SMALL COMPUTER SYSTEM INTERFACE(SCSI)
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SCSI is actually an acronym for Small Computer System Interface and it is pronounced as “skuzzy”. It is the second-most popular hard disk interface used in PCs today. It’s a high-speed, intelligent peripheral I/O bus with a device independent protocol for transferring data between different types of peripheral devices. The SCSI bus connects all parts of a computer system so that they can communicate with each other. The bus frees the host processor from the responsibility of I/O internal tasks. A SCSI bus can be either internal, external, or cross the boundary from internal to external. The SCSI protocol is a peer-to-peer relationship: one device does not have to be subordinated to another device in order to perform I/0 activities. Only two of these devices can communicate on the bus at any given time.
Each SCSI bus can connect up to 8 or up to 16 peripherals; one of those devices will always be the computer or the SCSI card, because they too are devices on the SCSI. SCSI devices are designated as either initiators (drivers) or targets (receivers) and the interface to the host computer is called the host adapter. Every device connected to the bus will have a different SCSI ID, ranging from 0 to 7. The host adapter takes up one ID leaving 7 ID’s for other hardware. SCSI hardware typically consists of hard drives, tape drives, CD-ROMs, printers and scanners. .
The reason for the slow taking of SCSI is the lack of standard. Each company seems to have its own idea of how SCSI should work. While the connections themselves have been standardized, the actual driver specs used for communication have not been. The end result is that each piece of SCSI hardware has its own host adapter. So, due to the lack of an adapter standard, a standardized software interface, and a standard BIOS for hard drives attached to the SCSI. Adapter.
History & Evolution
In the beginning, one couldn’t even use a hard drive on the bus. This was mainly because the BIOS in those systems were designed to use the ST506/412 controller. With the IDE, the BIOS was easily changed because of the similarity to ST506/412 on the WD1003 controller. At the register level, though, SCSI was very different, and would have required an entirely new set of BIOS in the PC.
What we currently know of as the SCSI interface had its beginnings back in 1979. Shugart Associates, led by storage industry pioneer Alan Shugart (who was a leader in the development of the floppy disk, and later founded Seagate Technology) created the Shugart Associates Systems Interface (SASI). This very early predecessor of SCSI was very rudimentary in terms of its capabilities, supporting only a limited set of commands compared to even fairly early “true” SCSI, and rather slow signaling speeds of 1.5 Mbytes/second. For its time, SASI was a great idea, since it was the first attempt to define an intelligent storage interface for small computers. The limitations must be considered in light of the era: we are talking about a time when 8″ floppy drives were still being commonly used.
Shugart wanted to get SASI made into an ANSI standard, presumably to make it more widely-accepted in the industry. In 1981, Shugart Associates teamed up with NCR Corporation, and convinced ANSI to set up a committee to standardize the interface. In 1982, the X3T9.2 technical committee was formed to work on standardizing SASI. A number of changes were made to the interface to widen the command set and improve performance. The name was also changed to SCSI; I don’t know the official reason for this, but I suspect that having Shugart Associates’ name on the interface would have implied that it was proprietary and not an industry standard. The first “true” SCSI interface standard was published in 1986, and evolutionary changes to the interface have been occurring since that time.
It’s important to remember that SCSI is, at its heart, a system interface, as the name suggests. It was first developed for hard disks, is still used most for hard disks, and is often compared to IDE/ATA, which is also used primarily for hard disks. For those reasons, SCSI is sometimes thought of as a hard disk interface. However, SCSI is not an interface tied specifically to hard disks. Any type of device can be present on the bus, and the very design of SCSI means that these are “peers” of sorts–though the host adapter is sort of a “first among equals”. SCSI was designed from the ground up to be a high-level, expandable, high-performance interface. For this reason, it is frequently the choice of high-end computer users. It includes many commands and special features, and also supports the highest-performance storage devices.
Of course, these features don’t come for free. Most PC systems do not provide native, “built in” support for SCSI the way they do for IDE/ATA, which is one of the key reasons why SCSI isn’t nearly as common as IDE/ATA in the PC world. Implementing SCSI on a PC typically involves the purchase of a storage device of course, but also a special card called a host adapter. Special cables and terminators may also be required. All of this means that deciding between SCSI and IDE/ATA is an exercise in tradeoffs.
SCSI began as a parallel interface, allowing the connection of devices to a PC or other systems with data being transmitted across multiple data lines. Today, parallel or “regular” SCSI is still the focus of most SCSI users, especially in the PC world. SCSI itself, however, has been broadened greatly in terms of its scope, and now includes a wide variety of related technologies and standards, as defined in the SCSI-3 standard. Many high-end systems have built-in SCSI support. There is usually an adapter card or an adapter built in to the motherboard. This native support for SCSI was set in motion by IBM. Their example was followed by many manufacturers. As a result, SCSI integration is becoming very easy to work with and will get easier as technology progresses.
This post was written by: Rajendra Prasad
Rajendra Prasad is a professional blogger, web designer and front end web developer. Follow him on Facebook
