USB 2.0: A Storage Bus Whose Time Has Come.

The popularity of the Universal Serial Bus (USB) and changing market dynamics have made the USB a strong candidate as an interface for storage applications.

Some in the industry might think that the above statement borders on heresy. However, many of these same people predicted the demise of ATA hard disk drives (HDDs) with the arrival of IEEE 1394 (Firewire).

Today, that vision of doom and gloom has proven to be shortsighted. For several reasons, ATA-based HDDs have actually grown in popularity. The low cost of ATA drives (providing the ability to sell a PC for under $1000), faster ATA standards, and emerging embedded storage applications have breathed new life into the ATA market.

For these same reasons, the concept of the USB as a viable interface needs to be revisited. Easy to install, low in cost, and with a new higher performance standard, USB is ideal for a certain segment of the storage market.

An Overview Of The USB

USB was developed differently than other standards. The ATA/ATAPI, SCSI, and Firewire standards were first developed; then the ability to hot-plug-and-play was added. In contrast, the USB specification anticipated and incorporated hot-plug-and-play from the beginning.

The USB was designed as an interface that would make the installation of peripheral devices (up to 127) to a PC very easy. Initially, acceptance of the USB was delayed by the lack of full operating-system support on some platforms. However in the last year, with most platforms now supporting USB, it has experienced a remarkable turn-around in popularity and viability.

Today, the consumer can install a new USB peripheral on a PC and expect it to work immediately. This is a remarkable accomplishment for a new standard and an exception for the PC industry. However, as more and more users plug all types of devices into their PC's USB, the demand is growing for more bandwidth than the l2Mbit/sec offered by USB Specification 1.1.

A Preview Of The USB 2.0 Standard

There is a legitimate concern about using USB 1.1 for storage applications because its l2Mbit/sec bandwidth limitation may prevent a device from immediately accessing the bus. The market has been quick to respond to this concern, however, as well as the need for more bandwidth to accommodate applications such as video cameras.

The USB Implementers Forum has proposed a new standard called the USB 2.0, which calls for the available bandwidth to be increased to 45OMbit/sec. (Test results have shown that a future version capable of 700Mbit/sec is possible.) At the same time, the new specification allows the USB 2.0 to be backward compatible with the USB 1.1.

The USB 2.0 Vs. Other Storage Interconnect Standards

The new USB 2.0 specification allows a USB to support data transmission on a par with the ATA/ATAPI, SCSI, and Firewire interface standards. In fact, with a maximum transfer rate of 450Mbit/sec (equivalent to 56MB/sec), the USB 2.0 has more than enough speed to function well as a storage interface bus.

Incidentally, the most popular IDE/ATA drives today are the Ultra ATA 33 drives, which have a maximum transfer rate of 33MB/sec, despite the availability of higher performance Ultra ATA 66 drives. For most PC users, the difference in maximum transfer rates is insignificant in actual experience.

Not only does the USB 2.0 match most transfer rates of other interfaces, it also offers benefits that ATA/ATAPI, SCSI, and Firewire cannot provide. These benefits include:

* Ease of installation

* Hot plug and play

* Cross platform compatibility

* Portability

Ease Of Installation

Adding peripherals (such as a Zip drive, a CD-ROM drive, or other storage) to a computer has always been a problem, even for the knowledgeable user. With ATA/ATAPI, SCSI, and Firewire, the user has to choose the right interrupt level and PCI cable, configure the setup, and overcome any conflicts with other peripheral devices already attached to the computer.

Because USB 2.0 is designed to be hot-plug-and-play, installing a storage device is usually as simple as plugging the standard USB cable connector into the computer. The computer's operating system automatically handles all the interrupts, cable, formatting, and conflicts without user intervention.

Cross-Platform Compatibility

Unlike other interfaces, the USE 2.0 provides users with the ability to move peripherals from one platform to another with confidence. No matter what the platform is, USB devices will be compatible.

The USB And The Increasing Market For Portable Storage

Information in electronic format is expanding exponentially. As users become more dependent on the Internet, the need to download and store information is quickly increasing.

At the same time, society and the workplace are becoming more mobile. Today, more and more people are using notebooks--many as their only PC. These users are limited to storing data onto their notebook HDD or onto multiple floppy, Zip, or CD-R discs. None of these options are good because of cost, size limitations, and the performance penalty when running a program.

The market is ready for an easy-to-use, portable storage product. The features of a USB make it ideal for portable peripherals, especially storage applications. Couple this with the ability of the USB 2.0 to effectively compete with the performance of other storage interconnect standards and you have an interface that can dramatically change the storage marketplace.

Marrying ATA And The USB 2.0

ATA-based HDDs currently have the lowest cost and the most established, widely accepted storage-interconnect standards in the market. The USB is the most cost-effective, compatible, easy-to-use interface available. Marrying the ATA HDD to the USB 2.0 is quick and easy and provides definite advantages in the market.

ATA HDD manufacturers can quickly develop USB 2.0 HDD products by combining their existing back-end ATA interface with a standard USB 2.0 device for the front-end interface. In a single swoop, the manufacturer can offer a product that will challenge Iomega's current domination over the removable storage market. Keep in mind that these new products will provide real-time execution and access of data. Iomega products cannot do this.

The USB Storage Bus In The Brave New World

Computer makers dream of manufacturing a PC that is very easy to install and use. In this vision, the USB could be the ideal storage bus. Consumers would simply buy the storage formats they want--DVD, floppy disk, HDD, or Zip drive--and then plug them into the PC chassis or frame. Bingo, the PC would be ready to use.

Seattle-based Stratos Product Development (www.stratos.com)is actually doing this now. Recently, they introduced a new USB connectivity center that allows external peripherals to be easily attached to a computer or hub in any orientation without cables. The user simply selects the peripherals and plugs them in. This kind of hot-plug-and-play is exactly what PC users are looking for.

Increasingly, the largest storage markets consist of mobile users and consumers, segments that are more concerned with convenience than a slightly higher maximum datatransfer rate. Those of us in the storage industry need to "think outside of the box" and give these users the storage products they want and need--at a price they are willing to pay. With its combination of low cost, a 45OMbit/sec transfer rate and benefits that no other standard can offer, the USB 2.0 and, specifically, its integration with ATA is perfect for these users.

Bob Salem is the product manager of semiconductors at CMD Technology, Inc. (Irvine, CA).

                       Maximum
Standard  Mode      Transfer Rate
                      (MB/Sec)
ATA/ATAPI PIO-0          3.3
          PIO-1          5.2
          PIO-2          8.3
          PIO-3          11.1
          DMA 0          16.7
          DMA 1          25.0
          DMA 2          33.3
          DMA 3          44.4
          DMA 4          66.7
SCSI      Narrow         5.0
          Fast           10.0
          Fast/Wide      20.0
          Ultra          40.0
          Ultra 2        80.0
1394      400            50
          800            100
USB       1.1            1.5
          2.0            56.3