Disk controller

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It has been suggested that this article or section be merged into Host adapter. (Discuss)

The disk controller (or "hard disk controller") is the circuit which allows the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.

Early disk controllers were identified by their storage methods and data encoding. They were typically implemented on a separate controller card. Modified frequency modulation (MFM) controllers were the most common type in small computers, used for both floppy disk and hard disk drives. Run length limited (RLL) controllers used data compression to increase storage capacity by about 50%. Priam created a proprietary storage algorithm that could double the disk storage. Shugart Associates Systems Interface, (SASI) was a predecessor to SCSI.

Modern disk controllers are integrated into the disk drive. The interface between the computer and hard disk is called a host adapter. The most common types in use are IDE, Serial ATA, and SCSI host adapters. Most home personal computers use an IDE or Serial ATA interface. High end PCs, workstations and network file servers mostly have SCSI adapters.

The correct term for the component that allows a computer to talk to a SCSI bus is host adapter or host bus adapter. The component that allows a disk to talk to a SCSI bus is called a controller. In this case SCSI bus is the bus which lies between disk controller and host adapter which converts signals from SCSI bus into motherboard's bus suitable format. Disks called "SCSI disks" have built-in SCSI controllers. In the past, before most SCSI controller functionality was implemented in a single chip, separate SCSI controllers interfaced disks to the SCSI bus. There are other ways of partitioning the functionality that can make applying this terminology difficult or inappropriate.

In the IDE standards, the terminology is like that of SCSI.

[edit] Hardware RAID

A RAID controller is a device which manages the physical storage units in a RAID system and presents them to the computer as logical units. The term is sometimes used to refer to a host bus adapter (HBA), but it properly refers to the hardware which implements the RAID logic. This controller may be integrated with an HBA, or it may be part of an independent enclosure, such as a disk array, a network-attached storage (NAS) server, or a Storage Area Network (SAN).

External disk arrays are usually purchased as an integrated subsystem of RAID controllers, disk drives, power supplies, and management software. RAID adapters for use internal to a PC or server are sold either as embedded systems in the computer or as separate expansion cards.