PC Hardware

SCSI-3 Drives

To speed up the pace of development, the SCSI Committee approved a "fast track" system for the SCSI-3 standard. It let a subcommittee handle most of the work, and new subsections were adopted without waiting for the publication of the entire SCSI definition.

That bright idea, plus the advent of the PCI bus and mature Plug and Play operating systems, has made it easy to install components and given users excellent control and flexibility. All SCSI-3 cards have ways to support existing SCSI-2 devices. Some of the highlights of current state-of-the-art SCSI technology on the desktop include the following seven features:

High-Performance Products

The success and stability of the SCSI standard makes it an ideal platform for developing high-performance products. SCSI's robust, reliable interface and advanced commands allow manufactures to build "best-of-breed" products to take advantage of its power. The fastest hard disk drives and CD-ROM devices traditionally show up first, sporting a SCSI interface. The most advanced scanners are SCSI-based, and many optical products come only in SCSI versions. Even when non-SCSI versions reach the market, they generally under-perform their SCSI siblings.

Plug and Play Installation

Well-designed SCSI cards are recognized and drivers are installed automatically with Plug and Play operating systems such as Windows 98, Windows NT, 2000, and the Macintosh OS. Most SCSI-based peripherals provide Plug and Play setup. The first time the system is booted up after they are added to a SCSI chain, the system notices the new device and asks for the product's setup disk.

Simple Expansion

Adding external devices is as simple as connecting an industry standard cable and power cord. If users decide to add additional host adapters, they can share the same drivers, reducing system overhead.

Advanced Management

SCSI products generally offer a range of tools to tune the bus and devices attached to it. For example, many host adapters have firmware that provides the ability to format and inspect hard disk drive reliability and define custom settings for each device on the chain. Operating-system utilities are provided to check the status of a device and enable advanced features.

SCAM Support

This SCSI acronym stands for "SCSI configured auto-magically." Most new SCSI products are SCAM enabled, meaning that the user does not have to worry about setting the ID numbers for them, because they will configure themselves, using an open ID position on the SCSI chain.

Even if a hard disk drive is SCAM enabled, you might have to set an ID on multidrive PCs, because the host adapter will need it to determine which drive is the normal boot device.


This command allows a SCSI device handling a large amount of data or performing complex operations to disengage from the host adapter's bus while performing the task, allowing other devices free access until it is finished.

Tag Command Queuing

SCSI devices with this feature can reorder how blocks of data are moved on the bus to speed transfer. The way it functions can be compared to letting a shopper with only a few items move to the head of the checkout line, to reduce the average wait time per shopper.

SCSI and IDE Compared
Devices per channel 7/15 per chain 2 per chain
Maximum potential throughput for major classes of SCSI and IDE 160 MB per second (Ultra 160)
80 MB per second (Ultra2)
40 MB per second (Wide SCSI)
66 MB per second (UDMA)
33 MB per second (UDMA)
16.7 MB per second(Fast ATA)
Connection types Internal and external Internal only
True bus mastering Yes No
Operate more than one I/O device at a time? Yes No
Advanced commands (such as tag command queuing, connect/disconnect) Yes No