A SCSI adapter, or SCSI HBA, facilitates connectivity and data transfer between a host and a peripheral device or storage system as defined by the SCSI set of American National Standards Institute standards for I/O interconnects. A plug-in HBA card typically initiates and sends service and task management requests to a target device, such as a storage drive or array, and receives responses from the target. The terms SCSI adapter and SCSI HBAgenerally refer to parallel SCSI, the predecessor to SAS.
Parallel SCSI devices are connected to a shared bus. The maximum parallel SCSI speed of 320 megabytes per second (MBps) is considered too slow to address the demands of modern computing systems, and performance often degrades as more devices are added to the shared bus. Parallel SCSI HBAs are viewed as outdated technology. Major manufacturers have discontinued production of parallel SCSI HBAs.
SAS was developed to address the limitations of traditional parallel SCSI and allow for higher speed data transfers to and from computer storage devices. Like parallel SCSI, SAS uses the SCSI command set, but the method of data transfer is different. SAS is a point-to-point serial data transport protocol. The introduction of SAS ushered in new terminology to describe the adapters, cables and connection options.