SCSI Types

SCSI has three basic specifications:
  • SCSI-1: The original specification developed in 1986, SCSI-1 is now obsolete. It featured a bus width of 8 bits and clock speed of 5 MHz.

  • SCSI-2: Adopted in 1994, this specification included the Common Command Set (CCS) -- 18 commands considered an absolute necessity for support of any SCSI device. It also had the option to double the clock speed to 10 MHz (Fast), double the bus width from to 16 bits and increase the number of devices to 15 (Wide), or do both (Fast/Wide). SCSI-2 also added command queuing, allowing devices to store and prioritize commands from the host computer.

  • SCSI-3: This specification debuted in 1995 and included a series of smaller standards within its overall scope. A set of standards involving the SCSI Parallel Interface (SPI), which is the way that SCSI devices communicate with each other, has continued to evolve within SCSI-3. Most SCSI-3 specifications begin with the term Ultra, such as Ultra for SPI variations, Ultra2 for SPI-2 variations and Ultra3 for SPI-3 variations. The Fast and Wide designations work just like their SCSI-2 counterparts. SCSI-3 is the standard currently in use.

Different combinations of doubled bus speed, doubled clock speed and SCSI-3 specifications have led to lots of SCSI variations. The chart below compares several of them. Many of the slower ones are no longer in use -- we've included them for comparison.

Name Specification # of Devices Bus Width Bus Speed MBps
Asynchronous
SCSI		 SCSI-1 8 8 bits 5 MHz 4 MBps
Synchronous
SCSI		 SCSI-1 8 8 bits 5 MHz 5 MBps
Wide SCSI-2 16 16 bits 5 MHz 10 MBps
Fast SCSI-2 8 8 bits 10 MHz 10 MBps
Fast/Wide SCSI-2 16 16 bits 10 MHz 20 MBps
Ultra SCSI-3
SPI		 8 8 bits 20 MHz 20 MBps
Ultra/Wide SCSI-3
SPI		 8 16 bits 20 MHz 40 MBps
Ultra2 SCSI-3
SPI-2		 8 8 bits 40 MHz 40 MBps
Ultra2/Wide SCSI-3
SPI-2		 16 16 bits 40 MHz 80 MBps
Ultra3 SCSI-3
SPI-3		 16 16 bits 40 MHz 160 MBps
Ultra320 SCSI-3
SPI-4		 16 16 bits 80 MHz 320 MBps

In addition to the increased bus speed, Ultra320 SCSI uses packeted data transfer, increasing its efficiency. Ultra2 was also the last type to have a "narrow," or 8-bit, bus width.

All of these SCSI types are parallel -- bits of data move through the bus simultaneously rather than one at a time. The newest type of SCSI, called Serial Attached SCSI (SAS), uses SCSI commands but transmits data serially. SAS uses a point-to-point serial connection to move data at 3.0 gigabits per second, and each SAS port can support up to 128 devices or expanders.


SCSI controller

All the different SCSI varieties use controllers and cables to interface with devices. We'll look at this process next.