SCSI is a type of computer link mainly utilized by fast hard disks. This is so that SCSI, as opposed to the widely used IDE disk link, can enable quicker data transmission speeds. Additionally, daisy-chaining devices are supported by SCSI, allowing multiple SCSI hard disks to be attached to a single SCSI interface with little to no speed loss.

The following is a summary of the various SCSI port types:

• SCSI-1: uses an 8-bit interface and can handle 4 MBps of data transmission.
• SCSI-2: uses a 50-pin connection rather than a 25-pin adapter and is compatible with numerous products. It is one of the SCSI protocols that is most frequently used. The average data transmission rate is 5 MBps.
• Wide SCSI: To enable 16-bit data transmissions, a larger wire (168 cable lines to 68 terminals) is used.
• Fast SCSI: uses an 8-bit interface but increases the CPU speed to handle 10 MB/s data transmission rates.
• Fast Wide SCSI: allows data transmission rates of 20 MBps using a 16-bit interface.
• Ultra SCSI: uses an 8-bit interface and can handle 20 MBps of data.
• SCSI-3: uses a 16-bit interface and 40 MB/s transmission speeds are supported. Also named Extremely Wide SCSI.
• Ultra2 SCSI: uses an 8-bit interface and 40 MB/s data transmission rates are supported.
• Wide Ultra2 SCSI: allows data transmission rates of 80 MBps using a 16-bit interface.
• Ultra3 SCSI: allows data transmission speeds of 160 MBps using a 16-bit interface. Likewise called Ultra-160.
• Ultra-320 SCSI: uses a 16-bit interface and can handle 320 MB/s of data transmission.
• Ultra-640 SCSI: allows data transmission rates of 640 MBps using a 16-bit interface.

While SCSI is still used for some high-performance devices, modern ports have completely supplanted SCSI in certain uses. For attaching external hard disks, Firewire and USB 2.0 have become popular choices. Internal hard disks are now connected quickly using Serial ATA, or SATA.