KarbosGuide.com. Module 5b5.

About the UDM/66 interface

The contents:

  • Ultra DMA ATA/66
  • Chipset support to UDMA/66
  • ATA/100
  • Serial ATA

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  • Ultra DMA ATA/66

    In 1997-98 Intel and Quantum created the new Ultra DMA standard called ATA/66. This gives a theoretical bandwidth of 66 MB/sec.

    The new system requires a new cable with 80 conductors. The 40 new conductors are used for grounding. In the old version, only 7 cables were used for grounding. The new and very effective grounding removes the so-called crosstalk (i.e. noise remaining in the cable after a transmission).

    In ATA/66 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, the transmissions can follow one by one without delay, and the bandwidth goes up.

    The ATA/66 protocol is fully compatible with ATA/33. You may use both type of drives on motherboards either with ATA/33 or ATA/66. Of course you only get ATA/33 performance using a ATA/66 drive on a ATA/33 motherboard. You may upgrade your motherboard with an PCI-based ATA/66 adapter. This is quite cheap.

    The new cables also use the same old 40-pin plugs:

    If you use a ATA/66 system with a 40-pin cable, the protocol will automatic switch to ATA/33.

    What is required?

    According to Western Digital (who had some of the fastest UDMA/66 drives), to use the ATA/66 technology a PC system must have:

  • Ultra ATA/66 compatible logic either on the system motherboard, or on an adapter card

  • Ultra DMA compatible BIOS

  • DMA-aware device driver for the operating system

  • Ultra ATA/66-compatible IDE device such as a hard drive or CDROM drive

  • 40-pin 80-conductor cable

    See this white paper from WD

    ATA/66 was very neccessary for further development of the EIDE harddisks. With the increasing data density the media data transfer rates are going up and up. Therefore the host data transfer rates also must increase. All the time the controlling logic must have a better transfer rate than the media or else performance is reduced. The new EIDE disks coming out from IBM and other vendors delivered such a powerful data output that the old UDMA ATA/33 standard could not cope with it.

    Chipset support to UDMA/66

    VIA's Apollo Pro+ chipset fully supports UDMA/66. The best performance should be gained from the T82C686A super south bridge controller.

    Intel supports it with the 810 (Whitney), 820 (Camino) and 840 (Carmel) chip sets.

    Other setups include special logic chips to includefull UDMA/66 performance tofor instance a BX-based motherboard.

    Promise FastTrak66

    Promise produces a PCI-based controller called FastTrak66 that does the job. It controls UDMA/66 disks at full speed, and it even allows doubling or quadroubling the speed using RAID techniques. See the description of the older Promise FastTrak controller.

    Promise FastTrak66


    In spring 2000 the new IBM disks became so fast, that ATA/66 was out of business. The disks use a new protocol called ATA/100, being developed by Quantum, who holds the Ultra ATA patents.

    The ATA interface started in 1996 with ATA/33, which in 1998 was upgraded to ATA/66. Two years later the ATA/100 was released.

    A kind of ATA/66 Second Edition

    Where ATA/33 gave a very powerful boost in the bandwidth between controller and harddisk, the ATA/66 gives a minor gain in performance. On the other hand it solves a lot of compatibility problems by improving timings and other parameters in the specification.

    As a result, the ATA/100 is reported being more simple to implement in the chipset logic. It is cheaper to produce and fully compatible with both ATA/33 and ATA/66.

    There is an upper limit of disksizes at 137 GB in the ATA/100 interface. However there has been made some workarounds to this problem in some comtrollers.

    100 Megabyte per second

    The ATA/100 interface have theoretical bandwidth of 100 MB/sec. This is more than any harddisk can deliver at present. However the harddisk technology is improving in very high speed, so the disk soon will reach this limit. Hence the Serial ATA will be needed.

    Another technology we will see more and more is ATA-based RAID. Using two or four cheap ATA-disks you can have a very powerfull disk system with doubled bandwidth. This requires a RAID-controller like the Promise FastTrak or a motherboard with onboard controller. Theserequires higher bandwiths in the ATA protocols to show real powerful performances.


    In 2001 some hardware vendors introduced ATA/133 as new version of the interface.

    Serial ATA

    Another more interesting new technology is the Serial ATA. Intel, Dell, IBM, Maxtor, Quantum and Seagate and other partners are about to replace ATA/100 with a faster drive interface.

    The new Serial ATA interface, can pump out 160 MB per second in the first version (Serial ATA 1x or SA1X). Later version promises bandwiths up to 528 MB per second. This will give us headroom for the next five years of harddisk technology improvements.

    Even more promising is the new cable design of Serial ATA. Instead of 40/80 conducters the cables only holds four conductors. This thinner cabling is great news for everyone putting together his own PC. I also expect the number of onboard ATA-channels to increase from 4 to 8.

    Serial ATA probably will kill the remaining hope for generel use of the IEEE 1394/FireWire interface for PC's harddisks. This was never really supported by Intel.

    Hopefully the new interface will operate with a command queue, which has been a great lack in ATA-design compared to SCSI.

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    To learn more

    Read more the boot process and system bus in Module 2b

    Read about file systems in module 6a

    Read about I/O buses in module 2c

    Read about the motherboard chip set in module 2d

    Read about RAM in module 2e

    Read Module 5c about SCSI, USB etc.

    Read module 7a about monitors, and 7b on graphics card.

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