How SCSI Works

For any device to connect to the computer there is a requirement for buses. Universal serial bus (USB) is most commonly used for these purposes. The USB aids in carrying data and power required for any device to operate and communicate with the computer. However, USBs are not powerful enough to support an entire computer. SCSI serves this purpose!


Small Computer System Interface or SCSI is an extremely fast bus, which is able to handle more than one device at a single time, also aiding these devices to connect with the computer. Almost all the devices such hard drives, CD-ROM/RW, scanners, tape drives or printers can be simultaneously connected to the computer using SCSI. SCSI has a built in controller that receives and send data and power to the SCSI-enabled devices like printers and hard drives.

Different categories of SCSI

SCSI is extremely fast with a speed of 320 megabytes per second (Mbps). However, it has some of the technical problems such as limited BIOS support, while having to be configured separately for separate computers. Then again, SCSI comes in different widths and speeds and hence, no common software interface exists. SCSI can be found in three categories: SCSI-1, SCSI-2 and SCSI-3, all showcasing various virtues such as “fast”, “ultra” and “wide”.

  • SCSI-1 was the original most built specification and is completely obsolete now. SCSI-2 is a more recent interface and included the Common Command Set (CCS), which comprises of a group of eighteen commands that are absolutely necessary for any computer. This one also has the option to double the speed to 10 MHz, while also enhancing the bandwidth of the device from 8 to 16 bits (hence, showcasing the fast and wide proponents).
  • SCSI-2 gave the users opportunity to connect up to 15 devices at one time.
  • SCSI-3 is the most recent interface.

Along with the specifications as presented by SCSI-2, this also offers additional SCSI Parallel Interface (SPI). Many more variations of SCSI have been formed by joining various combinations of bus speeds, clock speed and SCSI-3 specifications.

The Working

All the SCSI types work according to a mechanism – using controllers and cables to connect with the devices. The SCSI controllers are what coordinate the computer and SCSI buses. These SCSI controllers or host adapters can be in the form of a card or built-in form in the motherboard. The software needed to access and controlled vices is placed on the controller and is termed as SCSI BIOS. In order for the SCSI devices to work properly and in an orderly manner, each of the devices is assigned a unique identifier (UI). Now while the external devices attach themselves to the controller using a thick round cable, internal devices connect to the SCSI controller using a ribbon cable. Various SCSI variations use different connectors with varying pin numbers such as 50, 68 or 80 pins. Once all the devices are installed on the bus and allotted their own UI, the end of the bus should be closed; else the signal in the bus might reflect back interfering with the communication between other devices and controller. A resistor circuit is used for the same purpose.

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