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EIDE vs. SCSI : The Hard Drive Jumble

by Bill Santry
April 19, 2003

They're everywhere, those strange jumbles of letters that define your computer's hardware. RAM, DVD, CD-ROM, USB, and on and on. What the heck is all this? We will begin to help you make sense out of this high-tech alphabet soup in the following article by defining and explaining the differences between the two hard drive interfaces found in PCs, IDE/EIDE and SCSI. Hey, it's a start.

In this article, you will find:

  • A brief review of a hard drive
  • Information about IDE/EIDE
  • An introduction to SCSI
  • Pros and cons

What's the C stand for?

You probably know it best as the C: drive. That crucial letter that holds your programs, games, documents, and other vital data. Your hard drive is actually two pieces that make up one unit: the hard disk and the hard disk controller. The hard disk is the medium that holds your data on it's magnetic skin. The hard disk controller directs the action, retrieving data or recording it as necessary. The interaction between your computer's operating system, hardware devices, hard disk controller, and hard disk is managed by one of the interface standards described below.

More of the hard drive alphabet: IDE/EIDE and SCSI

While the functioning of a hard drive is generally the same from manufacturer to manufacturer, with differences in the method of reading and writing or caching information, there are two main types of interface for hard drives: IDE/EIDE and SCSI.

IDE (Integrated Drive Electronics) was the standard electronic interface used between a computer's motherboard and its disk device(s). IDE managed the flow of data to and from the disk, known as the "data paths".

Past tense is used because IDE was replaced with EIDE (Enhanced Integrated Drive Electronics). The limitation of IDE was that it could not manage hard drives with storage capacities of more than 528 megabytes. EIDE brought with it faster access to your disk and the ability to manage additional drives (hard disks, CD-ROM drives, or tape drives). In addition, EIDE maintains a Logical Block Address (LBA), which stores details about the physical location (cylinder, track, and sector) of data on a disk. Think of it as a highly detailed whitepages for your hard disk.

So if EIDE can do all that, what is SCSI (pronounced SKUH-zee) all about? SCSI (Small Computer System Interface) allows personal computers to communicate with all types of hardware such as disk drives, CD-ROM drives,tape drives, printers, and scanners faster and with more flexibility than EIDE. Devised by Apple Computer, SCSI is standard in Macintosh products. SCSI ports, however, are built into most high-end PCs and the interface is supported by the major operating systems.

Getting SCSI-er by the day

Unlike EIDE, SCSI is an evolving interface that is increasing the speed at which data can be transferred between devices. The important benefit of SCSI is not in boosting the performance of a single piece of hardware, but in moving data between devices more quickly than with EIDE. SCSI manages data flow so that devices which are not active do not take up resources.

The original SCSI, now known as SCSI-1, evolved into SCSI-2. Its widespread adoption meant SCSI-2 would eventually be referred to as "plain SCSI." SCSI-3 brought with it command sets and procedures for specific device types. SCSI-3 treats a CD-ROM or scanner differently than a hard drive or printer. These command sets meant SCSI could talk across parallel and serial ports.

The latest SCSI standard is Ultra-2. It can push through data at 80 megabits per second. Longer cables can now be used to connect devices because Ultra-2 uses two wires to relay data instead of the one wire present in previous SCSIs. By using two wires, a lower voltage is needed to send data and the wire can be longer without fear of signal degradation. Normal Ultra-2 allows up to 7 device connections, but Wide SCSI Ultra-2 can increase this number to 15. This process for connecting hardware is described as "daisy-chaining". The term Wide is used because the data cable is 68 wires wide and 16-bit, rather than the 50 wire, 8-bit cable of Ultra-2.

SCSI: Too good to be true?

Two serious drawbacks to SCSI-based systems are expense and uniformity. SCSI versions of hard drives or other devices are always more expensive than their EIDE counterparts. SCSI systems must also be uniform. If you want the benefits of a SCSI hard disk, you must also invest in a SCSI CD-ROM drive, floppy drive, etc. Unlike EIDE, SCSI relies on a separate device (a controller card) to manage the interactions. Most motherboards do not come with integrated SCSI controllers.

Newer incarnations of SCSI are generally compatible with older SCSI devices, although the device will work at the slower data rate.

Bottom Line

So which is best for you, the consumer? If you have mucho dinero to spend on a system, you might consider the high-performance SCSI setup. Be aware, however, that in addition to the initial expense of the SCSI system, upgrades will also take a bigger bite out of your wallet when compared to EIDE purchases. Not an issue for Mac owners. You will also be tempted to move up to the latest standard of SCSI with each evolutionary step in the interface. EIDE may not be as glitzy as SCSI, but the savings and simplicity of the interface may be your best bet.

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