ARCHIVED: What are SCSI standards, interfaces, and connectors?

This content has been archived, and is no longer maintained by Indiana University. Information here may no longer be accurate, and links may no longer be available or reliable.

On this page:


SCSI (Small Computer Systems Interface) is a smart bus, controlled with a microprocessor, that allows you to add up to 15 peripheral devices to the computer. These devices can include hard drives, scanners, printers, and other peripherals. High-end single SCSI boards have two controllers and support up to 30 peripherals on a single expansion card. An advantage of SCSI is that you can connect several peripherals to one host adapter, using only one slot in the bus.

Uses for SCSI

SCSI is widely used in workstations, servers, and mainframes; it is less commonly used in desktop PCs. The advantage of SCSI in a desktop PC is that you can add a scanner and several other drives (for example, CD-Rs, DVD-RAM, Zip drives), as well as hard drives, to one SCSI cable chain. This has become less important as alternate interfaces such as USB and FireWire have become popular.

SCSI is useful in network servers, where several hard drives can be easily set up as a RAID configuration. If one drive fails, it can be removed and a new one inserted, without loss of data, while the system is still operational. This feature of RAID hardware is called hot-swapping.

You can install SCSI hard drives in a PC that already contains one or more IDE disk drives. The IDE drive will still be the boot drive, and the SCSI drives will provide additional storage. An IDE device is always the default boot device even if a SCSI hard drive is installed and configured as SCSI device number 0. For example, if a SCSI hard drive is on the same computer with an IDE CD-ROM drive, the computer will always boot to the CD-ROM drive; the only way around this is to replace the IDE CD-ROM with a SCSI one.

Connecting SCSI devices

SCSI devices are daisy-chained together. External devices have two ports, one for the incoming cable and another for the outgoing cable to the next device. An internal device has a single port that attaches to a ribbon cable with multiple connectors. Some higher-end SCSI cards may have multiple internal ports that allow you to attach multiple ribbon cables.

Each SCSI device must have a unique ID number; normally you can set these numbers by flipping rotary switches on external devices or by setting jumpers on internal ones. The SCSI ID determines the device order, which runs from 7 to 0 and then from 15 to 8. The host adapter defaults to the highest priority, which is 7.

You must terminate the device at the end of a SCSI chain by either setting a switch or plugging a resistor module into the open port. Usually, host adapters default to terminated. If devices are connected both internally and externally, you must remove the host adapter termination and apply termination to the ends of both chains.

There are adapters that allow SCSI peripherals to be connected via the parallel port. The parallel port's transfer rate is considerably less than that of the SCSI host adapter, but it does provide a means to hook up SCSI devices to laptops. Not all SCSI devices will work on a parallel adapter, and some SCSI devices have their own parallel port adapters. In general, expect transfer rates of around 1MBps when using a SCSI-to-parallel-port adapter.

SCSI support

Windows 95, 98, NT, Me, 2000, and XP as well as most older Macintosh computers provide internal support for SCSI, but Windows 3.1 and DOS do not. Newer Macintosh computers support FireWire rather than SCSI for high-performance interfaces. To install SCSI in a Windows 3.1 or DOS computer, you must add the appropriate SCSI driver.

More information

This information was adapted from TechEncyclopedia at:

You can find more information on SCSI at:

SCSI standards

  • SCSI-1: Uses an 8-bit bus and a 25-pin Centronics-style connector. Supports data rates of 4-5MBps and can support up to 7 devices
  • SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what is most commonly meant as plain SCSI. Can support up to 7 devices
  • Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10MBps. Uses a 50-pin connector and can support up to 7 devices
  • Wide SCSI or Fast Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers. Supports data rates of 20MBps and can support up to 15 devices

    Note: The term "wide" refers to the number of cable lines, not to the physical width of the cable, which is actually smaller than a 50-pin "narrow" SCSI cable.

  • 8-bit Ultra SCSI-3: Uses an 8-bit bus, and supports data rates of 20MBps
  • 16-bit Ultra SCSI-3: Uses a 16-bit bus. Supports data rates of 40MBps and can support up to 15 devices. Also called Ultra Wide SCSI
  • 8-bit Ultra-2 SCSI-3: Uses an 8-bit bus. Supports data rates of 40MBps and can support up to 8 devices. This is also the first generation of SCSI to use a "low voltage differential" bus, which means that you will sometimes see Ultra-2 referred to as "LVD" SCSI.
  • Wide Ultra-2 SCSI: Uses a 16-bit bus. Supports data rates of 80MBps and can support up to 15 devices
  • Ultra3 SCSI: Not a specific protocol, but rather a label that the SCSI Trade Association created to apply to any SCSI technology that combines Ultra2 SCSI with one or more of five new features as defined by the ANSI SCSI-3 Parallel Interface - 3 (SPI-3) specification. Since Ultra3 does not restrict which features must be included, there could be as many as 63 variations that could qualify as Ultra3 SCSI. It would also be possible that two Ultra3 SCSI devices might not communicate at 160MBs because they would not include the same set of features.
  • Ultra 160 SCSI: This is Adaptec's and other SCSI manufacturers' implementation of their interpretation of the Ultra3 specification. It was created in response to a controversy arising from the fact that a manufacturer can implement only one of the five key features of the SPI-3 standard created by ANSI and still call itself "Ultra 3 SCSI". Adaptec and the other manufacturers wanted to distinguish their product by advertising its maximum possible speed of 160MBps. Their standards includes Double Transition Clocking (sending info on both the rise and decay of a clock cycle, which speeds data transfer without increasing the controller's clock speed), Cyclical Redundancy Checking (CRC), and Domain Validation (optimum speed negotiation). Ultra 160 has become the de facto standard, in place of Ultra 3; later technologies did not follow the "Ultra 3" naming paradigm by getting named "Ultra 4" and instead were named "Ultra 360" and "Ultra 640".
  • Ultra 160+: This is simply Ultra 160 with all five of the ANSI SPI-3 features built in, instead of just one to four of them.
  • Ultra 320 SCSI: This is a bit more than simply an extension of the Ultra 160. This is a newer generation of SCSI attempting to implement the SCSI-3 Parallel Interface - 4 (SPI-4) standards. It has a faster (80MHz) bus with a wide (16-bit) data path. As you can tell from the name, 320MBps is the expected top speed of this interface.
  • Ultra 640 SCSI: An extension of the previous standard, this time to 640MBps. This standard never became popular, due to the severe cable length limits required to meet the speed. Most manufacturers have skipped over this standard and have chosen to adopt Serial Attached SCSI instead.
  • Serial Attached SCSI (SAS): This is the latest standard attempting to switch over to a serial from a parallel interface; all the standards mentioned here before this one are parallel interfaces (i.e., multiple wires and connectors side-by-side). This does mean that SAS will be the first non-backwards-compatible SCSI standard, at least where the connector is concerned. It is meant to use the best features of SCSI, Serial ATA (SATA), and fiber channel disk interfaces, and is expected to have some compatibility with SATA (one way only; an SAS controller will recognize a SATA drive, but a SATA controller will not recognize an SAS drive). Transfer speeds start at 3GBps (gigabits per second), and the standard calls for that to increase to 10GBps by the year 2010.

SCSI connectors and interfaces

  • Centronics 50-pin connector: The Centronics 50-pin connector was once the most widely used SCSI connector. An external connector only, the Centronics is a SCSI-1 connector that looks the same as the Centronics cable that attaches to a parallel port printer. The Centronics 50-pin cable comes in male and female styles, and gender changers and cable converters are commonly available. Although used on older SCSI devices and external drive enclosures, this interface is no longer heavily used, due to its slow speed and short cable lengths.
  • High-density 50-pin connector: The high-density 50-pin connector is used on scanners and Jaz drives. It is one of the more common SCSI connectors and is usually used to connect SCSI-2 devices. Both ends of the cable are usually 50-pin male, while the sockets on the host adapter and external devices are 50-pin female.
  • DB 25-pin connector: The DB 25-pin or D Sub 25 is by far the most widely used connector. This connector is used for parallel and serial printers in addition to the many other devices available. Both ends of the cable are usually 25-pin male, while the sockets on the host adapter and external devices are 25-pin female. This cable is almost always an external connector.

    Note: DB-25 SCSI cables are not compatible with and should not be used as serial or printer cables; serial cables and printer cables should not be used or attached to DB-25 SCSI adapters. You can short out the SCSI host adapter or the motherboard by using the wrong cable. Marking cables is the best way to avoid this.

  • IDC50 connector: The IDC50 is the most common internal SCSI connector. It is very similar to the standard IDE internal ribbon cable. The IDC50 SCSI cable is considerably wider then an IDE ribbon cable; in fact, it is usually the widest standard internal cable in use. This is a standard SCSI-2 10MBps internal SCSI cable. Many low-end cables have only two or three connectors, allowing for one or two devices to be attached to the cable. Seven-device cables are available, though they are often expensive and require a large case, as the cables may be four or five feet long.
  • High-density 68-pin connector: The high-density 68-pin connector is the SCSI connector of choice for SCSI-3 host adapters and peripherals. There is an internal ribbon cable version that looks very similar to the IDC50 connector. Many low-end cables have only two or three connectors, allowing for one or two devices to be attached to the cable. Seven-device cables are available, though they are often very expensive and require a large case, as the cables may be three or more feet long. Both ends of the external cable are usually 68-pin male, while the sockets on the host adapter and external devices are 68-pin female.
  • SCA 80-pin Micro-Centronics connector: SCA stands for Single Connector Attachment, a type of disk drive connector that includes connection pins for the power cables as well as the data wires. A SCA connector uses an 80-pin plug and socket to connect peripherals. This connector combines power, data channel, and ID configuration for fast installation and removal. SCA connectors are typically found only on high-end SCSI hard disks. The SCA interface was designed to provide a standard connection for systems using drives that can be hot-swapped. SCA makes swapping SCSI hard drives much easier than with traditional SCSI cables, plugs, and sockets. An adapter enables SCA drives to fit into standard SCSI enclosures.
  • Serial Attached SCSI SFF 8482: Also called "4x internal" by some vendors. This is a connector with the same form factor as SATA with the addition of a "bump" to key it specifically for SAS. (SATA drives can be plugged into SAS controllers, but SAS drives will not function with a SATA controller; hence, the necessity for the key bump on the connector.) As the name says, it's meant to be used internally, i.e., inside the computer case.
  • Serial Attached SCSI SFF 8484: Also called "32-pin" or "MultiLane". This is a high density connector usually intended to plug into the motherboard, controller, or backplane itself. Cables with this connector on one end usually have four individual SFF 8482 connectors on the other.
  • Serial Attached SCSI SFF 8470: Also called "4x external" by some vendors. This is simply a version of the SFF 8484 that's meant to be used with external (i.e., not located within the case) drives.

You can find more information about SCA connectors at:

You can find more information about SCSI interfaces and connectors at:

This is document aiqw in the Knowledge Base.
Last modified on 2018-01-18 12:41:57.