SCSI
A SCSI host adapter connects host system to boot from a SCSI device, but also facilitates configuration of the host adapter. Typically a device driver, linked to the operating system, controls the host adapter itself.
In a typical parallel SCSI subsystem, each device has assigned to it a unique numerical ID. As a rule, the host adapter appears as SCSI ID 7, which gives it the highest priority on the SCSI bus (priority descends as the SCSI ID descends; on a 16-bit or "wide" bus, ID 8 has the lowest priority, a feature that maintains compatibility with the priority scheme of the 8-bit or "narrow" bus).
The host adapter usually assumes the role of SCSI initiator, in that it issues commands to other SCSI devices.
A computer can contain more than one host adapter, which can greatly increase the number of SCSI devices available.
Major SCSI adapter manufacturers are HP, ATTO Technology, Promise Technology, Adaptec, and LSI Logic. LSI, Adaptec, and ATTO currently offer PCIe SCSI adapters which fit in new Apple Mac, on Intel PCs, and low-profile motherboards from companies like HP (which now lack SCSI support due to the inclusion of SAS and/or SATA connectivity).
Read more about this topic: Host Adapter