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About Drives

About SCSI Drives


(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 board called a "SCSI host adapter" that plugs into the motherboard, typically using a PCI slot. SCSI peripherals are daisy chained together. They all have a second port used to connect the next device in line. SCSI 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 from desktop PCs to mainframes, although most desktop PCs come with IDE drives. The advantage of SCSI in a desktop PC is that a scanner and several other drives (CD-Rs, DVD-RAM, Zip drives, etc.) as well as hard drives can be added to one SCSI cable chain. However, this has become less important as alternate interfaces such as USB and FireWire have become popular.

Until the late 1990s, SCSI hard disks were the only ones used in RAID configurations which provide improved performance and/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.

Windows 95/98/NT/2000 and the Macintosh provide internal support for SCSI, but Windows 3.1 and DOS did not. Installing SCSI in a Win 3.1 or DOS machine required adding the appropriate SCSI driver.

A SCSI Chain: The advantage of SCSI is that several peripherals can be daisy chained to one host adapter, using only one slot in the bus.

ASPI and CAM: Because internal support for SCSI was not provided by DOS and Windows 3.x, there was no benchmark for a standard implementation. As a result, hooking up two SCSI devices often meant plugging in two host adapters, negating SCSI's advantage of connecting multiple peripherals.

ASPI and CAM were created to resolve these differences and provide common interfaces between the drivers and the host adapters. Almost all SCSI products are ASPI and CAM compliant. Windows 95 and higher does support popular SCSI host adapters directly. It also supports the ASPI and CAM standards so that older applications and drivers will run even if Windows does not support that peripheral with a native driver.

SCSI Is Like a LAN: SCSI is a bus structure itself and functions like a mini-LAN connecting eight or 16 devices. The host adapter counts as one device, thus up to seven or 15 peripherals can be attached depending on the SCSI type. SCSI allows any two devices to communicate at one time (host to peripheral, peripheral to peripheral).


Type Bus Width (bits) Max Dev Transfer Rate
Bus Lengths, Meters for Device Types: Pins
Single- ended LVD HVD
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 16 40 1.5 - - 68
Wide Ultra SCSI 16 16 40 3 - - 68
Ultra2 SCSI 8 8 40 - 12 25 50
Wide Ultra2 SCSI 16 16 80 - 12 25 68
Ultra3 SCSI, aka
Ultra160 SCSI
16 16 160 - 12 - 68
Ultra4 SCSI, aka
Ultra320 SCSI
16 16 320 - 12 - 68

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 (

Version Compatibility: 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.

SCSI and IDE Drives: You can install SCSI hard disk drives in a PC that already contains one or two IDE disk drives. The IDE drive will still be the boot drive, and the SCSI drives will provide additional storage. Follow the instructions in your SCSI host adapter manual carefully to make the correct settings. Some SCSI host adapters provide floppy disk control, which can be disabled.

IDs and Termination: SCSI devices are daisy chained together. External devices have two ports, one for the incoming cable and another for the outgoing cable to the next device. An internal 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 SCSI ID determines the device priority, which starts at 7 and goes to 0 and then from 15 to 8. The host adapter defaults to the highest priority, which is 7.

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.

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

Parallel to SCSI: There are adapters that allow SCSI peripherals to be connected via the parallel port. Although the parallel port's transfer rate is considerably less than the SCSI host adapter, it does provide a means to hook up SCSI devices to laptops without PC Card slots or desktop machines without available bus slots.

LUNs: Each SCSI device can be further broken up into eight logical units, identified by logical unit numbers (LUNs) 0 to 7. Although most SCSI disks contain only one disk inside and are addressed as LUN 0, CD-ROM and optical disk jukeboxes contain multiple units. Each disk in these devices can be addressed independently via LUN numbers; for example, a four-disk jukebox could be assigned LUN 0 to 3.

Single Ended, Differential and Low Voltage Differential:
There are three types of SCSI signaling. Single-ended SCSI allows devices to be attached to a total cable length of 6 or 3 meters for Fast and Ultra SCSI. Single-ended SCSI is not defined for Ultra2 SCSI and higher.

Differential SCSI, or High Voltage Differential SCSI (HVD), is used when devices are spread across a room, because the total cable length is increased to 25 meters. Differential devices cost more than single-ended ones.

Ultra2 SCSI introduced Low Voltage Differential signaling (LVD or LVDS) that supports cable lengths up to 12 meters. Single-ended SCSI uses a data line and ground. Both HVD SCSI and LVD SCSI use data low and data high lines to increase transmission distance. However, LVD requires less power and is less costly, because the transceivers are built into the controller chips.

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