CD/DVD Drive Interface

The drive's interface is the physical connection of the drive to the PC's expansion bus. The interface is the data pipeline from the drive to the computer, and its importance shouldn't be minimized. Five types of interfaces are available for attaching a CD-ROM, CD-R, or CD-RW drive to your system:

  • SCSI/ASPI (Small Computer System Interface/Advanced SCSI Programming Interface)

  • ATA/ATAPI (AT Attachment/AT Attachment Packet Interface)

  • Parallel port

  • USB port

  • FireWire (IEEE-1394)


SCSI (pronounced scuzzy), or the Small Computer System Interface, is a name given to a special interface bus that allows many types of peripherals to communicate.

A standard software interface called ASPI (Advanced SCSI Programming Interface) enables CD-ROM drives (and other SCSI peripherals) to communicate with the SCSI host adapter installed in the computer.

SCSI offers the greatest flexibility and performance of the interfaces available for CD-ROM drives and can be used to connect many other types of peripherals to your system as well.

The SCSI bus enables computer users to string a group of devices along a chain from one SCSI host adapter, avoiding the complication of installing a separate adapter card into the PC bus slots for each new hardware device, such as a tape unit or additional CD-ROM drive added to the system. These traits make the SCSI interface preferable for connecting a peripheral such as a CD-ROM to your PC.

Not all SCSI adapters are created equal, however. Although they might share a common command set, they can implement these commands differently, depending on how the adapter's manufacturer designed the hardware. ASPI was created to eliminate these incompatibilities.

ASPI was originally developed by Adaptec, Inc., a leader in the development of SCSI controller cards and adapters who originally named it the Adaptec SCSI Programming Interface before it became a de facto standard. ASPI consists of two main parts. The primary part is an ASPI-Manager program, which is a driver that functions between the operating system and the specific SCSI host adapter. The ASPI-Manager sets up the ASPI interface to the SCSI bus.

The second part of an ASPI system is the individual ASPI device drivers. For example, you would get an ASPI driver for your SCSI CD-ROM drive. You can also get ASPI drivers for your other SCSI peripherals, such as tape drives and scanners. The ASPI driver for the peripheral talks to the ASPI-Manager for the host adapter. This is what enables the devices to communicate together on the SCSI bus.

The bottom line is that if you are getting a SCSI interface CD-ROM, be sure it includes an ASPI driver that runs under your particular operating system. Also, be sure that your SCSI host adapter has the corresponding ASPI-Manager driver as well. Substantial differences exist between SCSI adapters because SCSI can be used for a wide variety of peripherals.

Low-cost, SCSI-3-compliant ISA or PCI adapters can be used for CD-ROM interfacing. In contrast, higher-performance PCI adapters that support more advanced SCSI standards, such as Wide, Ultra, UltraWide, Ultra2Wide, and so on, can be used with both CD-ROM drives and other devices, such as CD-R/CD-RW drives, hard drives, scanners, and tape backups.

To help you choose the appropriate SCSI adapter for both your CD-ROM drive and any other SCSI-based peripheral you're considering, visit Adaptec's Web site. The SCSI interface offers the most powerful and flexible connection for CD-ROMs and other devices.

It provides better performance, and seven or more drives can be connected to a single host adapter. The drawback is cost. If you do not need SCSI for other peripherals and intend to connect only one CD-ROM drive to the system, you will be spending a lot of money on unused potential. In that case, an ATAPI interface CD-ROM drive is a more cost-effective choice.


The ATA/ATAPI (AT Attachment/AT Attachment Packet Interface) is an extension of the same ATA (AT Attachment) interface most computers use to connect to their hard disk drives. ATA is sometimes also referred to as IDE (Integrated Drive Electronics). ATAPI is an industry-standard ATA interface used for CD/DVD and other drives.

ATAPI is a software interface that adapts the SCSI/ASPI commands to the ATA interface. This enables drive manufacturers to take their high-end CD/DVD drive products and quickly adapt them to the ATA interface.

This also enables the ATA drives to remain compatible with the Extensions that provide the CD/DVD drive with a software interface with DOS. With Windows 9x and later, the CD-ROM extensions are contained in the CD file system (CDFS) VxD (virtual device) driver.

ATA/ATAPI drives are sometimes also called enhanced IDE (EIDE) drives because this is an extension of the original IDE (technically the ATA) interface. In most cases, an ATA drive connects to the system via a second ATA interface connector and channel, leaving the primary one for hard disk drives only.

This is preferable because ATA does not share the single channel well and would cause a hard disk drive to wait for CD/DVD commands to complete and vice versa. SCSI does not have this problem because a SCSI host adapter can send commands to different devices without having to wait for each previous command to complete.

The ATA interface represents the most cost-effective and high-performance interface for CD-ROM drives. Most new systems that include a CD and/or DVD drive have it connected through ATA. You can connect up to two drives to the secondary ATA connector; for more than that, SCSI is your only choice and provides better performance as well.

Many systems on the market today can use the ATA/ATAPI CD/DVD drive as a bootable device, which allows the vendor to supply a recovery CD that can restore the computer's software to its factory-shipped condition.

Later, you'll see how bootable CDs differ from ordinary CDs and how you can use low-cost CD-R/CD-RW drives, along with mastering and imaging software to make your own bootable CDs with your own preferred configuration.

Parallel Port

In the past, some external CD-ROM or CD-RW drives were available in versions that connected to a PC's parallel port. USB has for the most part replaced the parallel port for this type of use.

If you use a parallel port drive, for best performance you should set your printer port to use IEEE-1284 standards, such as ECP/EPP or ECP, before connecting your parallel-port CD-ROM. These are bidirectional, high-speed extensions to the normal Centronics parallel port standard and provide better performance for virtually any recent parallel device.

If your operating system supports Plug and Play (PnP) (such as Windows 9x/Me or 2000/XP does), simply plugging a PnP drive into the parallel port enables the OS to detect the new hardware and load the appropriate driver automatically.

With the popularity, performance, and ease of use of USB, I recommend a USB external drive over a parallel port version. USB drives are much faster, more compatible, and easier to install and use. If you need parallel port support for some systems, look for drives that feature parallel and USB interfaces, such as the Backpack product line from Micro Solutions, Inc. See the Vendor list on the DVD for Web site and contact information.

USB Interface

Universal Serial Bus (USB) has proven to be extremely flexible and has been used for everything from keyboards and joysticks to CD/DVD drives from several vendors. USB 1.1 and earlier drives provide read and write transfer rates that match the fastest rates possible with IEEE-1284 parallel ports, with read rates on typical 6x models ranging from 1,145KBps to 1,200KBps.

USB 2.0 and later provide a transfer rate up to 60MBps, which is 40 times faster than USB 1.1 and yet is fully backward-compatible. USB also provides benefits that no parallel port drive can match: for example, hot-swappability, which is the capability to be plugged in or unplugged without removing the power or rebooting the system.

Additionally, USB devices are fully Plug and Play, allowing the device to be automatically recognized by the system and the drivers automatically installed. For Windows 98/Me or Windows 2000/XP systems with USB ports, USB-based CD-RW drives are an excellent solution for backup and archiving of data onto low-cost, durable optical media.


In addition, external CD/DVD drives are now available on the market with a FireWire (also called IEEE-1394 or iLink) interface. FireWire is a high-performance external interface designed mainly for video use. It evolved as an Apple standard and is used primarily on Macintosh systems.

Because few PCs include FireWire ports as a standard item—whereas all PCs include USB—I usually recommend the more universally supported USB for external CD/DVD drives. Make sure any external drives you purchase use the faster USB 2.0 (also known as Hi-Speed USB), which is faster and far more readily available than FireWire versions.

FireWire drives can be useful if you work in a two-platform environment (both PCs and Macs). However, because most Macs also support USB (and you can easily add a USB interface to those that don't), if your primary platform is the PC, I'd still recommend USB over FireWire.

If you do want to use a FireWire drive and your system does not include FireWire integrated into the motherboard, you can easily add a FireWire interface card to your PC. Additionally, some video and sound cards are available with FireWire ports as an option.