# host adapter

> device which connects a computer, which acts as the host system, to other network and storage devices

**Wikidata**: [Q585585](https://www.wikidata.org/wiki/Q585585)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Host_adapter)  
**Source**: https://4ort.xyz/entity/host-adapter

## Summary
A host adapter is a piece of computer hardware—usually an expansion card—that lets the host computer talk to storage or network devices over a shared bus. It is the “middle-man” controller that plugs into the computer and presents a standard interface (SCSI, SAS, Fibre-Channel, etc.) to the outside world.

## Key Facts
- Classified in Wikidata as a subclass of expansion card, controller, computing adapter and computer hardware (sitelink count: 18).
- Common market synonyms: host bus adapter, HBA, host controller, host-side controller.
- Physically contains at least one electrical connector and is operationally centered on a bus that links the network-host computer to peripheral devices.
- Differentiated from a disk controller because the host adapter sits inside the host system, not inside the peripheral.
- Opposite role to a peripheral controller: the host adapter is host-side, the peripheral controller is device-side.
- Part-of relationship: computer acting as network host.
- Has dedicated subclasses such as SCSI host adapter, converged network adapter, caching SAN adapter and USB-to-serial adapter.
- Image example: LSI 9207-4i4e PCIe-based SAS HBA expansion card.

## FAQs
### Q: Is a host adapter the same as a disk controller?
A: No. A disk controller lives in or on the disk drive itself, while a host adapter lives in the host computer and presents a bus interface that many kinds of peripherals—including disks—can plug into.

### Q: Do I need a host adapter if my motherboard already has SATA ports?
A: Only if you want to attach devices that use a different protocol (SAS, Fibre-Channel, NVMe-over-Fabric, etc.) or need more ports than the motherboard supplies; the on-board SATA controller already functions as a built-in host adapter for SATA drives.

### Q: What does “HBA” stand for?
A: Host Bus Adapter; the term is used interchangeably with host adapter in enterprise storage contexts.

### Q: How is a converged network adapter different from a plain HBA?
A: A converged network adapter (CNA) is a host adapter that simultaneously provides storage (e.g., FCoE or iSCSI) and standard Ethernet networking on the same card, saving slots and switch ports.

## Why It Matters
Without a host adapter, a computer cannot extend its internal bus beyond its case to reach external storage arrays, tape libraries, or high-speed networks. The adapter translates the CPU’s native bus protocol (PCIe, PCI, etc.) into the protocol spoken by storage devices (SCSI, SAS, NVMe, Fibre-Channel), ensuring plug-and-play interoperability and allowing hot-swapping, RAID, and clustering. By offloading low-level signalling and command queuing to dedicated hardware, HBAs free the host CPU for application work and reduce latency for mission-critical databases, virtualization farms, and media-production servers. In short, host adapters turn a stand-alone computer into a network-capable storage host.

## Notable For
- First mass-market appearance in 1980s SCSI cards that let desktop PCs connect to scanners, early hard disks, and CD-ROM towers.
- Modern HBAs deliver 32 Gbit/s Fibre-Channel or 22.5 Gbit/s SAS-4 speeds, matching or exceeding on-board controller bandwidth.
- Enterprise caching SAN adapters add onboard DRAM or flash to accelerate storage-area-network traffic, something a standard NIC cannot do.
- SCSI host adapters introduced the concept of LUN (logical unit number) abstraction now common across all storage protocols.
- PCIe-based SAS HBAs remain the standard way to attach large JBOD disk enclosures to rack servers.

## Body
### Physical Form Factor
Most host adapters are PCIe plug-in cards, though some older units used PCI or ISA. They expose one or more external ports: SFF-8644 mini-SAS HD, LC fibre, or RJ-45 for iSCSI/CNA models.

### Functional Stack
The adapter contains a controller chip (ASIC or FPGA) that implements the target protocol, DMA engines to move data directly into host memory, and firmware that presents a standard driver interface (Linux scsi_host, Windows StorPort, etc.).

### Subclasses in Use
- SCSI host adapter – parallel SCSI, up to 320 MB/s Ultra-320.
- SAS HBA – serial attached SCSI, 12 Gb/s per lane today.
- Fibre-Channel HBA – 8/16/32 Gb/s optical links.
- Converged network adapter – combines FCoE/ROCE with 10/25/40 GbE.
- USB-to-serial adapter – simpler, converts USB to RS-232; still a host adapter because it lets the host reach a peripheral bus.

### Relation to RAID
A plain HBA offers “IT” (initiator-target) mode and passes disks through to the OS. Many cards can be flashed to “IR” (integrated RAID) mode, adding hardware RAID 0/1/10 without a separate RAID card.

### Market Leaders
Broadcom (LSI), Microchip (Adaptec), Marvell QLogic, and Emulex dominate OEM sales; consumer add-in cards often use ASMedia or JMicron controllers.

## Schema Markup
```json
{
  "@context": "https://schema.org",
  "@type": "Thing",
  "name": "host adapter",
  "description": "A hardware expansion card that connects a host computer to storage or network devices over a shared bus.",
  "sameAs": [
    "https://www.wikidata.org/wiki/Q1638028",
    "https://en.wikipedia.org/wiki/Host_adapter"
  ],
  "additionalType": "https://schema.org/ComputerHardware"
}

## References

1. Freebase Data Dumps. 2013
2. BabelNet