Ultra320 SCSI Basics
For many years, SCSI (Small Computer System Interface) has been regarded as the most professional interface for mass-storage devices. Compared to UltraATA and Serial ATA, SCSI works fundamentally differently since up to 15 devices can be operated on one "channel" - internally and externally.
A special controller and a 68-pin cable with that many connections is used to operate the individually identified drives. Terminators are provided at the start and finish to prevent signal reflections on the SCSI bus.
The great advantage of a SCSI subsystem is its independence. The actual controller is generally far more complex than the logic device required for UltraATA or Serial ATA, yet it functions independently of the system processor. And, thanks to its command of interesting features, it also offers far more intelligence.
For example, there's the CRC (Cyclic Redundancy Check) on the bus, which replaces the simple parity check in connection with the Ultra160 SCSI (Double Data Rate Transfers). Since signals on parallel buses are subject to mutual influence, it had to be ensured that the bits arrive correctly.
SCSI also makes good use of tagged command queuing, also known as command queuing and reordering. This enables a device to receive not one, but up to 256 commands, and to process them in optimized sequence. It's easy to imagine how that translates into performance on a multi-hard drive SCSI system in a multi-user server environment.
|Type||Speed||Hard drive/peripheral connections|
|Ultra320 SCSI(16-bit Wide)||320 MByte/sec||State-of-the-art hard drives|
|Ultra160 SCSI(16-bit Wide)||160 MByte/sec||Hard drives|
|Ultra2 SCSI(16-bit Wide)||80 MByte/sec||Hard drives|
|Ultra Wide SCSI(16-bit Wide)||40 MByte/sec||Hard drives and tape drives|
|Ultra SCSI(8-bit Narrow)||20 MByte/sec||CD-R, CD-RW, tape, removable storage (Jaz), and DVD drives|
|SCSI-2, Fast SCSI(8-bit Narrow)||10 MByte/sec||Scanners, Zip drives, and CD-ROM|