SCSI is an entirely different interface than the more popular IDE. It is more of a system level interface, meaning that it does not only deal with disk drives. It is not a controller, like IDE, but a separate bus that is hooked to the system bus via a host adapter. A single SCSI bus can hold up to eight units, each with a different SCSI ID, ranging from 0 to 7. The host adapter takes up one ID, leaving 7 ID’s for other hardware. Typical SCSI hardware includes hard drives, tape drives, CD-R/RW drives, scanners, etc.

SCSI’s popularity is increasing. Speed seems to be the main reason for this, although I will show further down that this really isn’t anything to get excited about. One advantage is that there are a multitude of hardware types that can use a SCSI bus. The interface is very expandable, whereas IDE is pretty much limited to hard drives and CD-ROMs.

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, and the software drivers for the device cannot work with an adapter made by someone else. 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, SCSI is pretty much a mess for the end-user. Don’t get me wrong, here, though. SCSI is a relatively easy thing to implement, should you wish to.

SCSI Evolution

SCSI has come a long way. 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. Newer PC BIOS versions have been designed with built-in SCSI support, but this is not always included on a motherboard. Not to fear, though. There is an extension BIOS on the host adapter. 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 easier to work with and will get easier as technology progresses.

SCSI Standards

There is more than one version of SCSI out there:

• SCSI-1. SCSI-1 was standardized by ANSI in 1986. While this outlined the physical and electrical traits of SCSI, it failed to outline a common set of commands so that all manufacturer’s hardware would work together. The industry, then, decided to agree on a minimum set of 18 basic commands. This command set was called the Common Command Set (CCS). All SCSI hardware supported the CCS. CCS became the basis for SCSI-2…


• SCSI-2. A more advanced version of the original SCSI that provided extra commands for other types of devices. SCSI-2 also provided extra speed with options called Fast SCSI (basically a version clocked at 10 MHz rather than the normal 5 MHz) and a 16-bit version called Wide SCSI (as opposed to the normal 8-bit bus width). Fast/Wide SCSI was a version that used the 16-bit bus width while also running at 10 MHz. SCSI-2 had a feature called command queuing which gave the SCSI device the ability to execute commands in an order that would be most efficient. This is most useful on hard drives using OSes that are multitasking. The standard for SCSI-1 and SCSI-2 is somewhat clouded. Almost all features and commands of SCSI-1 are supported in SCSI-2, and most SCSI-1 hardware is called SCSI-2. Many manufacturers boast that their equipment is SCSI-2. This makes it seem better, but in reality, it may not support the extra features that were included in the true SCSI-2 revision. This also means that SCSI-1 adapters will work with SCSI-2 hardware. SCSI-1 and SCSI-2 compliant hardware is the same.


• SCSI-3. Debuting in 1995, the SCSI-3 standard is really a classification for a broader set of different SCSI types. Because of the constant evolution, SCSI-3 is really not considered a standard, but more a set of different specs that have been “officially adopted” by certain manufacturers. Each spec is built upon the SCSI Parallel Interface (SPI), which defines the way in which SCSI devices communicate with each other. The different specs mostly start with the word “Ultra”, with “Ultra” using SPI1, “Ultra2″ using SPI2, and so on. The Fast and Wide terms are assigned as defiend above.

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