SCSI
The Small Computer System Interface (SCSI, sometimes spoken [ skʌzi ] ) has emerged as a standard parallel port for the connection and data transfer between peripherals and the computer bus. Compared to ATA / ATAPI is an essential feature of the historical parallel interface SCSI is the possibility of more than two devices to be connected. Since the SCSI-3 standard SCSI is, however, defined as a protocol that can make use of different transport mechanisms. Thus there are, for example, the Serial Attached SCSI ( SAS ) with similar cables as Serial ATA ( SATA), as well as Fibre Channel or iSCSI over TCP / IP and Ethernet.
- 4.1 Compatibility
- 4.2 Installation
- 4.3 roles
- 4.4 Further developments
- 5.1 connector for external SCSI devices
- 5.2 Connectors and cables for internal SCSI devices
- 5.3 SCSI devices
- 5.4 SCSI detail photos
Formation
Founded by Alan Shugart after his departure from Shugart Associates Shugart Technology company led SCSI 1979 under the name SASI ( Shugart Associates System Interface). After 1981 other companies - particular NCR - had decided to support SASI, SASI was renamed SCSI. NCR also initiated the beginning of the following year standardization process, and 1986, the SCSI specification was standardized as X3.131 - 1986 by the ANSI. Since then, SCSI has become the industry standard, which could be used in almost any computer system (there's even SCSI implementations for the Commodore 64 home computer ).
Use
To connect to a computer SCSI devices, a host bus adapter ( HBA short ) requires that controls the data transfer on the SCSI bus. The device to be connected has a SCSI controller, for transmitting the data via the bus and to communicate with the host bus adapter. The SCSI host bus adapter may be integrated on the motherboard, but is often retrofitted as a plug-in card. SCSI is most often used for connecting hard disks 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 device-independent, so that, theoretically, each SCSI peripheral device can use.
Chance are SCSI and VHDCI ( Very High Density Cable of interconnect, a miniaturized version) connectors used in industrial control technology and Ein-/Ausgabeports of devices for analog and digital data acquisition and output, for example to PXI modules.
While in the 1990s and early 2000s SCSI had a certain distribution especially at workstations, and servers, it is now used as a SAS almost exclusively in servers. While previously had (hard drives, CD drives, CD burners, scanners ) used ambitious private users SCSI devices, today there is only an extremely small group of users, which operates partly antiquated equipment for technical and nostalgic reasons. Had established, especially since the wide availability of powerful devices with USB interface and since SATA drives and drives as standard, SCSI devices disappeared almost completely from the retailer shelves and are almost exclusively to refer online yet. Then as now, SCSI devices are more expensive than those with IDE / SATA. This is due mainly to the extremely low numbers. In addition, for example, SCSI hard drives are designed for continuous operation and thus have a longer life than comparable IDE / SATA disks. This is also reflected in the price. Further costs arise due to the addition to be incorporated SCSI controller (quasi all consumer motherboards have only IDE / SATA controller onboard).
The various SCSI standards
SCSI has been further developed over the years. The following standards (in chronological order) are defined:
SCSI-1 (1986)
The original SASI ( Shugart Associates Systems Interface ) derived and published by the ANSI standard in 1986. SCSI-1, also called narrow SCSI, offers a bus with 8 -bit width and parity checking, asynchronous with about 3.5 MB / s or synchronous with 5 MB / s runs, the maximum cable length amounts to 6 meters and it can be connected to seven devices ( the competing ATA interface at that time was limited to 1.5 m cabel length and four devices). A variation of the SCSI -1 standard ( differential SCSI ) used a differential signal levels based on transmission technology, and allowed a cable length of 25 m. To distinguish it from the modern low-voltage differential interface ( LVD) is called the old technology today 's high-voltage differential ( HVD ). HVD was expensive, electrically incompatible and was mainly used in a professional environment to building local, combined memory computer networks.
SCSI-2 (1989)
This standard was adopted in 1989 and formed the basis for the variants Fast SCSI and Wide SCSI. Fast SCSI (8-bit, F ) doubled the bus clock, which is a transfer rate of up to 10MB / s meant. Due to the high bus clock wiring was only allowed to be more than 3 meters long. Because Fast SCSI using the same cable as SCSI-1, it spread very quickly. Existing installation were easily upgradeable device by device, the mixing operation was possible.
Wide SCSI retained the bus clock (and hence the allowable cable length) at, but doubled the bus width to 16 bits. This also led to 10 MB / s, but new, 68 -pin cable were needed. Pure Wide SCSI had no market significance. SCSI-2 also specified a 32- bit version of Wide SCSI, which used two 16 -bit bus per cable. This technique has been ignored by SCSI device makers mostly, and therefore abolished with SCSI-3 again.
To get to 20 MB / s, we combined Fast SCSI ( high bus clock ) and Wide SCSI ( dual bus width ) to each other. This variant was very common and is often called Fast Wide SCSI. When it is spoken today by Wide SCSI, Fast Wide is almost always meant as pure Wide had no meaning.
Ultra - SCSI ( 1992)
Ultra SCSI was introduced in 1992 as part of the comprehensive SCSI-3 standard. The original official name was almost 20 (SCSI ) or Wide Fast 20 The bus speed was doubled again to 20 MB / s for " narrow " (8 bits, U) systems and 40 MB / s for the wide- version ( 16-bit, UW). The maximum cable length stayed with up to four devices 3 m, with five to eight, she was reduced to max. 1.5 m, as was increased to the cable quality, which U- SCSI won an undeserved reputation to be very sensitive to cable length and environmental conditions the request. ( Most wore inferior cables and plugs, old Wide SCSI cable or passive termination resistors ( terminators), which were officially no longer allowed on these issues debt. The cable ends had to echo cancellation are terminated active. ) Longer cables were still possible with HVD devices (eg Adaptec 2944 UW differential controller).
SCSI-3 (1993)
SCSI-3 standard is a set of stand-alone document which also contains records for alternative transfer techniques such as IEEE 1394 ( FireWire standard Apple ), and Fiber Channel for the first time. Also included is VHDCI ( Very High Density Cable of Interconnect), a 68-pin connector system with 0.8mm pitch, which also appears under the brand CHAMP.
Ultra -2 SCSI ( 1997)
This standard was introduced in 1997 and brought a new differential bus with low signal level with him ( Low Voltage Differential LVD). Therefore, ultra -2 is sometimes also referred to as LVD SCSI. The conventional transmission technique is referred to, in contrast, as an SE SCSI ( Single Ended SCSI). It has by the LVD technique possible to increase the length of cable to 12 m and in much better noise immunity. At the same time, the transfer rate to 40 MB / s (narrow, 8-bit, U2) and 80 MB / s (wide, 16-bit, U2W ) was increased. Ultra -2 SCSI only had a short life as it was soon replaced by Ultra 3 ( Ultra 160) SCSI.
Ultra - 160 (1999)
This version was introduced in late 1999, and is a partial implementation of the Ultra -3 SCSI standards [ The degree of conformity Umsetzung/Ultra-3 SCSI hung from the manufacturer ], abbreviated U160 or in later versions with full Ultra SCSI -3 compatibility U3. Basically, it was an improvement of the Ultra - 2 standards by the transfer rate through the introduction of the double edge clock ( Double - Edge Clock) to now 160 MB / s was doubled. In this method, a data word is transferred both on the rising as in the falling edge of the clock signal. Ultra -160 SCSI also offers new features such as a cyclic redundancy check ( CRC, Eng. CRC) and domain validation. In the latter, test data to the device and which are sent back during the initialization of the bus. If errors occur, the speed is reduced until the transmission works flawlessly. From Ultra -160 SCSI, there were only 16 bit wide buses.
Ultra - 320 (2002)
Ultra -320 is an ultra -160 dar. with a 320 MB / s transfer rate doubled and represents the completion of the development of the parallel SCSI data transfer
The introduction of ultra -640 (Fast -320 ) had once again doubled the speed to 640 MB / s, according to this standard but no devices were built. Instead, the industry relies on Serial Attached SCSI ( SAS).
A total of 16 devices can be connected or 15 devices plus the host bus adapter.
SCA
SCA/SCA-2 ( Single Connector Attachment) is not a stand-alone SCSI standard, but a 80 -pin connector that is commonly used in hot-plug caddy. This connection format is available for both the SE, LVD, HVD and for the transmission format. In contrast to the other SCSI connectors SCA includes the power supply ( 5 and 12 volts ) and the control lines for the SCSI ID and the LED displays.
General information about the SCSI standards
Compatibility
Ultra 2, Ultra 160 and Ultra 320 devices can be mixed on the LVD bus with no performance loss because the host bus adapter, the speed and other management decisions with any device denies individually. Single-ended devices should not be connected to the LVD bus, as the forces the whole bus over to the " single-ended " - with the known limitations of speed ( 40 MB / s ) and the cable length ( 3 m). There are SCSI bridges that bypass by an electrical breakdown of the bus this limitation on some HBAs even incorporating such.
Generally SCSI devices are backward compatible, which means that it is possible to use an Ultra 3 hard drive to an Ultra 2 host bus adapter (but with reduced speed and without specific Ultra - 3 commands ).
HVD SCSI devices ( including terminators ) are in principle not compatible with SE or LVD SCSI devices, but can also be adapted with converters / bridges.
Installation
Each SCSI device (including the host bus adapter ) must be configured with a unique ID number. The host bus adapter or controller has been generally associated with the ID = 7. Thus, each device on the SCSI bus are clearly identified and set the priority of the devices. The priority of the IDs, in descending order of 7-0 and then 15 to 8, may have restrictions on the part of the BIOS or operating system for the allocation of ID numbers. Any device with an ID has configured in addition include at least one LUN (Logical Unit Number). With SCAM (SCSI Configured Automatically ), efforts were made to simplify this sometimes complicated configuration. So SCAM enables a largely automated configuration. For newly attached devices did not have to manually create a SCSI ID to be entered, for example; SCAM completed the self- employed. However, SCAM has never gained practical importance.
Each SCSI channel must be terminated with exactly two terminators - at each end of a physical line. Usually the host bus adapters provide the ability to schedule a side of the bus, so that usually only one plug terminator is required. There are both active and passive terminators, should be with the active type with preference given (for Ultra SCSI and LVD buses it is imperative; Single-Ended and LVD terminators are different, they are there but also very 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 a " narrow " when the wide devices are connected directly behind the host bus adapter and the narrow devices at the end of the bus. A cable is needed, terminates the the " wide" part of the bus and the narrow loops through. This is also called high- 9- or half - scheduling. Special commands allow the host bus adapter to determine which width of the bus to a device.
Roll
In the devices participating in the communication, one distinguishes between the SCSI initiator and the SCSI target. The SCSI initiator initiates the connection by sending a SCSI command. A SCSI Target does not perform connection setup, but waits for requests from the SCSI initiator. The SCSI target then provides one or more logical unit numbers (LUNs) for addressing the instruction and data stream.
Usually, the computer in the role of the SCSI initiator and a peripheral device ( storage, printer or the like. ) In the role of the SCSI target. When Initiator-/Target-Beziehung is a Client-/Server-Beziehung. Typically, each SCSI ID is assigned on the bus one of these roles firmly.
Only in very exceptional cases a device to perform both role. Thus, in older plants (eg Commodore 8000 ) it was possible to send a file from a floppy drive directly to a printer on the same bus without passing the data on the computer. In this case, the computer sends (as initiator) a command to the floppy drive ( as a target ), and then the floppy drive changed the role and connect to the printer initiated.
Further developments
In the past, SCSI was widespread on all types of computers. The same is true today for high-performance workstations, servers and high-end peripherals yet. Desktop and notebook computers, however, used mainly the slower ATA or use the Serial ATA interface since around 2004 with their drives, and USB ( USB uses SCSI -like commands for some operations ) for other devices, since these interfaces, although they are less universally applicable, in the implementation cost less.
The original SCSI standards specified the physical properties of the buses and the electrical signaling as well as a set of instructions that defined the different commands that could run the SCSI devices. This instruction set is also independent of the SCSI bus very useful because it is mature and there are a large number of users and developers familiar with it. Therefore, parts of the dive SCSI command set up in other standards such as ATAPI, Fibre Channel, Serial Storage Architecture, InfiniBand, iSCSI, USB, IEEE 1394, Serial Attached SCSI.
Some observers expect the iSCSI standard, an embedding of SCSI-3 over TCP / IP, will replace the long run Fibre Channel, because currently they have achieved with Ethernet data rates grow faster than Fibre Channel or other connection technologies achievable rates. iSCSI can therefore serve both the low- and the high-end market with a cost-effective solution. iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged.
The most important data at a glance
Connector for external SCSI devices
50 -pin " Centronics " connector, 8 -bit bus width
High Density, 8 and 16-bit bus width ( 'Half Pitch DB50 ' and ' DB68 ')
VHDCI connector
Connectors and cables for internal SCSI devices
68 -pin standard cable, 16-bit bus width
68 -pin LVD cable with twisted wires and Terminator, 16-bit bus width
SCSI devices
SCSI hard disk
SCSI CD burner
SCSI detail photos
SCA adapter ( SCA connector is on the reverse )
80 -pin Ultra320 connecting a hard drive