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small computer system interface (SCSI)

Small ComputerSystemInterface (SCSI) is a storage bus introduced by Shugart in 1982, which was standardized by ANSI in 1986 and is especially suitable for PC peripherals such as hard disks and tape drives. In its original form, it was an 8-bit wide, parallel bus for connectingmass storagedevices and other peripherals. The SCSI bus has since disappeared from PC technology, but it is used in workstations, servers and with Fibre Channel (FC) for data exchange between servers and storage devices.

The SCSI bus, which can be implemented on the motherboard in the form of traces or as a multicore flexible cable, provides transmission control with the SCSI protocol. The SCSI bus must be terminated and ends in the SCSI interface, which is implemented by a multi-pin Sub-D connector or Centronics connector. The continuation of the bus to the peripheral devices takes place in daisy chaining via the SCSI cable.

SCSI ribbon cable for internal devices

SCSI ribbon cable for internal devices

With SCSI, the connected drives are not entered in the Basic Input/Output System (BIOS) of the computer, as is the case with the IDE bus, for example, but are addressed via their own ID number, which is assigned to the respective peripheral device. A total of up to 15 devices can be addressed, which may be divided into a maximum of 256 subdevices. These sub-devices are the Logical Unit Numbers (LUN), which are addressed via sub-IDs.

The development of SCSI technology

Bus widths and data rates of the SCSI standards

Bus widths and data rates of the SCSI standards

SCSI technology is subject to constant further development. The various SCSI versions differ in terms of bus width, transfer rate, the number of peripheral devices that can be connected, the cable length of the SCSI cables and the connectors. SCSI uses the Sub-D connector, the SCA connector and the Centronics connector as connectors.

SCSI-1: SCSI-1 is the original standard with the interface to disk drives. It is an 8-bit bus with asynchronous (3 MB/s) and optional synchronous (5 MB/s) transmission. SCSI-1 uses the 25- and 50-pin DB connectors and the Centronics connector in standard design.

SCSI-2: SCSI-2 offers higher data transfer rates, and the functions and technical options have also been significantly expanded. SCSI-2 uses the 50-pin miniature version of the Centronics connector with 1.27 mm contact spacing.

SCSI

connector versions

connector versions

SCSI-3:SCSI-3 supports data transfer rates of 20 MB/s. Transfers on SCSI-3 can use single-ended and differential signals. SCSI-3 uses the 68-pin miniature version of the Centronics connector

In addition, there is Fast-SCSI with 10 MHz clock and 10 MB/s transfer rate with 8 bit bus width and 20 MB/s with Wide-SCSI with 16 bit/32 bit bus width, Ultra-SCSI with 20 MHz clock and 20 MB/s and in the Wide-SCSI version with 40 MB/s data transfer rate, Ultra-2-SCSI with 40 MB/s (8 bit) and 80 MB/s (16 bit) and Ultra-3-SCSI with 160 MB/s (Ultra-160-SCSI) as well as Ultra-320-SCSI with 320 MB/s. Theoretically, Ultra-640-SCSI with 640 MB/s would also be possible, but the implementation effort is very high.

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Englisch: small computer system interface - SCSI
Updated at: 12.04.2019
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