Single Connector Attachment

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

Since hard disk drives are among the components of a server computer that are the most likely to fail, there always has been a demand for a possibility to replace a erroneous drive without having to shut down the whole system. This technique is called Hot_swapping and is one of the main motivations behind the development of SCA. In connection with redundant array of independent disks 5, 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 to be set in the BIOS of the system. Drives employing SCA only have one plug which carries both data and power and also allows them to receive their parameters from the host adapter. The plug for SCA has 80 pins, as opposed to the 68 pin interface found on most modern SCSI drives. Some of these pins on the host side 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 could get connected before its corresponding ground_(electricity) reference pin. To put another emphasis on their hot-plugging capability, SCA drives usually are installed into drive bays 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.

SCA is available both for the single-ended and low voltage differential signaling flavours of SCSI.