Single Connector Attachment, or SCA, is a type of connection for the internal cabling of SCSI

SCSI

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

 systems. There are two versions of this connector: the SCA-1, which is deprecated, and SCA-2, which is currently in use in most systems. In addition there are Single-Ended (SE) and Low Voltage Differential (LVD)

Low voltage differential signaling

Low-voltage differential signaling, or LVDS, is an electrical digital signaling system that can run at very high speeds over inexpensive twisted-pair copper cables. It was introduced in 1994, and has since become very popular in computers, where it forms part of very high-speed networks and...

 types of the SCA.

Since hard disk drives are among the components of a server computer that are the most likely to fail, there has always been demand for the ability to replace a faulty drive without having to shut down the whole system. This technique is called hot-swapping

Hot swapping

Hot swapping and hot plugging are terms used to describe the functions of replacing computer system components without shutting down the system...

 and is one of the main motivations behind the development of SCA. In connection with RAID, for example, this allows for seamless replacement of failed drives.

Normally, hard disk drives make use of two cables: one for data and one for power, and they also have their specific parameters (SCSI ID etc) to be set using jumpers

Jumper (computing)

In electronics and particularly computing, a jumper is a short length of conductor used to close a break in or bypass part of an electrical circuit...

 on each drive. Drives employing SCA have only one plug which carries both data and power and also allows them to receive their configuration parameters from the SCSI backplane. The SCA connector for parallel SCSI

Parallel SCSI

Parallel SCSI is one of the interface implementations in the SCSI family. In addition to being a data bus, SPI is a parallel electrical bus: There is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it...

 drives has 80 pins, as opposed to the 68 pin interface found on most modern parallel SCSI drives. Serial SCSI drives have SCA connectors with a smaller number of pins.

Some of the pins in SCA connectors are longer than others, so they are connected first and disconnected last. This ensures the electrical integrity of the whole system. Otherwise, the angle at which the plug is inserted into the drive could be the reason for damage because, for instance, the pin carrying the voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 could get connected before its corresponding ground

Ground (electricity)

In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 reference pin.
The additional length also provides what is known as a pre-charge which provides a means whereby the device is alerted to a pending power surge. That allows a slower transition to full power and thereby makes the device more stable.

To make better use of their hot-plugging capability, SCA drives usually are installed into drive bay

Drive bay

A drive bay is a standard-sized area for adding hardware to a computer. Most drive bays are fixed to the inside of a case, but some can be removed....

s into which they slide with ease. At the far end of these bays is the backplane of the SCSI subsystem located with a connector that plugs into the drive automatically when it is inserted.

Full hot-swappable functionality still requires the support of other software and hardware components of the system. In particular the operating system and RAID layers will need hot-swap support to enable hard drive hot-swapping to be carried out without shutting down the system.

See also

• SCSI connector
  SCSI connector
  A SCSI connector is used to connect together computer parts that use a system called SCSI to communicate with each other. Generally, two connectors, designated male and female, plug together to form a connection which allows two components, such as a computer and a disk drive, to communicate with...
  
  , for a description of other SCSI connectors
• Fibre Channel electrical interface
  Fibre Channel electrical interface
  The Fibre Channel electrical interface is one of two related standards that can be used to physically interconnect computer devices. The other standard is a Fibre Channel optical interface, which is not covered in this article....
  
  , for details of the SCA-40 connector
• SAF-TE
  SAF-TE
  In computer storage, a SCSI Accessed Fault-Tolerant Enclosure is an industry standard to interface an enclosure to a SCSI subystem to gain access to information or control concerning* temperature* fan status* slot status...
  
  - active backplanes for sensoring and swap assistance

Standards

SCSI devices and connectors are specified in SPI-2, SPI-4 and SPI-5 as part of the SCSI-3 Standards Architecture: http://www.t10.org/scsi-3.htm

Organization responsible for drafts of the specification documents is the Technical Committee T10: http://www.t10.org/