Sarasota PC Monitor
Tech Talk (3/97)
IDE. EIDE & SCSI
by Brian K Lewis Ph D
Member of the Sarasota Personal Computer Users Group, Inc.The alphabet soup in the title refers to the different types of hard drive controllers that are available for most of today's computers. Stick around and I'll try to explain why I prefer the SCSI or "scuzzy" version.
When IBM moved from the original PC to the PC-AT, they included a new method of controlling hard drives. These were drives where most of the intelligence resided on the hard drive itself instead of being on the controller card. So IDE means Integrated Drive Electronics. These drives were a considerable improvement over the older MFM and RLL drives. Installation was simpler since the IDE drives were low-level formatted by the manufacturer. These drives were also capable of some internal sector translation so their Capacities could be increased beyond the limits set by PC/MSDOS. Finally, their data handling was greatly improved so that they could read or write 1-2 megabytes per second on the average.
However, as time passed, computer systems required larger and faster drives. The latest EIDE specification increased the rate of data transmission to a theoretical 16MB per second. In actual practice, the rate tends to be 8-12MB per second. Still, that is a considerable improvement over the IDE specification. A maximum of four of these drives can be installed in newer systems which have two IDE ports (primary and secondary using IRQ's 14 and 15). That's probably more than any home user would want. But, it also means that an IDE CD-ROM can be installed in a separate port with the working drives on the primary IDE port. If you install the CD-ROM on the same port with an IDE drive, it will usually result in slowing the hard drives read/write functions. This is especially true with Windows 95 which may not be able to use a 32-bit driver for the hard drive. The 32-bit Windows 95 driver works in protected mode and contributes to faster data handling. These drivers are much more efficient than DOS mode drivers.
The other drive controller system is the Small Computer Systems Interface or SCSI. These drives have been the standard in the Apple Macintosh since it was first introduced. They have had a much slower acceptance in the PC world, except in large network servers. I believe part of this reluctance to accept the SCSI drive has been related to two misconceptions. One is that SCSI drives are more expensive, which is true to a certain extent. The other is that they are difficult to install and maintain, which is certainly not true.
The SCSI system has a definite advantage over IDE in that you can connect seven devices to one SCSI controller card. That's not just hard drives. The devices can include scanners, tape drives, optical drives, etc. Another advantage is that you can add additional SCSI controller cards to your system. All it requires is that you have the necessary IRQ's for the cards. The data transfer rate for SCSI systems is much higher than for the EIDE drives. The early systems could handle up to about 10MB per second, while the newer SCSI II systems can handle up to 20MB per second. The next step was Wide SCSI which can handle up to 40MB per second. Some wide SCSI controllers can handle 14 devices, just as if they were two controller cards.
How about problems in installing SCSI drives? Two problems seem to exist in many peoples minds. The first is called termination and the other is device ID. Neither of these ia a real problem. When you install SCSI devices internally you connect a cable to the controller card and then to each device in turn. This resembles the IDE/EIDE installation. It is usually called a daisy chain. The next step is that the first and last device in the chain must have terminators. These are usually two or three resistors plugged into the controller card each SCSI device. On some newer SCSI devices, the termination is automatically sensed by the device and turned off and on depending on the device location on the chain. So,we have two hard drives, a SCSI tape drive, and a SCSI CD-RPM connected to the controller via a single cable. The first device in the chain is the controller card and the last device at end of the cable is the CD-ROM. So only these two devices need to be terminated. The resistors can be removed from the other devices. That's not too tough, is it? Next Step add an external SCSI scanner. This plugs into the external port on the SCSI controller card via a cable. The card is no longer at the beginning of the SCSI daisy chain. So you have to remove the terminators from the controller and make sure the scanner is terminated. My scanner has an autotermination setup so it knows when it is the last device on the chain. As you can see, deciding which devices need terminaton is not really difficult. The problem comes when the documatation has printing errors. My controller card has two jumppers instead of terminating resistors. The manual said remove the jumpers to add termination to the card. When I added the jumpers to remove the termination, the only SCSI device my computer could see was the scanner. Since you can't boot from a scanner, it looked like I had a drive problem. However, when I removed the jumpers, the compuyer was able to identify all of the SCSI devices and booted normally. So much for believing what you read.
So how about this ID business? Each device on the must have a different ID number, ranging from 0 to 6. The SCSI controller card is usually factory set at 7. The primary hard drive is also customarily Set at 0. All the other devices can be set to any unused number. The setting of the number has been greatly simplified by the addition of a number "wheel" on most external SCSI devices. In a few cases,the number is set by jumpers. In either case, the only problem is making certain that no two devices are set to the same number.
Now about cost. In the case of hard drives, part of the initial cost is the price for the controllercard. However, you can attach 7 devices to the card, you don't have another controller to buy whatever the type of device you want add. In addition, many removable drives, such as the Iomega and Syquest are SCSI drives which can be easily added SCSI system. SCSI tape drives, optical drives and scanners are much faster than the IDE/parallel port or floppy controller units. In the long run, adding onto a SCSI system can be less expensive and easier than adding to an IDE/EIDI system. Try it. You might like it. I do. My main system is all SCSI.
Dr. Lewis, a former university and medical school professor, is currently a computer consultant. He teaches computer courses at Sarasota County Technical Institute. Additionally, he is available on a fee basis as an on-site consultant to help you with your home or business computer problems. He can be reached via e-mail at bklew@ix.netcom.com or voice mail at (941) 925-3047.
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Copyright 1997. This article is from the March 1997 issue of the Sarasota PC Monitor, the official monthly publication of the Sarasota Personal Computer Users Group, Inc., P.O. Box 15889, Sarasota, FL 34277-1889. Permission to reprint is granted only to other non-profit computer user groups, provided proper credit is given to the author and our publication. We would appreciate receiving a copy of the publication the reprint appears in, please send to above address, Attn: Editor. For further information about our group, email: spcug@netline.net Web: http://www.spcug.org/
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