Difference Between SAS & Ultra SCSI

The Small Computer Systems Interface, known as “SCSI,” is a means to connect hard drives and other high data rate equipment to computers. The SCSI standard defines the various connectors, cabling and software needed for reliable hard disk operation. Initially developed in 1986, SCSI has undergone several evolutionary changes, the most recent being SAS, short for “Serial Attached SCSI.” Among other improvements, SAS has faster data transfers than traditional SCSI.

Data Rate

The term “Ultra SCSI” covers several generations of the SCSI standard, beginning with SCSI 3, having a data transfer rate of 160 Megabits/second and progressing through Ultra-640 with a rate of 5.12 Gigabits/second. The original SAS interface standard has a transfer rate of 3 Gb/second; as of the time of publication, some devices support speeds of 6 Gb/second.

Point to Point vs. Daisy Chain

An SAS controller has a number of ports, each of which services a single device such as a hard drive. This point-to-point connection simplifies the electrical signals passing between the computer and hard drive, permitting faster data communication and simplifying the isolation of defective drives. By comparison, each Ultra SCSI controller supports up to eight or 16 devices connected one after another in a daisy-chain configuration. If a drive fails anywhere in the chain, a technician must disconnect the set and test each one to find the problem.

Hot Plugging

The SAS standard supports “hot plugging,” which allows you to connect new devices to a computer without shutting down the operating system and turning the computer off. When you connect an SAS drive to the controller, the controller recognizes the drive, and the operating system readies it for use. Only those Ultra SCSI controllers and drives having the 80-pin SCA connector support hot-plugging.

Clock Skew

Ultra SCSI’s daisy-chain connection produces an electronic effect called clock skew, where high-frequency pulses used to synchronize data become increasingly delayed the further the device is on the chain. This problem limits the data transfer rate of Ultra SCSI devices. Because SAS devices are point-to-point and don't share a common bus-type connection, they don't suffer from clock skew.