Data Clinic Knowledgebase: Data Recovery and Hard Disk reference section > SCSI hard disks - Primer

What is SCSI?

The SCSI acronym stands for Small Computer Systems Interface and is pronounced "skuzzy". It's a standardized way of connecting hardware peripherals to a computer using standardized hardware and control commands. The standard can be divided into SCSI-1 (standard SCSI) and SCSI-2 (SCSI fast, SCSI wide, and SCSI wide and fast). The most recent standard as of this writing is SCSI-2 but the SCSI-3 standard is really a family of standards and is defined in that context.

The devices on the SCSI bus talk to the computer through one of the devices connected to the SCSI bus. That device is called the controller and has an interface to one of the data buses of the computer. On modern PCs the SCSI controller is usually connected to the PCI bus either as an on-board solution on motherboards or as a separate card in a PCI slot. Other buses are becoming more viable these days as the carrier of the SCSI protocol, like FireWire (IEEE 1394) and USB. Besides disk drives, todays SCSI standard supports several other devices. Examples are removable media devices (CD-ROM, CD-R, Zip drives), video frame grabbers, scanners and tape streamers (DAT, DLT).

Each SCSI device can have multiple logical sub-units (LUNs) which can be quite handy when using CD changers (both readers and burners) or tape arrays, but most units have only one logical unit like hard drives and single disk CD-ROMs. All devices have the ability to release the bus after receivng requests to perform time consuming operations not requiring the availability of the bus. This leaves it free for other devices to use for transferring data or receiving commands.

Standards

ANSI defines the SCSI standards which describe the characteristics and capabilities of the interface. There are currently two main standards, SCSI-1 and SCSI-2. SCSI-3 is a family of standards that is currently under development.

SCSI-1

This is the original SCSI standard from 1986 and defines cabling, command sets and transfer modes. It uses an 8-bit wide bus where data transfers are done with a 5 MHz clock resulting in a 5 MB/s peak transfer rate. At most, 8 devices can be connected to the bus as a direct consequence of the bus width as each device is addressed using one of the 8 data lines. Most devices used on this bus are hard drives as the command set doesn't explicitly support other media in the command set.

SCSI-2

The limited transfer rate and device support forced the development of the SCSI-2 standard which was approved by ANSI in 1990. Greater transfer rates were accomplished by doing two things:
- Increasing the data clocking rate to 10 MHz. This is called Fast SCSI.
- Increasing the bus width to 16 bits from the original 8 bits allowed doubling the transfer rate. This is called Wide SCSI and requires different cabling compared to normal SCSI. A side effect of this is that a wide SCSI bus can use 16 devices.

Enhancements to the command set were completed. This made it possible to connect devices to the SCSI bus that previously required proprietary controllers like CD-ROMs. Cable specifications had to change to support the higher data clock.
Command queueing is one of the strong points of SCSI and allows multiple outstanding requests between devices on the bus. A maximum of 256 commands on each LUN (Logical Unit Number) of a SCSI device can be queued.

SCSI-3

In order not to stall development it was decided that future development of SCSI should be divided into different layers and command sets. This has made it possible to develop physical transports using new technology like fibre channel for massive increases in data transfer rate, while at the same time keep and develop the older technologies like the parallell interface.
The most common SCSI interface today is the parallel interface and the improvements in SCSI-3 made compared to SCSI-2 are again higher data clocking speed and better cabling to handle the higher speed. The higher speeds used on the parallel interface are named Ultra (20 MHz), Ultra2 (40 MHz) and Ultra3 (40 MHz double transition clocking).

Cabling

All of the SCSI standards evolved through defining new and improved ways of interconnecting the SCSI devices to achieve safe data transfer. The parallel interface (ribbon cable) that was defined in the first SCSI standard have always included termination of the ends of the bus to prevent ringing and can currently safely clock data at 40MHz using low voltage differential signals on the data lines. The SCSI-3 family of standards includes FireWire and other new technologies to further increase connectivity.

Future for SCSI

From the beginning the SCSI standard was carefully planned and evolved over some years before beeing defined as an official ANSI standard. Because of this the SCSI architecture is much more mature than competing interfaces and can evolve and expand further. This is obvious by looking at the SCSI-3 family of standards which quickly has adopted new technologies. It will undoubtedly continue to do so in the future.

Because it is used in more demanding applications SCSI devices usually have a lot of "intelligent" logic to increase performance and are usually of better build quality and this makes them the obvious choice for the high-performance aware that requires reliability and state of the art performance. SCSI devices usually have a higher price tag than their counterparts in the market place but we'd like to point out that you get what you pay for. SCSI will always have niche in the computer business because of this.