2.3 Hardware Components
2.3.9 EIDE and SCSI controllers
The internal hard drive (discussed later in this chapter) is connected to a disk controller with a cable. The hard drive and other devices can use one of two types of interface controllers to work with the computer. These include the Enhanced Integrated Drive Electronics (EIDE), and Small Computer System Interface (SCSI, pronounced "scuzzy") connections.

EIDE Drive Interface Controllers
Enhanced IDE (EIDE) is the "new and improved" Integrated Drive Electronics (IDE) drive interface. Not confined to IDE's 528 MB of data, the EIDE interface can handle up to 8.4 GB or more. While IDE can support only two drives, EIDE can support up to four devices using two IDE cables. Additionally, EIDE supports non-disk peripherals that follow the ATAPI protocol. The EIDE interface is often described as an ATAPI (AT Attachment Packet Interface), or a Fast ATA (Fast AT Attachment) interface. ATAPI (ATA Packet Interchange) is the protocol used by enhanced IDE devices such as IDE CD-ROM drives, IDE external tape backup drives, etc.

One type of EIDE interface is ATA-2. This newer ATA-2 standard brought significant upgrades to the drive controller and BIOS. The ATA-2 specification was developed in 1994 and subsequently modified to final form in 1995. The EIDE/ATA-2 specification covers the interface signals on the 40-pin connector, the drive commands issued by the BIOS, cable specifications, and drive configuration circuitry.

Sector translation and continuous technological improvements have allowed hard drives to grow larger and larger in size. Today, typical drive sizes can go up to 50 gigabytes or more.

SCSI Interface Controller
The Small Computer Systems Interface (SCSI ) controller evolved from the Shugart Associates Standard Interface (SASI). Like EIDE, SCSI devices have the controlling electronics on each of the drives. However, SCSI is a much more advanced interface controller than ATA-2/EIDE. It is ideal for high-end computers, including network servers.

A standard SCSI interface will allow up to seven devices to be connected to one SCSI adapter or controller. If the SCSI card is counted, it makes eight SCSI devices. Such devices may include hard drives, CD-ROM drives, taped drives, scanners, and removable drives. SCSI devices are typically connected in a series, forming a chain that is commonly referred to as a daisy chain. The cables used are either described as internal or external, depending on whether they are used inside or outside the computer. Additionally, the cables are characterized by the number of pins the connectors have. There are several types:

  • The 50-Pin connector is also called a type A cable, it has an 8-bit (wide) data path.
  • The 68-Pin connector is also called a type P cable, it has a 16-bit data path.
  • The 80-Pin connector is a SCSI cable variety cable that has a 16-bit data path.

Some additional important points students need to learn about SCSI devices and the workings of this high-speed controller are summarized in Figures and .