What is SCSI?

The SCSI (Small Computer System Interface) defines interfaces and protocols for data transfer between mass storage devices and computer systems. The original variant was a parallel bus system that was replaced by faster transmission standards. The SCSI protocol is still used today in standards such as ATAPI, Fiber Channel or Firewire.

SCSI is a collection of standards that define interfaces and protocols for transferring data between computer systems and mass storage media such as hard disks or optical drives. Peripheral devices such as scanners can also be connected to a computer via the Small Computer System Interface.

In the original variant, the Small Computer System Interface specified electrical signals, cables, and connectors for a parallel bus as well as protocols and commands for communication between the various systems. The first version of the Small Computer System Interface appeared in 1986. Other versions of the standard followed. The classic parallel bus was very successful for many years and was used in many computers. The physical parallel bus system is hardly of any importance today because it has been replaced by higher-performance interfaces. However, the SCSI protocol is still used in transmission standards such as ATAPI, Fiber Channel, Firewire, iSCSI, SCSI Express or SAS.

The classic parallel SCSI bus and its typical properties

On the classic parallel bus, each device identifies itself with a unique ID (a number between 0 and 15). The host bus adapter always has the ID 7. The bus lines must be terminated with passive or active terminators. The host bus adapter controls and monitors the exchange of data on the bus. In many cases it was a plug-in card installed in the computer.

The first version of the Small Computer System Interface from 1986 enabled data rates of a maximum of five megabytes per second on an eight-bit bus. The maximum cable length could be up to six meters. Subsequent versions of the standard increased the clock rate and the width of the bus to increase the data rate. For example, Ultra-320-SCSI from 2002 allowed up to 320 megabytes per second with cable lengths of up to twelve meters.