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 SCSI Disk
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SCSI Interface by David Risley
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SCSI Interface

SCSI Interfaceby David Risley
last update: March 30, 200 ©PC Mechanic

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 defined above.

Name Specification # of Devices Bus Width Bus Speed MBps
SCSI-1 8 8 bits 5 MHz 4 MBps
SCSI-1 8 8 bits 5 MHz 5 MBps
SCSI-2 16 16 bits 5 MHz 10 MBps
SCSI-2 8 8 bits 10 MHz 10 MBps
SCSI-2 16 16 bits 10 MHz 20 MBps
8 8 bits 20 MHz 20 MBps
8 16 bits 20 MHz 40 MBps
8 8 bits 40 MHz 40 MBps
16 16 bits 40 MHz 80 MBps
16 16 bits 40 MHz 160 MBps


The Layout of the SCSI System

The most basic SCSI bus consists of a controller (or host adapter), a SCSI cable and a SCSI device.

The Layout of the SCSI System

The most basic SCSI bus consists of a controller (or host adapter), a SCSI cable and a SCSI device.

The Layout of the SCSI System

The most basic SCSI bus consists of a controller (or host adapter), a SCSI cable and a SCSI device.

In SCSI setups in which the controller sits in the middle of the chain (it can sit on any position in the chain), then termination must be provided on both ends of the chain. Where the controller is at one end of the chain, it can serve as termination on that end.

There are a variety of different cable types used, depending on the type of SCSI being used. They are:

  • DB-25 (SCSI-1)
  • 50-pin internal ribbon (SCSI-1, SCSI-2, SCSI-3)
  • 50-pin Alternative 2 Centronics (SCSI-1)
  • 50-pin Alternative 1 high density (SCSI-2)
  • 68-pin B-cable high density (SCSI-2)
  • 68-pin Alternative 3 (SCSI-3)
  • 80-pin Alternative 4 (SCSI-2, SCSI-3)


SCSI drives aren't that hard to configure. Each device must have a SCSI ID. On SCSI setups supporting 8 devices, the devices are labeled with IDs 0-7. The host adapter takes one ID, usually the highest one. Most adapters are usually factory-set to ID 7, which is the highest-priority ID. Many adapters require that any SCSI boot drive be configured to a certain ID. With the newer ones, it doesn't usually matter. The ID is configured by some type of switch or jumper on the drive, much like the master-slave jumper on an IDE setup. There are three jumpers used to describe the SCSI ID. Instead of making this simple, manufacturers decided to make the ID # a result of a binary representation of the jumpers. For example, setting all three jumpers off gives a binary of 000, meaning SCSI ID 0. Below is a table of jumper settings:

SCSI ID Jumper Settings
0 off / off / off
1 off / off / on
2 off / on / off
3 off / on / on
4 on/ off / off
5 on / off / on
6 on / on / off
7 on/ on / on

Depending on the manufacturer, the order of these jumpers may have been reversed. In this case, just flip the order of the jumper settings around. For example, ID 4 above is on-off-off. On a reversed setup, it would be off-off-on. As always, use the manual as the primary reference for these settings. Many drives do not require you to set the ID number manually, instead using plug-and-play technology to assign an available ID automatically.

SCSI in Use

SCSI is nowhere near as popular as IDE. For one, SCSI costs more to setup than IDE. Secondly, it is more involved to get running, especially when you consider the fact that IDE controllers are part-and-parcel of every motherboard on the market, whereas SCSI support built-in to a board is more of an extra. IDE devices, too, are gaining speed on SCSI, so raw speed between the two interfaces is not as large an issue as it once was. When it comes to external devices, USB and Firewire have both marginalized SCSI as an interface. USB 1.1 has more than enough bandwidth potential for most devices, and USB 2.0 and Firewire take the slack for devices requiring higher bandwidths. USB 2 and Firewire both provide more bandwidth potential than SCSI. SCSI is great in multi-tasking environments because the bus operates independently of the CPU, but for external devices, both USB and Firewire have abilities to operate independently.

SCSI is most often seen in servers and workstations. This is due to the high workload of these systems and the fact that many processes are often going on simultaneously. Also, SCSI RAID arrays are often set up in servers in order to provide extra speed as well as data redundancy using mirroring.