SCSI - technical definition

(Small Computer System Interface) Pronounced "scuzzy." SCSI is a hardware interface that allows for the connection of up to 15 peripheral devices to a single PCI board called a "SCSI host adapter" that plugs into the motherboard. SCSI uses a bus structure and functions like a mini-LAN connecting 16 devices, but the host adapter counts as one device. SCSI allows any two devices to communicate at one time (host to peripheral, peripheral to peripheral).

Host adapters are also available with two controllers that support up to 30 peripherals. Introduced in 1986 and originally developed by Shugart Associates (see SASI), SCSI is widely used in servers, mainframes and storage area networks (SANs).

The Daisy Chain
SCSI peripherals are daisy chained together. Each device has a second port used to connect the next device in line. An earlier advantage of using SCSI in a desktop PC was that a scanner and several drives (CD-Rs, Zip, hard disks, etc.) could be added to one SCSI cable chain. However, this became less important since the advent of the USB interface.

RAID Was Originally SCSI
Until the late 1990s, SCSI hard disks were the only ones used in RAID configurations for improved performance or fault tolerance. Since the advent of IDE RAID controllers, SCSI and IDE have become more equalized, although SCSI continues to be the drive interface of choice in the server market. See RAID, SAS and SCSI switch.

SCSI Signaling

The total length of the SCSI chain is based on the signaling used. Following are the three types.

Single Ended
Single-ended SCSI allows devices to be attached to a total cable length up to six meters or only three meters, depending on type (see below). Single-ended signaling uses data and ground lines.

High Voltage Differential Signaling (HVDS)
Also called just "differential SCSI" because it was available before there was a second differential option, HVDS supports cable lengths up to 25 meters. In order to increase distance, differential signaling uses data low and data high lines and costs more than single-ended (see differential signaling).

Low Voltage Differential Signaling (LVDS)
Ultra2 SCSI introduced LVDS, which supports cable lengths up to only 12 meters. LVDS requires less power and is less costly than HVDS, because the transceivers are built into the controller chips.

SCSI Versions

The different SCSI types provide backward and forward compatibility. If a new SCSI host adapter is used with an older SCSI drive, the drive will run at its maximum speed. If an older SCSI host adapter is used with a newer drive, the drive will run at the host adapter's maximum speed. Following are the various SCSI types:

 SCSI SPECIFICATIONS
                           Tfr   Bus Length In
                  Bus      Rate  Meters For
                  Width Max  MB  Device Types:
 Type            (bits) Dev Sec   SE LVD HVD  Pins
 SCSI-1             8    8    5    6  12* 25    25

 SCSI-2             8    8    5    6  12* 25    50

 Fast SCSI          8    8   10    3  12* 25    50

 Wide SCSI, aka
 Fast Wide SCSI     8   16   20    3  12* 25    68

 Ultra SCSI         8    8   20    3   -   -    50

 Wide Ultra SCSI   16   16   40    -  12* 25    68
 Wide Ultra SCSI   16    8   40  1.5   -   -    68
 Wide Ultra SCSI   16    4   40    3   -   -    68

 Ultra2 SCSI        8    8   40    -  12  25    50
 Wide Ultra2 SCSI  16   16   80    -  12  25    68

 Ultra3 SCSI, aka
 Ultra160          16   16  160    -  12   -    68

 Ultra4 SCSI, aka
 Ultra320          16   16  320    -  12   -    68

 Serial Attached
  SCSI (SAS)        1   16K 300   See SAS.


 12* - LVD was not part of these specs; however,
       if all devices are LVD, 12 meters applies.
       If any device is single ended, then length
       in SE column applies.

 Information for this chart was obtained from the
 SCSI Trade Association (STA), San Francisco, CA
 (www.scsita.org).

ASPI and CAM

The Advanced SCSI Programming Interface (ASPI) and Common Access Method (CAM) provide common software interfaces between drivers and SCSI host adapters. ASPI was developed by Adaptec, and CAM is an ANSI standard. Most SCSI products are ASPI or CAM compliant. Prior to ASPI and CAM, hooking up two SCSI devices often meant plugging in two host adapters, negating SCSI's advantage of connecting multiple peripherals.

IDs, LUNs and Termination

External SCSI devices have two ports, one for the incoming cable and another for the outgoing cable to the next device. An internal SCSI device has a single port that attaches to a ribbon cable with multiple connectors. Each device must be set to a unique ID number, which is normally done by flipping rotary switches on external devices or by setting jumpers on internal ones. The ID determines the device priority, which starts at 7 and goes to 0 and then from 15 to 8. The host adapter defaults to 7, the highest priority.

SCAM Sets IDs Automatically
A subset of Plug and Play, called "SCSI Configured Automatically" (SCAM), allows IDs to be set by software rather than manually. Both the host adapter and peripheral must support this.

Logical Unit Numbers (LUNs)
Each SCSI device can be further broken up into logical units, identified by logical unit numbers (LUNs) 0 to 7 for 8-bit devices and 0 to 15 for 16-bit devices. Although most SCSI drives contain only one disk inside and are addressed as LUN 0, optical disc libraries and RAID arrays contain multiple drives, each of which can be addressed independently via LUN numbers. See LUN.

Termination
The device at the end of a SCSI chain must be terminated by setting a switch or plugging a resistor module into the open port. Usually, host adapters default to terminated. If both internal and external devices are used, the host adapter termination must be removed, and termination must be applied to the ends of both chains.