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Small Computer System Interface (SCSI)

Definition - What does Small Computer System Interface (SCSI) mean?

A small computer systems interface (SCSI) is a standard interface for connecting peripheral devices to a PC. Depending on the standard, generally it can connect up to 16 peripheral devices using a single bus including one host adapter. SCSI is used to increase performance, deliver faster data transfer transmission and provide larger expansion for devices such as CD-ROM drives, scanners, DVD drives and CD writers. SCSI is also frequently used with RAID, servers, high-performance PCs and storage area networks SCSI has a controller in charge of transferring data between the devices and the SCSI bus. It is either embedded on the motherboard or a host adapter is inserted into an expansion slot on the motherboard. The controller also contains SCSI basic input/output system, which is a small chip providing the required software to access and control devices. Each device on a parallel SCSI bus must be assigned a number between 0 and 7 on a narrow bus or 0 and 15 on a wider bus. This number is called an SCSI ID. Newer serial SCSI IDs such as serialattached SCSI (SAS) use an automatic process assigning a 7-bit number with the use of serial storage architecture initiators.

Techopedia explains Small Computer System Interface (SCSI)

Peripheral devices are attached to the CPU through buses and interfaces, and SCSI is the most common interface for attaching these devices. SCSI's efficiency is the main reason it is so widespread. SCSI was revolutionary in regards to data transfer and compatibility when compared to the parallel data transfer interfaces used in earlier days. SCSI also allows backward compatibility where devices were compatible with earlier version of SCSI. These devices can still be attached to a newer version of SCSI, but the data transfer rate will be slower. The original SCSI used a SCSI parallel bus.

Serial SCSI architecture was introduced in 2008, which is a lot faster and more reliable than the SCSI parallel bus. The Internet protocol used is the Internet SCSI. This interface has no physical attributes and uses TCP/IP to transmit data. SCSI was established in 1978 by the Shugart Associates System Interface and industrialized in 1981. The pioneer of the technology was fathered by Larry Boucher who worked at Shugart Associates and later at Adaptec, a company producing SCSI, serial ATA, serial attached SCSI, and supporting host adapters. The SASI was designed as an interface between a hard disc drive and a host PC for data communication. It featured a 50-pin flat ribbon connector using an 8-bit parity bus and supported up to 8 devices. The SASI sent data in blocks with a clock speed of 5 MHz and ran asynchronously at 3.5 MBps or 5 MBps in synchronous mode.

By 2000 the Ultra 640 SCSI had a clock speed of 160 MHz, which caused issues with parallel cabling. To remedy the problem the serial SCSI was adapted. Device connections were now hot swappable plus compatible with serial advanced technology attachment at a lower cost. With the use of fiber channel arbitrated loop and optical fiber cables the clock speed went up to 4 GHz. The SCSI can support external and internal SCSI devices using one connector. Internal devices are connected by a single ribbon cable. The internal parallel SCSI ribbon cable generally has two or more 50, 68 or 80–pin connectors. External devices use a port. The external cable is often shielded and has 50 or 69–pin connectors on each end, depending on the SCSI bus standard. There is also a single connector attachment, which is an internal connection including in two versions.

All SCSI devices and the host adapter support a single daisy chain. A daisy chain connects the devices in a series of nodes one after the other by using hardware configuration. The SCSI interface supports various devices depending on the SCSI version. The advantage of a daisy chain is the ability to add an additional node anywhere on the chain. Each device in the chain can adjust one or more signals before transmitting to the next device. The SCSI-2 supports 16 devices, ultra SCSI supports 5 to 8 and the ultra-320 SCSI supports 16. The serial attached SCSI adapted in 2010, can support up to 16,256 addressable devices per port with a transfer rate up to 3 Gbps.

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