and UNIX marketplace, where an assortment of devices were developed to take advantage of SCSI's ability to daisy-chain
multiple devices under a single interconnect scheme, controlled by a sin- gle host adapter. The kinds of devices that can be interconnected
include:
• Hard disk drives and disk arrays • Tape backup devices
• Scanners • Printers
• External storage devices, such magneto-optical cartridge drives and removable cartridge drives
SCSI Considerations
SCSI also supports the use of additional host adapters installed in a single system to provide multiple daisy chains of devices. This capability can prove useful in a production environment or in situations where many different types of peripherals are required. Graphics arts studios, with their needs for scanning equipment, near-line storage, and high-resolu- tion printers, are one possible
While SCSI has made inroads in the PC marketplace, its use is more prev- alent in higher end machines, including business and professional com- puters. The higher cost of SCSI components has led many manufacturers to use less expensive interface methods, such as USB and ATAPI IDE, to reach larger markets. Even stalwart SCSI advocates such as Plextor have expanded their drive options to include IDE equipment. The less robust device handling facilities of IDE have been to some degree offset by recent advances in microprocessor speeds. The extra processing power helps to make up for some of the bus-handling issues, minimizing con- flicts when servicing multiple IDE devices.
Sometimes you can find a PC-based system that includes built-in SCSI support, but generally you have to add a circuit board to serve as the SCSI host adapter. Once you have a SCSI-capable system, however, SCSI makes it easy to add equipment—especially external equipment. Owing to the prevalence of SCSI on each of the major computer platforms, its multi- tasking capabilities, and its ability to maintain high data-transfer rates, many of the current CD recorders on the market use SCSI interfaces.
SCSI Connectors SCSI connectors, unfortunately, come in many different sizes and shapes. External connectors of SCSI-1 vintage usually have 50 pins and resemble Centronics printer connectors of years past. Macintosh SCSI-1 connectors, however, have 25 pins and use a D-shell type connector, simi- lar to the connector type used for RS-232 serial connections. SCSI-2 con- nectors, the kind found most frequently on CD recorders, often use a 50- pin, half-pitch connector, much smaller than most connectors, with a unique clamp-in fastener to lock it to the drive or adapter. There are also a variety of proprietary connectors that inevitably require special cabling to attach to anything. Avoid these, if possible, to simplify your intercon- nection tasks.
The original SCSI-1 standard can support data transfers up to 5MB per second. Since recording to a quad-speed CD-R drive only requires data transfer rates of 600KB per second, SCSI-1 is sufficient for most CD-R applications. However, if a system configuration includes many different types of devices on the SCSI bus, bus traffic may create slowdowns and
prevent the data flow to the CD-R from being maintained at the appro- priate rate.
Most current CD recorders use the more modern variation of the SCSI standard: SCSI-2. Advances in the SCSI standard have led to the introduc- tion of a whole series of successors, including: FastSCSI, Ultra SCSI, UltraWide SCSI, and the current speed champion, Ultra2 SCSI.
A faster version of SCSI can help reduce bus traffic concerns. The Fast SCSI version of SCSI-2 supports transfer rates up to 10MB per second over an 8-bit or 16-bit bus. Most mainstream SCSI host adapters now use 16-bit or 32-bit buses, and 64-bit buses have become available with the introduction of the PCI-X bus. Ultra SCSI and UltraWide SCSI accelerate transfers to 20MB and 40MB per second, respectively. These standards use 16- or 32-bit buses for transfers, which are becoming more common in CD-recordable equipment designs. For the ultimate in SCSI perfor- mance, the Ultra2 SCSI implementation offers data transfer rates of 80MB per second. Increasing the clock rate on the bus to 40MHz made it possible to achieve this level of sustained data transfer. This newest release is designed to be backwards compatible with all earlier SCSI devices.
Another advantage of Ultra2 SCSI is the adoption of a Low Voltage Dif- ferential (LVD) I/O interface that supports longer cable lengths by improving signal transmissions. With a simple point-to-point connection between one SCSI host adapter and a CD-R drive (or other device) the maximum cable length can be extended to 82 feet. On a fully loaded cable with multiple SCSI devices, the maximum length is 39 feet. Up to 15 devices can be interconnected on an Ultra2 SCSI daisy chain. Adaptec is working on new versions of the SCSI standard, such Ultra160/m SCSI with a sustained transfer rate of 160MB per second. Looking ahead into the forseeable future, SCSI appears to have a ongo- ing role in handling high-performance I/O applications.