Sarasota PC Monitor


Tech Talk (08/01)

IDE, EIDE and UDMA

by Brian K. Lewis, Ph.D.*
Member of the Sarasota Personal Computer Users Group, Inc.

last month's TechTalk was all about hard drive size.

This month I decided to cover hard drive data transfer speed. More and more ads are appearing referring to UDMA/100 or ATA/100 drives. You may have also seen some ads for UDMA/66 or ATA/66 drives. So, what does this mean, and what are the advantages or disadvantages relative to your computer system? Enhanced IDE (EIDE) is the interface standard for the inexpensive, high performance hard disks used in PCs. This is a registered name owned by Western Digital Corporation. They also own the name "IDE." Other companies like Seagate, IBM, Quantum and Maxtor use the term Advanced Technology Attachment (ATA). EIDE and ATA refer to the same interface. However there are many different data transfer protocols included in these terms.

Ultra DMA (UDMA) also refers to the hard drive interface for the transfer of data to and from the hard drive. This is an extension of the original Integrated Data Electronics (IDE) interface that first appeared in the original IBM AT computer and the Enhanced IDE interface. The term AT is simply an abbreviation for Advanced Technology. This original specification was followed by EIDE and then by the UDMA improvements. Within the specification for the UDMA interface there are now three additional protocols: UDMA/33, UDMA/66 and UDMA/100.

All Pentium system boards since 1995 have an EIDE controller built into the chip set. This allows the hard disk and other EIDE units to be connected directly to the system board. The EIDE standard is a great improvement over the old IDE. For example, the EIDE hard disk can exceed the 528MB IDE limit that was discussed last month. The most important feature is that the interface connects directly to the PCI bus. This allows for transfer speeds that far exceed those of the older hard disk controllers.

The original EIDE interface allowed for a maximum data transfer rate of 16 megabytes per second (MB/sec). The next data transfer improvement was accomplished with the introduction of the Ultra DMA or Ultra ATA interface. This is an interface patented by Quantum but supported by all motherboard and disk drive manufacturers. The technology involves an improvement in the governing electronics that deliver the hard disk data to the system board. Quantum succeeded in reducing the bottleneck that occurs in transferring data to/from the EIDE hard disks. The UDMA hard disk is no faster, but the data paths have been optimized. With the new protocol, the speed is doubled by allowing twice the data transfer per clock cycle. This is referred to as UDMA/33 (UDMA mode 2) and supposedly has a maximum data transfer rate of 33MB/sec. However, in practice, this maximum speed is rarely achieved.

In 1997-98 Intel and Quantum created another Ultra DMA standard called ATA/66 or UDMA/66 (UDMA mode 4). This protocol has a theoretical bandwidth of 66MB/sec. It also requires a cable with 80 conductors instead of the original 40. The 40 additional conductors are used for grounding. In the older cables, only seven conductors were used for grounding. This improved grounding removes the noise remaining in the cable after a transmission (crosstalk). In the UDMA/33 protocol the controller had to wait for noise in the cable to disappear before the next transmission. With the new cables the noise is dramatically reduced, so there is less delay in data transmission.

The UDMA/66 protocol is fully compatible with UDMA?33. You may use both types of drives on motherboards that support either UDMA/33 or UDMA/66. Of course, you only get UDMA/33 performance using an UDMA/66 drive on an UDMA/33 motherboard. If your motherboard supports only the older EIDE standard (UDMA mode 0 or PIO mode 4), then you are limited to 16.6MB/sec transfer rates. If you have an older motherboard and you wish to install a UDMA/66 drive, you need to install a PCI-based UDMA/66 adapter to achieve maximum performance.

In spring of 2000, the IBM hard disks became so fast, that UDMA/66 became inadequate. These fast disks use a protocol called UDMA/100, developed by Quantum, who holds the Ultra DMA patents. Where UDMA/33 gave a very powerful boost in the bandwidth between controller and hard disk, UDMA/66 gives less of a gain in performance. On the other hand it solves a lot of compatibility problems by improving timings and other parameters in the UDMA specification. The UDMA/100 specification is reportedly simpler to implement in the chipset logic. Therefore it is cheaper to produce and fully compatible with both UDMA/33 and UDMA/66. It also provides another significant increase in hard drive performance.

The UDMA/100 interface has a theoretical bandwidth of 100MB/sec. This is more than any hard disk can deliver at present. However, the hard disk technology is improving rapidly, so disks may soon reach this limit. When that occurs, further improvements in data transfer technology will be needed. That new technology may be Serial ATA. Intel, Dell, IBM, Maxtor, Quantum and Seagate, and other partners are about to replace UDMA/100 with this new interface. The Serial ATA interface can transfer data at a rate of 160MB per second in its initial version (Serial ATA 1X or SA1X). Later versions are expected to provide bandwidths of up to 528MB per second. This will give us room for the next five years of hard disk technology improvements. It may also put the IDE technology ahead of SCSI for the first time.

Even more promising is the new cable design of Serial ATA. Instead of 40/80 conductors, the cables have only four conductors. This thinner cabling should reduce the airflow problems in many of the smaller computer case designs. There may also be an increase in the number of onboard hard drive controller channels from 4 to 8. Serial ATA will probably kill the remaining hope for use of the IEEE 1394 (FireWire) interface in PC's. It may still be used for video camera connections or other external devices. Intel has never really supported it for hard drive use.

In order to get maximum performance from a UDMA drive, your motherboard chip set, BIOS, and operating system must support the UDMA protocol. Many of the more recent motherboards support UDMA/66, but not UDMA/100. For this latter protocol you may need to install a third party I/O card that supports it. For the UDMA/100 (ATA/100) drives, the drive manufacturer will generally provide drivers to work with Windows 98, 2000 or ME. However, the driver is of no value if the motherboard chipset doesn't support this faster protocol. So, before upgrading your hard drive, check with the motherboard manufacturer, or your owner's manual, to determine which UDMA specification the board supports. When you purchase your drive, be certain that it is supplied with the correct 80-conductor cable required for UDMA/100 and UDMA/66 hard drives.

So now you know that before upgrading an older computer, you need to check out several parameters. First, can your system support a hard drive greater than 40GB in size? Second, what UDMA protocol does it support? The answers to these questions will determine just what you can reasonably expect to install in your system. Although third party I/O cards are available, they do add to the upgrade cost. You must also ensure that you have a PCI slot available for this card. Once you have installed your new drive, you will have reduced, but not totally eliminated, one bottleneck in your computer's data handling system.

*Dr. Lewis is a former University and Medical School professor. He is available to help you with your computer hardware and software problems. He may be contacted via e-mail at brian_klewis@hotmail.com or voice mail at (941) 925-3047. :

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Copyright 2001. This article is from the August 2001 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: admin@spcug.org/ Web: http://www.spcug.org/

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