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SCSI vs. Enhanced IDE (EIDE)

IDE was designed as a low-cost, easy to use interface. Although EIDE has broken the original IDE 528 MB limitation, EIDE transfer rates are still limited to between 9 and 16 MB/s (33 MB/s Ultra DMA) and can only support 4 devices. These devices can only be two hard disk drives, a CD-ROM and a tape drive. Conversely, SCSI is designed for optimum performance and flexibility. Wide Fast SCSI subsystems are capable of transferring data at up to 20 MB/s, 40 MB/s (Wide Ultra SCSI) and 80 MB/s (Wide Ultra2 SCSI) by utilizing an intelligent SCSI adapter. Disk throughput has always been important in high-end, multi-user systems but until recently has not been an issue with single-user PCs. With the availability of the Windows 95 operating system and the popularity of software using CD-ROM, the task of keeping up with the data requirements of these applications has become more challenging. Faster SCSI drives coupled with bus mastering SCSI adapters have begun to look more attractive to users of EIDE drives.

Other advantages of SCSI over EIDE include its ability to process multiple overlapped commands, support for command queuing and support for scatter/gather data transfers. These features combine to optimize and maximize throughput. Since EIDE is still tied to the old WD1003 (ST506) interface, overlapped I/O and command queuing functions cannot be executed and only single task operations are possible.


Device Support: Hard Disk, CD-ROM, Tape, Optical, Scanner, WORM, etc. Hard Disk, CD-ROM
(EIDE only)
Max. No. of Devices: 15 (16-bit Wide SCSI) 4
External Device Support: Yes No
Max Data Transfer Rate: 40 MB/s
(Wide Ultra SCSI)

80 MB/s
(Wide Ultra2 SCSI)
16 MB/s
(Mode 4)

33 MB/s
(Ultra DMA)
Command Queuing: Yes No
Overlapped I/O: Yes No
Multi-tasking Support: Yes No
Error Checking via Parity: Yes No

The performance disadvantages of EIDE, although not so apparent under DOS, severely limit the performance of multi-tasking operating systems such as Windows 95, NetWare, Unix, Windows NT and OS/2. These operation systems benefit significantly from SCSIs ability to perform overlapped I/O and command queuing.

Unlike EIDE, SCSI supports devices connected to the computer externally. With EIDE all connected devices must reside inside the computer box. This can present some obvious configuration and capability limitations. SCSI also offers parity-based error checking to maximize the probability of error-free data transmission. Additionally, the choice of EIDE devices is limited currently to hard disk drives and CD-ROMs, while SCSI devices include hard disk drives, CD-ROMs, WORMs, optical devices, scanners, tape drives, and many others.

In consideration of SCSIs feature and performance advantages over EIDE, it is the view of industry analysts today that EIDE will continue to be dominant on the low-end, price-sensitive DOS-based PCs, while SCSI will be the peripheral interface of choice of all other platforms, particularly multi-user systems and those that are sensitive to performance issues.


For personal users, if using just only one hard drive to operate, EIDE can be good enough for data transfer.  However, if users are looking for integrity and high performance I/O, we recommend to use two SCSI hard drives and one IOI PCI SCSI host adapter with software RAID (0, 1, and 0+1). 

To determine whether to mirror (RAID 1), stripe (RAID 0) and mirror and stripe (10) a volume, consider the following:

  • RAID 1: Mirror time-critical valuable data; data you need to access frequently which must be available at a moment's notice.
  • RAID 0: Stripe volumes are used with high performance applications, such as Digital Video and Photoshop.
  • RAID 10: Mirror your critical data and stripe your applications.

If you need further information regarding RAID, click here to obtain detail reference.

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Ultra2 SCSI (LVD/SE):
Low Voltage Differential
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       Featured Products:
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