CD-ROM Interfaces

CD-ROM drives have three main connections on the back: power, audio out to a sound card and a data interface.

These days it is increasingly common to find CD-ROM drives with an IDE data interface, which in theory can be connected to the IDE controller found on virtually every PC. The original IDE hard drive was designed for theAT Bus, and the older IDE interface allowed two hard drives to be attached, one as master and the other as slave. Later, the ATAPI specification allowed one of these to be an IDE CD-ROM drive. EIDE takes this a stage further by adding a second IDE channel for two more devices, and allows a mixture of hard discs, CD-ROMs and tape drives to be used.

An operation on one of these devices must be completed before any others can be accessed. Putting a CD-ROM on the same channel as a hard drive will affect performance as its much slower and will block access to the disc. In systems with two IDE hard discs, the CD-ROM drive should be isolated on the secondary IDE channel while the discs are fitted as master and slave on the primary channel. The discs have to contend with each other but at least the CD-ROM drive won’t get in the way. Other drawbacks of EIDE are that the number of devices which can be attached is limited to four and devices must be mounted internally, so expansion can be limited by the size of the PC.

The SCSI-2 standard allows a maximum of 14 devices can be attached to one host adapter card and these may be a mixture of internal and external devices. SCSI allows all devices on the bus to be active simultaneously, although only one can be transmitting data. Physically locating data on devices is more time-consuming, so while one device is using the SCSI bus, any other devices can be positioning their heads for read or write operations.

By 2000, Ultra160 SCSI – a subset of the larger Ultra3 SCSI standard – supported a maximum data transfer rate of 160 MBps, compared with Ultra ATA/100’s 100 MBps. Moreover, since SCSI devices have more built-in intelligence they are far less CPU intensive than IDE/ATA devices.

SCSI scores over IDE again with its more frugal use of the PC’s resources – namely IRQs. Due to the number of extra cards and devices, today’s multimedia, Internet-ready, networked PCs make heavy demands on IRQs, leaving little or no room for further upgrades. The primary EIDE interface is usually allocated IRQ14, while the secondary gets IRQ15, so four devices can be added for the cost of two interrupts. SCSI is far less resource hungry, as no matter how many devices are attached to the bus, only a single IRQ is required for the host adapter.

In summary, SCSI has greater expansion potential and better performance, but comes at a much higher cost than IDE. The current preponderance of EIDE internal drives would therefore seem to be more a matter of convenience and cost rather than technical superiority and SCSI remains the interface of choice for external CD-ROM drives.