Small Computer System Interface

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Hewlett-Packard 2 GB SCSI hard drive , 1999

The Small Computer System Interface ( SCSI , sometimes pronounced [ ˈskʌzi ]) is a family of standardized protocols and interfaces for the connection and data transfer between peripheral devices and computers.

Originally the data transfer was only parallel . Today, in addition to parallel SCSI (SPI) , SCSI defines other transmission paths such as Serial Attached SCSI (SAS), Fiber Channel and iSCSI .


Shugart Technology , founded by Alan Shugart after leaving Shugart Associates , introduced SCSI in 1979 under the name SASI (Shugart Associates System Interface) . After other companies - NCR in particular  - decided to support SASI in 1981 , SASI was renamed SCSI . NCR also initiated the standardization process that began the following year, and in 1986 the SCSI specification was standardized by ANSI as X3.131-1986 .

Subsequently, SCSI became the industry standard that could be used in almost every computer system (there are even SCSI implementations for the Commodore 64 home computer). The classic parallel bus ( SPI SCSI Parallel Interface ) has been supplemented over the course of time by a number of more powerful and flexible variants and has now been replaced.

The most important standards at a glance

interface Transmission
[MByte / s]
Bus width
Bus clock [MHz]
data rate
Cable length
of devices
Cable type (SE = Single-Ended,
HVD = High-Voltage-Differential,
LVD = Low-Voltage-Differential)
Parallel SCSI SCSI (SCSI-1) 5 8th 5 SE: 6
HVD: 25
8th 50-pin, SE or HVD
Wide SCSI (SCSI-2) 10 16 16 68-pin, SE or HVD
Fast SCSI (SCSI-2) 10 8th 10 SE: 3
HVD: 25
8th 50-pin, SE or HVD
Fast Wide SCSI (SCSI-2) 20th 16 16 68-pin, SE or HVD
Ultra SCSI (SCSI-3) 20th 8th 20th SE: 1.5-3 h
HVD: 25
5-8 50-pin, SE or HVD
Ultra Wide SCSI (SCSI-3) 40 16 SE: 5-8 a HVD: 16 68-pin, SE or HVD
Ultra2 SCSI (SPI-2) 40 8th 40 12 8th 50-pin, twisted pair, LVD or HVD
Ultra2 Wide SCSI (SPI-2) 80 16 16 68-pin, twisted pair, LVD or HVD
Ultra-160 SCSI (SPI-5) 160 16 40 GDR 12 16 68-pin, twisted pair, LVD
Ultra-320 SCSI (SPI-5) 320 16 80 GDR 12 16 68-pin, twisted pair, LVD
IEEE-1394 S3200 400 serial 3.2 Gbit / s 4.5 63
Serial Attached SCSI SAS-3 1200 serial 12 Gbit / s c d 25th 65.025 SFF 8484/8482 (typical)
SAS-4 2400 serial 22.5 Gbit / s g d 65.025
Fiber Channel 1 Gbit / s 98.4 serial 1.0625 Gbit / s c d FC-AL: 127,
FC-SW: 2 24
2 Gbit / s 197 serial 2.125 Gbit / s c d
4 Gbit / s 394 serial 4.25 Gbit / s c d
8 Gbit / s 788 serial 8.5 Gbit / s c d
10 Gbit / s 1181 serial 10.52 Gbit / s e d
16 Gbit / s 1575 serial 14.025 Gbit / s e d
iSCSI via Ethernet ( IPv4 , MTU  1500) per gigabit / s 119 serial, partly multilane practically
IPv4: 2 32 ,
IPv6 : 2 128
via Ethernet (IPv4, MTU 9000) 10 Gigabit / s each 1239 serial, partly multilane
ATAPI over PATA (UDMA-6) 133 16 66 0.45 2 Flat ribbon cable 80-pin, SE
ATAPI over SATA 6 Gb 600 serial 6 Gbit / s d f SATA: 1, eSATA: 2 15 with port multiplier Twinax
a There are 16 addresses available, but due to the short cable length and the specified minimum distance between two devices, only a correspondingly smaller number of devices can be connected.
b per port
cper link, full duplex, 8b10b coding
d Point-to-point, no bus
eper link, full duplex, 64b66b coding
f half duplex, 8b10b encoding
Gper link, full duplex, 128b150b encoding
H The bus can be up to 3 meters long if a maximum of 3 devices are connected to the SCSI controller.

Parallel SCSI


Basic sequence for establishing a connection between two SCSI devices

In order to be able to connect SCSI devices to a computer, a host bus adapter (HBA for short) is required which controls the data transfer on the SCSI bus. The device to be connected has a SCSI controller to transmit the data over the bus and to communicate with the host bus adapter. The SCSI host bus adapter can be integrated on the motherboard , but is often retrofitted as a plug-in card . SCSI is mostly used to connect hard drives and tape drives , but it can also be used with a number of other devices, such as scanners and optical drives . The SCSI standard is designed to be device-independent, so that theoretically every peripheral device can use SCSI.

Occasionally, SCSI and VHDCI (from Very High Density Cable Interconnect , a miniaturized design) connectors are also used in industrial control technology and on input / output ports of devices for analog and digital data acquisition and output, for example on PXI modules.

While SCSI was still somewhat widespread in the 1990s and early 2000s, especially in workstations and servers, today it is used as SAS almost exclusively in servers. While in the past even ambitious private users used SCSI devices (hard drives, CD drives, burners, scanners), nowadays there is only an extremely small group of users who operate some antiquated devices for technical and nostalgic reasons. Especially since the widespread availability of high-performance devices with USB interfaces, and since SATA hard disks and drives had established themselves as the standard, SCSI devices have almost completely disappeared from retailer shelves and are almost only available online. Then as now, SCSI devices are more expensive than those with IDE / SATA . This is mainly due to the enormously lower numbers. In addition, z. B. SCSI hard disks are designed for continuous operation and thus have a longer service life than comparable IDE / SATA disks. This is also reflected in the price. Additional costs result from the additional SCSI controller to be installed (virtually all consumer mainboards only have IDE / SATA controllers onboard).

The various SCSI standards

SCSI has evolved over the years. The following standards (in chronological order) are defined:

SCSI-1 (1986)

The original standard derived from SASI (Shugart Associates Systems Interface) and published by ANSI from 1986. SCSI-1, also known as Narrow SCSI, offers an 8-bit-wide bus with a parity check that runs asynchronously with approx. 3.5 MB / s or synchronously with 5 MB / s, the maximum cable length is 6 meters and seven devices can be connected (the competing ATA interface is limited to 1.5 m cable length and two devices per connection). A variation of the SCSI-1 standard (differential SCSI) used a transmission technology based on differential signal levels and thus enabled a cable length of 25 m. To distinguish it from the modern low-voltage differential interface (LVD), the old technology is now called high-voltage differential (HVD). HVD was expensive, electrically incompatible and was mainly used in the professional environment up to the construction of local, combined storage-computer networks.

SCSI-2 (1989)

NCR 53C94 SCSI-2 controller in the PLCC 84 - housing

This standard was adopted in 1989 and formed the basis for the Fast SCSI and Wide SCSI variants . Fast SCSI (8 bit, F ) doubled the bus clock, which meant a transfer rate of up to 10 MB / s. Because of the high bus frequency, the wiring could only be a maximum of 3 meters long. Because Fast SCSI used the same cables as SCSI-1, it spread very quickly. Existing installations could be upgraded device by device without any problems, mixed operation was possible.

Wide SCSI kept the bus speed (and thus the permitted cable length), but doubled the bus width to 16 bits. This also led to 10 MB / s, but new 68-pin cables were required. Pure wide SCSI had no market significance. SCSI-2 also specified a 32-bit version of Wide SCSI that used two 16-bit cables per bus. This technology was also largely ignored by SCSI device manufacturers, and therefore abolished again with SCSI-3.

In order to get to 20 MB / s, Fast SCSI (high bus speed) and Wide SCSI (double bus width) were combined. This variant was very common and is often called Fast Wide SCSI. When we speak of Wide SCSI today, we almost always mean Fast Wide, since pure Wide had no meaning.

Ultra SCSI (1992)

Host bus adapter with 50 pin cable

Ultra SCSI was introduced in 1992 as part of the comprehensive SCSI-3 standard. The original official name was Fast 20 (SCSI) or Wide Fast 20. The bus speed was doubled again to 20 MB / s for "narrow" (8 bit, U ) systems and 40 MB / s for the wide variant (16 bit , UW ). The maximum cable length remained 3 m for up to four devices, for five to eight it was reduced to max. 1.5 m, the demands on the cable quality were also increased, which earned U-SCSI the undeserved reputation of being very sensitive to cable length and environmental conditions. Inferior cables and connectors, old wide SCSI cables or passive termination resistors (terminators), which were officially no longer permitted, were mostly to blame for these problems. The cable ends must be actively terminated to avoid echoes. Larger cable lengths were still possible with HVD devices (e.g. Adaptec 2944 UW differential controller).

SCSI-3 (1993)

For the first time, SCSI-3 is a bundle of independent standard documents that also contains protocols for alternative transfer techniques such as IEEE-1394 (Apple's FireWire standard) and Fiber Channel. Also included is VHDCI (from Very High Density Cable Interconnect ), a 68-pin connector system with 0.8 mm pitch, which is also available under the brand CHAMP ( Tyco Electronics ).

Ultra-2 SCSI (1997)

Ultra-2-Wide SCSI host bus adapter

This standard was introduced in 1997 and brought with it a new differential bus with a low signal level (Low Voltage Differential, LVD). Hence, Ultra-2 is sometimes referred to as LVD SCSI . In contrast, conventional transmission technology is referred to as SE SCSI (Single Ended SCSI). The LVD technology made it possible to increase the cable length to 12 meters with significantly better noise immunity. At the same time, the transfer rate was increased to 40 MB / s (narrow, 8 bit, U2 ) or 80 MB / s (wide, 16 bit, U2W ). Ultra-2 SCSI only had a short life as it was soon superseded by Ultra-3 (Ultra-160) SCSI.

Ultra-160 (1999)

This version was introduced towards the end of 1999 and is a partial implementation of the Ultra-3 SCSI standard. The degree of implementation / Ultra-3 SCSI conformity depended on the manufacturer, abbreviated to U160 or, in later versions with full Ultra-3 SCSI compatibility, U3 . In principle, it was an improvement of the Ultra-2 standard in that the transfer rate was doubled to 160 MB / s through the introduction of the double-edge clock. With this method, a data word is transmitted on both the rising and falling edges of the clock signal. Ultra-160 SCSI also offers new functions such as a cyclic redundancy check (ZRP, CRC) and domain validation. With the latter, test data are sent back to and from the devices when the bus is initialized. Should errors occur, the speed is reduced until the transmission works without errors. From Ultra-160 SCSI there were only 16-bit wide buses.

Ultra-320 (2002)

Ultra-320 is an Ultra-160 with a transfer rate that has been doubled to 320 MB / s and represents the end of the development of parallel SCSI data transfer.

The introduction of Ultra-640 (Fast-320) would have doubled the speed again to 640 MB / s, but no more devices were built according to this standard. Instead, the industry relies on Serial Attached SCSI (SAS).

A total of 16 devices can be connected or 15 devices plus a host bus adapter.

General information about the parallel SCSI standards


Ultra-2, Ultra-160 and Ultra-320 devices can be mixed on the LVD bus without loss of performance, as the host bus adapter coordinates the speed and other management decisions with each device individually. Single-ended devices should not be connected to the LVD bus, as this forces the entire bus into “single-ended” operating mode - with the known restrictions of speed (40 MB / s) and cable length (3 m). There are SCSI bridges that circumvent this restriction by dividing the bus electrically, some HBAs even have one integrated (e.g. Adaptec AHA-2940U2W)

In general, SCSI devices are backwards compatible , which means that it is possible to use an Ultra 3 hard drive on an Ultra 2 host bus adapter (but at a reduced speed and without specific Ultra 3 commands).

HVD-SCSI devices (including the terminating resistors) are in principle not compatible with SE or LVD-SCSI devices, but can also be adapted with converters / bridges.


A SCSI terminator, here in Centronics design

Each SCSI device (including the host bus adapter) must be configured with a unique ID number. ID = 7 was generally assigned to the host bus adapter or controller. In this way, the individual devices on the SCSI bus are clearly identified and the priority of the devices is determined. The priority of the IDs is 7 to 0 and then 15 to 8 in descending order. There may be restrictions on the part of the BIOS or the operating system when assigning the ID numbers. Each device with an ID also has at least one LUN (Logical Unit Number) configured below it . With SCAM (SCSI Configured Automatically) efforts were made to simplify this sometimes complex configuration. In this way, SCAM enables largely automatic configuration. For example, a SCSI ID no longer had to be entered manually for newly connected devices; SCAM did this on its own. However, SCAM has never achieved practical importance.

Each SCSI line must be terminated with exactly two terminators - one at each physical end of the line. The host bus adapters usually offer the option of terminating one side of the bus, so that only one plug-in terminator is usually required. There are both active and passive terminators, although preference should be given to the active type (it is mandatory for Ultra-SCSI and on LVD buses; single-ended and LVD terminators are different, but they are also very different often combined as "SE / LVD"). Improper termination is one of the most common problems with SCSI installations.

It is possible to make a “wide” bus into a “narrow” one if the wide devices are connected directly behind the host bus adapter and the narrow devices at the end of the bus. This requires a cable that terminates the “wide” part of the bus and loops through the narrow one. One also speaks of high-9 or half termination. Special commands allow the host bus adapter to determine the width of the bus to a device.

Connectors for external SCSI devices

Connectors and cables for internal SCSI devices

SCSI connectors, SCA top, standard 68-pin connector

SCA / SCA-2 ( Single Connector Attachment ) is an 80-pin connector that is often used with hot-plug removable frames. This connection format is available for SE, LVD and HVD transmission formats. In contrast to the other SCSI connections, SCA also contains the power supply (+5 and +12 volts) as well as the control lines for the SCSI ID and the LED displays.

SCSI devices

SCSI detail photos


When it comes to the devices involved in communication, a distinction is made between the SCSI initiator and the SCSI target . The SCSI initiator starts the connection by sending a SCSI command. A SCSI target does not establish a connection, but waits for requests from the SCSI initiator. The SCSI target then provides one or more Logical Unit Numbers (LUNs) for addressing the command and data stream.

Usually the computer is in the role of the SCSI initiator and a peripheral device (memory, printer, etc.) in the role of the SCSI target. The initiator / target relationship is a client / server relationship. Usually each SCSI ID on the bus is assigned one of these two roles.

Only in rare exceptional cases can a device perform both roles. With older systems (e.g. Commodore 8000 series ) it was possible to send a file from a floppy disk drive directly to a printer on the same bus without routing the data via the computer. In this case the computer (as initiator) sends a command to the floppy disk drive (as target), whereupon the floppy disk drive changed roles and initiated a connection to the printer. This is not SCSI, it is IEEE-488 , but the functionality in this case is very similar.

Further developments

In the past, SCSI was widely used on all types of computers. Desktop computers and notebooks have meanwhile mainly used the slower ATA or have been using the faster Serial ATA interface for their drives and USB for other devices since around 2004 (USB uses SCSI-like commands for some operations).

The original SCSI standards specified the physical properties of the buses and the electrical signaling, as well as a command set that defined the various commands that the SCSI devices could execute. This instruction set is also very useful regardless of the SCSI bus, because it is mature and there are a large number of users and developers who are familiar with it. This is why parts of the SCSI command set also appear in other standards such as ATAPI , Fiber Channel , Serial Storage Architecture , InfiniBand , iSCSI , USB , IEEE 1394 and Serial Attached SCSI .

Some observers expect that the iSCSI standard, an embedding of SCSI-3 via TCP / IP , will replace Fiber Channel in the long term , since the data rates achieved with Ethernet are currently growing faster than those achieved with Fiber Channel or other connection technologies. iSCSI can therefore serve both the low-cost and the high-end market with a cost-effective solution. iSCSI keeps the basic SCSI paradigms , especially the instruction set, almost unchanged.

See also


  • Franz-Josef Lintermann, Udo Schaefer, Walter Schulte-Göcking, Klaas Gettner: Simple IT systems . Textbook / specialist book. 5, 1st corrected reprint edition. Bildungsverlag EINS, 2008, ISBN 978-3-8237-1140-7 (pages 88-93).

Web links

Commons : SCSI  - collection of pictures, videos and audio files

References and comments

  1. EMC Proven Professional (Ed.): Information Storage and Management . 2nd Edition. 2012, ISBN 978-1-118-23696-3 , pp. 15-25 .