The two faces of CD-R connections: SCSI and ATAPI

Emedia Professional, August, 1998 by Michael McMurdie, Rob Griffith

In 1998, CD-R and CD-RW are poised to become mainstream data storage technologies. Low-priced drives and media, combined with user-friendly software and increased advertising in mainstream publications and even prime-time television by key manufacturers, have increased public demand for recordable CD. Another factor that is instrumental in boosting CD-R's overall appeal to some OEM customers, such as computer manufacturers, is the introduction of a new generation of ATAPI CD-R and CD-RW drives. Until recently, virtually all CD recorders were SCSI-based peripherals, and required a computer with a SCSI host adapter and related software. However, most PC computer systems today include IDE-ATAPI as a standard feature, while SCSI is an optional extra. The availability of ATAPI-based CD-R technology is of enormous interest to manufacturers of computer systems who plan to include CD-R or CD-RW in their systems.

Recently, discussion and debate abound regarding the differences between SCSI and ATAPI for CD-R and CD-RW on the Internet and elsewhere. As is often the case in these kinds of discussions, people tend to take sides, and inaccurate information sometimes becomes the fuel of heated debates. The fact of the matter is that both SCSI and ATAPI are valid technologies for CD-R and CD-RW, but there are differences between the two.

When making a decision to purchase a CD-R drive, users weigh several factors, including cost, data transfer rate, ability to connect multiple devices, and whether to choose an internal or external drive. Exploring the history, architecture, and differences between SCSI and ATAPI will help the user to make an informed decision.

A BRIEF HISTORY OF SCSI AND IDE-ATAPI

In earlier generations of PCs, disk drives attached to the computer through proprietary add-on cards. These cards/ drives generally had their own command set which was different from most others cards. In order to send commands to these cards, the operating systems would work with the system BIOS (also called CMOS) to dispatch the appropriate commands to the card in question. As new hard drives were released with different characteristics, it became impractical to update the BIOS, or to develop a new controller card, creating migration and compatibility problems.

SCSI and IDE are two approaches to solving these problems. SCSI and IDE are both able to take generic commands from the operating system of the computer and translate them into commands that can be understood by a particular drive. The mechanism that each approach uses is somewhat however.

SCSI stands for Small Computer System Interface. It was originally developed as a way for small computers to interface with peripheral devices such as hard drives, scanners, and the like. In 1979, a precursor to SCSI called SASI (Shugart Associates Systems Interface) was developed, but it could only interface with two devices per system, and was designed mainly for floppy drives. An accredited standards committee for SCSI was formed in 1982, and published the first SCSI specification in 1986 (SCSI-1). SCSI-1 allowed up to seven peripheral devices, including hard drives, scanners, CD-ROM drives, and such, to be attached to a single SCSI bus.

Although useful, the original implementation of SCSI had certain limitations. Later, significant improvements were made to the specification for a variant of SCSI called SCSI-2, which added multitasking and rudimentary command queuing and other features. Variations of SCSI-2 include Fast-SCSI Wide-SCSI, and Fast-and-Wide SCSI. A new generation of SCSI is now evolving, including Ultra2 SCSI and Ultra3 SCSI, serial SCSI, including P1394 (also called Firewire, which is an Apple trademark), Serial Storage Architecture (SSA), and Fibre Channel under the umbrella of SCSI-3. SCSI has arguably become the standard for high-speed reliable data transfer to peripheral devices.

IDE, or Integrated Drive Electronics, uses an approach in which the controller for the drive is actually built into the drive itself. The specification for IDE was released in 1987 as a standard interface for connecting hard drives to computer systems. (It should be noted that the formal engineering name for the IDE interface is ATA, or AT Attachment Disk Drive Interface.) IDE became popular, and became a standard feature of most desktop PCs due to low cost. In earlier 286- or 386-based systems, this interface was provided by an add-on card, but if you buy a PC today, you will usually find an IDE interface built into the motherboard. An IDE hard drive can plug directly into this interface, with no intervening controller card required.

Originally, IDE/ATA was designed as an interface for hard drive drives. Early CD-ROM drives required SCSI controllers, or even proprietary controllers to interface with the computer system. However, since IDE interfaces had become a standard feature of most PC systems, engineers developed a specification, which allowed CD-ROM to interface with a computer system using an IDE connector. This specification is called ATAPI (AT Attachment Packet Interface). ATAPI is actually part of a larger evolutionary implementation of IDE called Enhanced IDE (EIDE or ATA-2/3/etc.). Enhanced IDE also includes additional enhancements to the original IDE specification, such as a faster data transfer rate, support for addressing of larger-capacity hard drives, and the ability attach up to four devices to a single EIDE bus using two channels. Like SCSI, IDE/ATA has evolved into several variations, including ATA-1, Fast ATA, and ATA-2.