Chapter 7. The SCSI Bus

Small Computer System Interface (SCSI; pronounced "scuzzy") is a general-purpose interface used for connecting many types of devices to a PC. This interface has its roots in SASI (Shugart Associates System Interface). SCSI is very flexible; it is not just a disk interface, but is a systems-level interface that allows many types of devices to be connected, including scanners and printers. SCSI's primary use today is as a host adapter for high-performance hard disk, tape-backup, and RAID arrays on servers.

SCSI is a bus that supports as many as 7 or 15 total devices. Multichannel adapters exist that can support up to 7 or 15 devices per channel. The SCSI host adapter might be installed as a PCI or as a PCI-Express card, or it might be built in to the motherboard.

The host adapter functions as the gateway between the SCSI bus and the PC system bus. Each device on the bus has a SCSI controller chip built in. The SCSI host adapter does not talk directly with actual devices such as hard disks; instead, it talks to the SCSI interface controller that is built in to the drive.

A single SCSI bus can support as many as 8 or 16 physical units, usually called SCSI IDs. One of these units is the SCSI host adapter in the PC; the other 7 or 15 can be other peripherals. You can have hard disks, a tape backup, or other devices connected to a single SCSI host adapter. Most systems can support up to 4 host adapters, each with up to 15 devices, for a total of 60 devices. There are even dual-channel adapters that could double that figure.

SCSI is a fast interface, generally suited to high-performance workstations, servers, or anywhere ultimate performance for a storage system interface is needed. The latest SCSI version, Ultra 5 (Ultra640), supports transfer speeds of up to 640MBps. By comparison, parallel ATA (PATA; also known as ATA/IDE) transfers at speeds up to 133MBps, first-generation Serial ATA (SATA) transfers at 150MBps, and 3G (second-generation) SATA transfers at 300MBps. Note that these figures are raw interface transfer speeds; the speeds of individual devices are normally much lower.

The real advantage of the SCSI bus over ATA/IDE is seen when multiple devices share the bus. Multiple SCSI devices can overlap reads and writes and more fully utilize the higher bandwidth available. The slower ATA buses allow only one or two devices, making higher throughputs unnecessary because they would be unusable.

When you purchase a SCSI device such as a SCSI hard disk, you are usually purchasing the device, device controller, and SCSI interface controller in one circuit; the device is ready to connect directly to the SCSI bus. This type of drive is usually called an embedded SCSI device because the SCSI interface is built in. For example, most SCSI hard drives are technically the same as or similar to their ATA counterparts except for the addition of the SCSI interface controller circuits (normally a single chip) added to the logic board. Your system cannot talk directly to the device controller as though it were plugged in to the system bus, as it could on a standard ATA drive. Instead, communications go through the SCSI host adapter installed in the system bus, then to the SCSI interface controller on the drive logic board, and finally to the device controller (also on the logic board) and disk drive. You can access the drive only through the SCSI protocols.

SCSI originally became popular in the PC-based workstation and server market because of the performance and expandability it offers. One block that stalled acceptance of SCSI in the early PC marketplace was the lack of a complete standard; the SCSI standard was originally designed by one company and then turned into a committee-controlled public standard, where it eventually became comprehensive and complete. Since then, no single manufacturer has controlled it.

Note

Low-end SCSI host adapters, such as those bundled with image scanners or rewritable CD or DVD drives, are not suitable for use with multiple SCSI devices and also lack support for bootable SCSI hard disks. These stripped-down host adapters are designed to support only the device they are packaged with. However, a full-featured SCSI host adapter can support multiple devices, and most SCSI host adapters sold at retail also support booting from a SCSI hard disk.

SCSI drives are controlled with a special SCSI BIOS that is built in to the SCSI host adapter; on servers with an integrated SCSI host adapter, the SCSI BIOS is incorporated into the system BIOS.