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Host adapter

In computer hardware, a host controller, host adapter, or host bus adapter (HBA) connects a host system (the computer) to other network and storage devices. The terms are primarily used to refer to devices for connecting SCSI, Fibre Channel and eSATA devices, but devices for connecting to IDE, Ethernet, FireWire, USB and other systems may also be called host adapters. Recently, the advent of iSCSI has brought about Ethernet HBAs, which are different from Ethernet NICs in that they include hardware iSCSI-dedicated TCP Offload Engines.

SCSI

A SCSI host adapter connects a SCSI bus to a computer. The host adapter bridges the physical and logical chasm that separates the SCSI bus from the host computer's internal bus. Modern host adapters contain all the electronics and firmware required to execute SCSI transactions, and often include a BIOS that not only allows the 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 HBA manufacturers are HP, ATTO Technology, Adaptec, and LSI. LSI, Adaptec, and ATTO Technology 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).

Fibre Channel

Today, the term host bus adapter (HBA) is most often used to refer to a Fibre Channel interface card. Fibre Channel HBAs are available for all major open systems, computer architectures, and buses, including PCI and SBus (obsolete today). Each HBA has a unique World Wide Name (WWN), which is similar to an Ethernet MAC address in that it uses an OUI assigned by the IEEE. However, WWNs are longer (8 bytes). There are two types of WWNs on a HBA; a node WWN, which is shared by all ports on a host bus adapter, and a port WWN, which is unique to each port. Major HBA manufacturers are Emulex, QLogic, LSI, Sun StorageTek and ATTO Technology. There are HBA models from different speed: 2Gbit/s, 4Gbit/s and 8Gbit/s.

InfiniBand

The term host channel adapter (HCA) is usually used to describe InfiniBand interface cards.

ATA

ATA host adapters are integrated into motherboards of most modern PCs. They are often improperly called disk controllers. The correct term for the component that allows a computer to talk to a peripheral bus is host adapter . A proper disk controller only allows a disk to talk to the same bus.

SAS and SATA

SAS or serial-attached SCSI is the current connectivity to replace the previous generation parallel-attached SCSI (PAS) devices. Ultra320 was the highest level of parallel SCSI available, but SAS has since replaced it as the highest-performing SCSI technology.

SATA is a similar technology from the aspect of connection options. HBAs can be created using a single connector to connect both SAS and SATA devices.

Major SAS/SATA HBA manufacturers are Adaptec, HP, QLogic, LSI and ATTO Technology.

eSATA

External Serial ATA disk enclosures and drives are increasingly common in the consumer computing market, but many SATA compatible motherboards and disk controllers do not include external SATA ports. As such, adapters to connect external SATA devices to ports on an internal SATA bus are commonly available.

Mainframe I/O channels

In the mainframe field, the terms host adapter or host bus adapter were traditionally not used. A similar goal is achieved since 1960s with a different technique: I/O channel, or simply channel, is a separate processor that can access main memory independently, in parallel with CPU (like later DMA in personal computer field), and that executes its own I/O-dedicated programs when pointed to such by the controlling CPU.

Protocols used by I/O channels to communicate with peripheral devices include ESCON and newer FICON.