Single Connector Attachment
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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 has always been a demand for a possibility to replace a faulty 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 RAID 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 (SCSI ID etc) to be set using jumpers on each drive. Drives employing SCA only have 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 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 could get connected before its corresponding ground 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.
It should be noted that 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.
SCA is available both for the single-ended and low voltage differential versions of parallel SCSI. See the SCSI connector article for a description of other SCSI connectors. See the Fibre Channel electrical interface article for details of the SCA-40 connector.
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